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Ask an Astrophysicist: Earth and Moon

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Earth and Moon

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Library of Past Questions and Answers

    Calendar and Seasons

When is the Sun at it's highest point in the sky? (Submitted July 14, 1997)

The Question

When is the Sun at its highest point and how did you determine the answer?

The Answer

Thank you very much for your question about the Sun, it was a pleasure to answer. The short answer to your question is simply "noon." Astronomical noon is defined to be the time of day when the Sun is highest in the sky. For a northern latitude of 40 degrees (typical of North America) the Sun's noon position ranges from about 40+23=63 degrees South of straight-up in Late December to about 40-23=17 degrees South of straight-up in late June. That is why it is hotter in the summer than the winter. (Note: The Earth's spin axis is tilted by 23 degrees with respect to its circular orbit around the Sun; that is where the 23 degrees comes in.)


TIME ZONES AND DAYLIGHT SAVING TIME

Now, Astronomical Noon is not always at 12:00 local time. In the winter when we are on Standard Time it is within an 1/2 hour or so of 12:00 -- closer if you live near the middle of your time zone. If you live on the western edge of your time zone, Astronomical Noon is a little later than 12:00 because the Sun moves from East to West during the day. If you live on the Eastern edge it is earlier. (It could even be a few hours off if you live in parts of Alaska.)

On the other hand, during Daylight Saving Time astronomical noon is at around 1:00 pm., because we change our clocks so we can have more daylight in the evening when we are awake, and so the Sun does not rise too early in the morning when we are asleep.


THE SUN'S PATH IN THE SKY

There is another astronomical effect you should know about that can change the time that the Sun is highest in the sky. This is a little hard to explain, but it also has to do with us watching the Sun from a tilted perspective. The Earth spins on its axis about 366 and 1/4 times each year, but there are only 365 and 1/4 days per year. This is because we define a day not based on the Earth's period of rotation, but based on the average time from noon one day to noon the next. Gradually over the course of a year the Sun appears to go 'backwards' (West to East) around the Earth compared to the far away stars (this is because we are really going around the Sun). Subtracting this 1 time backwards from the 366 and 1/4 times forward, we get the typical 365 and 1/4 days per year.

So back to the point of the Sun at noon. The Sun appears to go around the Earth in a circle from West to East once per year in an orbit that is tilted by 23 degrees from the Equator (because the Earth's axis is tilted). This tilt means that sometimes the Sun appears to move mostly West-to-East, and very little North or South (like in late Dec and late June). While at other times is appears to move a good deal North or South (like in late March or late September), so it appears to move slower West-to-East.

So how what does this have to do with time and the Sun at noon? The time standard is to set our clocks to 12:00 when the Sun is highest in the sky on March 21st, since this is when it crosses the equator, also known as vernal equinox. But as you check the Sun's position at the same time, by your watch, on subsequent days, it appears to move West. This is because it appears to be moving slower than average West-to-East.

Conversely, in late December the Sun looks like it is turning around from moving South to moving back North again, known as the winter (to people living in the northern hemisphere) solstice. It does not appear to move either North or South, but it is moving faster than average West-to-East. Now following this argument we see that right after the equinox, in April and May, the Sun will be a little West of South at 12:00 (or it will be highest in the sky at a little before 12:00). However, by late June (the summer solstice) the Sun would have caught-up so it is now highest in the sky at 12:00 again. In July and August it is now East of South at 12:00 (or it will be highest in the sky at a little after 12:00). And then by late September (the Autumnal Equinox) it has fallen back to being South at 12:00 again.

There is a chart, called the Analemma (URLs below) that puts together all these effects to show the position of the Sun at noon each day throughout the year. You can recognize the Analemma because it looks like a figure 8.

(see http://www.alpheratz.net/murison/SunAltAz/WashDC1997.html)


YET EVEN ANOTHER EFFECT

If you are even more interested in this, there are some smaller effects because the Earth moves slightly closer and farther away from the Sun throughout the year. (We are closest to the Sun in January and farthest away in July). The motion toward and away from the Sun is very small compared to the distance to the Sun, yet still large compared to the size of the Earth. When the Earth is farther away from the Sun, it moves slower and when it is closer it moves faster. It is this effect that makes the Analemma not perfectly symmetrical -- it has a bigger loop on the bottom than the top. If the Earth had a perfectly circular orbit, the Analemma would be a perfectly symmetrical Figure 8 with the cross-over point directly above the Equator. As you can tell from the figures (URLs above) it is not symmetrical.


I hope this leaves you lots to ponder. We really enjoyed your question because it is so simple -- yet it can be answered simply, in more detail or excessive detail.


HOW YOU CAN MEASURE THIS YOURSELF

One way to determine when the Sun is highest is to find the time when shadows are shortest. Find a flagpole or other vertical object surrounded by level ground. Every ten minutes or so, place an object, marked with the time, at the tip of its shadow. When the shadow has stopped shrinking and is starting to get longer again, the object closest to the flagpole will be marked with the time at which the Sun was highest. If what we said above is correct, the shadow should be due North. Go test this yourself and see if we got it right!

Sincerely,

Jonathan Keohane
For Ask an Astrophysicist

-- with much technical expertise from David Palmer and help from Paul Butterworth, Karen Smale and Tess Jaffe

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Question ID: 970714

What is the altitude of the Sun on Equinoxes and Solstices? (Submitted February 10, 1997)

The Question

What are the altitudes of the Sun on the days of the Vernal and Autumnal Equinoxes, and on the Summer and Winter Solstices, knowing that the latitude of where we are measuring from is 40.3 degrees?

The Answer

Let's first define altitude. "Altitude" is the height an object is above the horizon. The altitude of the Sun varies throughout the day, but it reaches its maximum altitude around noon-time. So the most meaningful answer to your question would be the maximum altitude of the Sun on these dates.

At a north latitude of 40.3 degrees, the maximum altitude of the Sun is 49.7 degrees on the Vernal and Autumnal Equinoxes. This is because the Sun is crossing the celestial equator on those days, and the maximum altitude of the celestial equator is simply 90 degrees minus your latitude.

On the Summer Solstice, the maximum altitude of the Sun as seen from a north latitude of 40.3 is 73.2 degrees; on the Winter Solstice, it's 26.2 degrees. This is because the Sun travels along the ecliptic, which is inclined by 23.5 degrees from the celestial equator. On the day of the Summer Solstice, the Sun is at the point on the ecliptic which is furthest above the celestial equator (i.e. add 23.5 degrees to the equinox value). On the day of the Winter Solstice, it's at the point which is furthest below the celestial equator (i.e. subtract 23.5 degrees from the equinox value).

The 23.5 degrees should sound familiar, as it's the angle by which the earth's axis is inclined to its orbit. The celestial equator is an extension of the earth's equator onto the sky. The ecliptic is a projection of the earth's orbit onto the sky. The earth's orbit projected onto the sky is equivalent to the Sun's path across the sky through the course of the year. The maximum altitude of the Sun is just a matter of the geometry of these two orbits.

Jim Lochner
for Imagine the Universe!

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Question ID: 970210b

What sequence of events is necessary for the completion of one month on our calendar? (Submitted April 08, 1997)

The Question

What sequence of events in the solar system is responsible for completing one month on our calendar?

The Answer

The month is roughly based on the time from full Moon to full Moon as seen from the Earth. This is between 29 and 30 days.

Now there are two catches:

1. There are not an even number of "Moons": in one year nor an even number of days in a "Moon," so we have a calendar that only approximates this by dividing up the year into 12 months.

2. The actual time it takes for the Moon to orbit the Earth is only about than 27 days, however because the Earth orbits the Sun the Moon has to go a little farther before we see it as full again. Therefore, there are about 12 "Moons" in a year, but the Moon goes around the Earth about 13 times

Thanks for asking.

Sincerely,

Jonathan Keohane
-- for Imagine the Universe!

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Question ID: 970408a

How did Gauss calculate the date Easter will fall on? (Submitted April 01, 1997)

The Question

Any idea how Gauss formulated his method of determining Easter within 3 days for any year of the Gregorian calendar?

The Answer

I'm not sure how Gauss derived his famous equation for the calculation of Easter. In general, calculating the dates of festivals which are partially defined in terms of astronomical events is not difficult. You have to calculate how a very predictable and regular thing (the astronomical event) combines with another very predictable and quite regular thing (a human calendar - Julian, Gregorian, Jewish, Islamic, Mayan, ...). Churches and other authorities have been calculating festivals well in advance for long periods. (Often they introduce some simplification of the astronomical event, such as considering the mean motion of a body, but that's fine - they are in charge of the festival and can set up the rules for its occurrence however they want to!). The achievement of Gauss was to come up with such a simple formula for the calculation of Easter. (His formula can be readily generalized to give the correct date in many different calendars and during any desired range of years). I looked at a number of books on Gauss to see if any had a detailed account of his work on the Easter formula, but found nothing beyond references to his almost unequaled ability to recognize patterns, upon which some of his other extraordinary accomplishments in mathematics were founded. One book (by Schaaf, see below) gave the following reference to Gauss's first publication of his Easter work, which isn't in our library and I haven't had time to pursue:

Monatliche Correspondenz, August 1800, page 121.

The books I looked at were:

"Carl Friedrich Gauss, prince of mathematicians" by W.L. Schaaf, and "Carl Friedrich Gauss, titan of science" by G.W. Dunnington.

There are several good web sites which discuss calendars.

Paul Butterworth
for the "Ask an Astrophysicist" team

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Question ID: 970401b

How does the tilt of the Earth's axis affect the seasons? (Submitted February 11, 1998)

The Question

How does the earth's tilt affect the changing of the seasons, and what different angles cause those different seasons?

The Answer

The bottom line for the changes from season to season is the average daytime temperature. This depends on the amount of heating that the earth receives in a single day throughout the year, and this depends on how many hours the Sun is above the horizon and exactly how long it spends at its highest elevation above the horizon. For every square meter on the surface of the earth, it will be heated by the Sun at a rate that depends on the 'cosine' of the angle of the Sun above the horizon. The higher the Sun gets, the less slanted the rays of light are that intercept each square meter, and so the efficiency with which these slanted rays can deliver energy to the surface gets better and better the higher up the Sun gets. When you add up during the daylight hours just how much heating this surface gets, it receives most of its heating from those times during the day when the Sun is the highest above the horizon. For a tilted earth, there will be some days during the year at a given latitude, where this heating rate is the highest and we call this summer. There will be other days when the Sun never gets very high above the horizon and so its heating ability is very low, and we call this winter. The details of just how hot and cold we get, and the exact dates, depend also on whether we are near water, or in the interior of a continent.

So, seasonal changes depend on the tilt of the earth's axis because they lead to changes in the amount of heat delivered to a square meter of surface, and the fact that there are a changing number of hours in the day when the Sun is above the horizon and high enough up that it can efficiently heat the surface over the course of a typical day. (from Ask a Space Scientist)

There is an activity on our StarChild website that illustrates how the earth's tilt affects the change in the seasons. Take a look at http://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level2/javascript/song.html

Jim Lochner and Maggie Masetti
for Ask an Astrophysicist

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Question ID: 980211f

Why is winter colder than summer? (Submitted February 21, 1998)

The Question

I have heard two conflicting reasons explaining why winter is cooler.

(1) Because of the slight pivot of Earth's on its axis, the Sun is farther away during winter because part of the planet is pointing away from the Sun, hence, less energy reaches that surface.

(2) The Sun is actually CLOSER to the surface during winter but light hits the planet at an obtuse angle which "skims" the surface. Direct rays are not hitting the surface which brings about cooler temperatures.

Which explanation is correct?

The Answer

The second. Winter is colder because the earth's axis is tilted. Winter occurs for the hemisphere which is tilted away from the Sun (the northern hemisphere in January, the southern in July). This has two main effects on the winter hemisphere. First, the Sun is above the horizon for fewer hours each day, so that hemisphere receives less heat from the sun. Also, sunlight strikes the ground at a shallower angle so that less energy per unit area is intercepted by the winter hemisphere. It is true that the earth is closest to the Sun in January. However, the distance from the earth to the Sun varies by only about 2% over a year. This causes a change of only 4% in the amount of solar radiation hitting the earth so this effect is not significant compared to the other two.

Damian Audley and Kevin Boyce
for Ask an Astrophysicist.

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Question ID: 980221a

What causes the time asymmetry of sunrises and sunsets? (Submitted January 16, 1998)

The Question

I have a question regarding sunrise and sunset. I realize that the winter solstice on 21Dec is the shortest day of the year. Since this date, the days have been gradually getting longer. Sunset has been getting gradually later as expected, however, sunrise continued to come later until the first week in Jan. My question is: what is the cause of this asymmetrical distribution of daylight between sunrise and sunset?

The Answer

This is due to a phenomenon called "the equation of time".

Solar day is the length of time between one local noon (when the Sun is highest in the sky) to the next. As it turns out, the length of the solar day is not always 24 hrs (its average over the course of a year defines 24 hrs). The solar day would always be 24 hrs if the Sun 'moves' east against the background of fixed stars at a constant rate (for convenience, astronomers have invented 'Mean Sun' to do exactly that). The real Sun moves at a variable rate, however,

  • Because of the tilt of the Earth rotation axis relative to its orbit around the Sun (the obliquity), the same reason as for the changing length of daytime hours.
  • Because the Earth's orbit is elliptical and so it moves faster at perihelion (around Jan 2) than at aphelion (Jul 3).

    Both effects combine to create an offset in the time of local noon (and those of sunrise and sunset) by as much as +/- 16 min: this is the equation of time. Around winter solstice, the daily change in the equation of time happens to be more important than the daily change in the length of the day, causing the phenomenon you so keenly observed.

    The equation of time is often represented by a figure 8. That figure is called an 'analemma'. There is an actual photograph of an analemma at http://sundials.org/links/local/pages/dicicco.htm, which was taken by Dennis di Cicco.

    I hope this helps,

    Koji Mukai, David Palmer, and Tim Kallman
    For the Ask an Astrophysicist Team

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    Question ID: 980116c

  • What is Sidereal time? (Submitted February 28, 1997)

    The Question

    Can somebody please tell me what sidereal time is?

    The Answer

    Sidereal time is based on how long it takes for a celestial object, like a star, to return to the same position in the sky after the Earth rotates. This is not quite the same as the typical 24 hour day because the Earth moves in its orbit around the Sun during the 24 hour day, and this motion causes an apparent shift in the positions of stars, etc. (from the Earth's point of view). It turns out that a sidereal day is about 4 minutes shorter than a 24 hour day.

    Andy Ptak
    for the Ask an Astrophysicist Team

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    Question ID: 970228c

    When does the new millennium start? (Submitted September 02, 1998)

    The Question

    Hi,

    I'm 17 years old. I'd like to know when the new millennium starts. Isn't it Jan 1st, 2001? Why do people get excited about 2000 then? How can I explain this to my friends? Please help.

    The Answer

    You are right that the millennium starts on Jan 1st 2001. There is no year zero, so the first millennium started on January 1, 1 C.E.*, the day after December 31, 1 B.C.E. The first millennium ended 1000 years later, on the night of Dec 31, 1000/morning of Jan 1, 1001, and the second millennium ends 1000 years after that, on Dec 31 2000/Jan 1 2001.

    However, the year 2000 is when you have to throw away all your old printed checks and get new ones. It's the year that there will be computer failures due to Y2K bugs making them believe they have just jumped back a century. When you buy a car, it typically has a few tenths of a mile on its odometer, but you still celebrate when it clicks to all zeros.

    The main reason people will celebrate the millennium on the night of Dec. 31 1999 is to hold big parties, and to hold them a year sooner than they would otherwise. I expect that, around February, 2000, people will start coming around to the belief that the millennium does indeed start with 2001, and plan their next party accordingly.

    *C.E. and B.C.E. Common Era, and Before Common Era, are the deity-neutral terms for what used to be called A.D. and B.C.

    David Palmer and Samar Safi-Harb
    for Ask an Astrophysicist

    P.S. Astronomically speaking, the end of one millennium and the beginning of the next does not have any special meaning. In particular, neither January 1, 2000 or January 1, 2001 has any special planetary or solar alignments. Even if they did, planetary alignments have no effect on the Earth.

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    Question ID: 980902d

    Can science confirm the missing day referred to in the Bible? (Submitted March 25, 1997)

    The Question

    Can the event that took place in Joshua 10:13 be confirmed, for example by counting the positions of heavenly bodies backward in time?

    The Answer

    According to the laws of physics, there are only two possible explanations for having the Sun stand still in the sky for a day: (1) the Earth would essentially have to stop spinning on its axis...for which there is no evidence. -or- (2) the Sun would have to start moving about in the solar system in a very specific way so that it appeared to us on our spinning Earth to be standing still. There is no evidence of this occurring either.

    We, too, have heard an "urban legend" about scientists at NASA GSFC finding the "missing day" in computer calculations of the motions of the planets. The legend has been around for longer than NASA itself, but turned into a NASA "event" sometime in the 60's. The story goes that some scientists were doing orbital mechanics calculations to determine the positions of the planets in the future, for use in determining the trajectories of future satellite missions. They realized they were off by a day. A biblical scholar in the lot remembered the passage from Joshua and all was set right. But these events, in fact, never occurred. It is easy to understand why:

    The "GSFC finds missing day" urban legend doesn't make sense for the following reason. If we want to know where the planets will be in the future, we use accurate knowledge of their initial positions and orbital speeds (which would be where they are located now), and solve for their positions for some time in the future. We solve a very well determined set of equations that describe their motions. The major dynamical component of any planet's orbital motion is determined by solving an equation (force is equal to the mass times the acceleration) which is the perhaps the most fundamental in classical physics. The validity and predictive power of this equation are well documented and can be seen every day: a recent example is the lunar eclipse that was visible to much of the world last Sunday. This calculation would not cover any time before the present, so some missing day many centuries ago, if it had occurred, could not be uncovered with this method.

    In general, trying to prove events that are said to have occurred in the Bible, using scientific principles, doesn't work. Most scientists draw a clear distinction between things that are taken on faith, and those that are testable and therefore falsifiable. Science deals with the latter, and religion with the former.

    Check out:

    Brunvand, Jan Harold (1984) The Choking Doberman and Other "New" Urban Legends. W. W. Norton and Company, pp. 198-199.

    Brunvand, Jan Harold (1991) "The Missing Day in Time," paper presented at the annual conference of the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP), Berkeley, California, May 4.

    Loftin, Robert W. (1991) Origin of the Myth About a Missing Day in Time. Skeptical Inquirer. vol. 15, no. 4, Summer, pp. 350-351.

    McIver, Tom (1986) Ancient Tales and Space-Age Myths of Creationist Evangelism. Skeptical Inquirer. vol. 10, no. 3, Spring, pp. 258-276.

    and

    Talk.Origins Archive Feedback for June 1998

    Has NASA Discovered Joshua's "Lost Day"?

    Regards,
    Padi Boyd and Laura Whitlock
    for Ask an Astrophysicist

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    Question ID: 970325g

    Do eggs only balance on end during the Spring Equinox? (Submitted March 21, 1997)

    The Question

    We did a first day of spring experiment. We were able to balance an egg on its end on one of the tables in our classroom. We don't know why this worked. Does anyone have an answer for us?

    The Answer

    I can suggest a good follow-up experiment --- pick a random day of the year and try again to balance an egg on its end on several different tables, including the one you had success with on the first day of spring. Try with several eggs, as the shapes of the individual eggs do matter. Mark the eggs to indicate which ones could be balanced, and save them in a fridge. Try the same experiments again a few days later, with those saved eggs. In this way, you'll be able to figure out what was the cause and what was just a coincidence.

    Best wishes,

    Koji Mukai
    for Imagine the Universe!

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    Question ID: 970321c

      The Earth

    Is there a source that one can peruse and purchase high altitude pictures of a particular area on earth? (Submitted October 08, 1997)

    The Question

    Is there a source that one can peruse and purchase high altitude pictures of a particular area on earth? (For example, one's home town.) I'm sure NASA has a source but I can't seem to find it.

    The Answer

    This outside of our field of high-energy astrophysics, but you might try the following link:

    http://ic-www.arc.nasa.gov/ic/projects/bayes-group/Atlas/Earth/

    Andy Ptak
    for Ask an Astrophysicist

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    Question ID: 971008a

    Why does the Earth spin on its axis? (Submitted November 07, 1996)

    The Question

    How does the Earth manage to spin on its axis?

    The Answer

    The Earth spins on its axis because of conservation of angular momentum. The classic example of this is a figure skater. When a figure skater pulls in her arms, she spins faster. The Earth formed when gas left over from making the Sun condensed into the planets. As this gas cooled and condensed, it started to spin faster. Now that it is spinning (and not condensing any more), it will keep spinning at a steady rate unless something stops it.

    Andy Ptak and the Ask an Astrophysicist team

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    Question ID: 961107a4

    How fast does the Earth rotate? (Submitted April 01, 1997)

    The Question

    I am trying to find out the speed of the turning of the Earth.

    The Answer

    Basic Answer

    The circumference of the Earth at the equator is 25,000 miles. The Earth rotates in about 24 hours. Therefore, if you were to hang above the surface of the Earth at the equator without moving, you would see 25,000 miles pass by in 24 hours, at a speed of 25000/24 or just over 1000 miles per hour.

    Multiply by cosine of your latitude to see how fast the Earth is rotating where you are.

    Earth is also moving around the Sun at about 67,000 miles per hour.

    Advanced Answer

    If by "turning" you mean the rotation of the Earth about its axis (where axis just means the straight line between the North and South poles) it is quite easy to figure out how fast any part of the Earth's surface is moving.

    The Earth rotates once in a few minutes under a day (23 hours 56 minutes 04. 09053 seconds). This is called the sidereal period (which means the period relative to stars). The sidereal period is not exactly equal to a day because by the time the Earth has rotated once, it has also moved a little in its orbit around the Sun, so it has to keep rotating for about another 4 minutes before the Sun seems to be back in the same place in the sky that it was in exactly a day before.

    An object on the Earth's equator will travel once around the Earth's circumference (40,075.036 kilometers) each sidereal day. So if you divide that distance by the time taken, you will get the speed. An object at one of the poles has hardly any speed due to the Earth's rotation. (A spot on a rod one centimeter in circumference for example, stuck vertically in the ice exactly at a pole would have a speed of one centimeter per day!). The speed due to rotation at any other point on the Earth can be calculated by multiplying the speed at the equator by the cosine of the latitude of the point. (If you are not familiar with cosines, I wouldn't worry about that now, but if you can find a pocket calculator which has a cosine button you might like to try taking the cosine of your own latitude and multiplying that by the rotation speed at the equator to get your own current speed due to rotation!).

    The Earth is doing a lot more than rotating, although that is certainly the motion we notice most, because day follows night as a result. We also orbit the Sun once a year. The circumference of the Earth's orbit is about 940 million kilometers, so if you divide that by the hours in a year you will get our orbital speed in kilometers per hour. We are also moving with the Sun around the center of our galaxy and moving with our galaxy as it drifts through intergalactic space!

    Paul Butterworth
    and David Palmer for the Ask an Astrophysicist Team

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    Question ID: 970401c

    What is the direction of rotation of Earth? Of our galaxy? (Submitted February 25, 1998)

    The Question

    If the moon goes around the earth from left to right, and the earth rotates from left to right around the Sun, does the the Sun go around in the galaxy from left to right, too? Is this a pattern? Is our galaxy going around something?

    Oh and by the way I'm in 6th grade and my whole science class (we rotate for science) has been debating about for a week.

    The Answer

    What an excellent question! The direction of rotation is very important in astronomy because it gives us some clue about the how things were formed in the first place. One thing to remember is that things can rotate in any direction. Think about throwing a ball with topspin, backspin, and sidespin. A good tennis player can hit a ball that is spinning in any direction (up, down, left, right, kind-of-up-and-right, etc ...). (Show this to your class with a real ball.)

    We often think of rotation using the "right hand rule". Take your right hand so your thumb on the axis of rotation and your figures point in the direction of the rotation.

    Example: A counter-clockwise spinning top will rotate "up" according to the "right hand rule". A clockwise spinning top rotates down.

    Exercise: Find a Globe and have everybody in your class put their hands in the right way for each of the different rotation compared to the globe.

    So lets go over it:

    1. The Earth rotates on its axis from West to East with its axis in the North/South direction (by definition of North and South). So it is rotating "due North" because of the right hand rule.

    2. The Earth revolves around the Sun about 23 degrees from "due north". (This is why we have seasons!)

    3. The Moon revolves around the Earth about 5 degrees from the direction the Earth revolves around the Sun. The Moon's rotation is about 1.5 degrees from the direction the Earth revolves around the Sun.

    4. All the planets revolve in the same general direction, with Pluto's orbit being the most inclined (17 degrees). Their axes of rotation are more diverse, Uranus and Pluto rotate 'on their sides' and Venus's axis points towards the South.

    5. Our Galaxy, on the other hand, is completely different. The Sun revolves around the galaxy in a totally different direction. Using that "right hand rule" you need to point your thumb toward the "South Galactic Pole." This is located above the southern hemisphere, at 27 degrees south latitude. So the rotation axis of the Galaxy is tilted by 117 degrees from the rotation axis of the Earth.

    You can see this at night, by noting that the Milky Way (the disk of the galaxy) is always across the sky in some funny direction. Never due East-West.

    6. Our Galaxy, the Milky Way, is also moving in some funny direction (completely unrelated to any of the other directions) as it orbits the other galaxies nearby.

    I hope this helps. And please, take out the globe and have each person in your class point his/her thumb in the right direction for each example (1-5) above.

    Also, you can take out a protractor and make a drawing of each of the 3 main axes. Remember, from the Earth's axis: 23 degrees and 117 degrees.

    Good luck!

    Jonathan Keohane
    for Ask an Astrophysicist

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    Question ID: 980225a

    How fast does the Earth travel around the Sun? Why, when the Earth moves at such a high rate of speed, don't we feel it? (Submitted October 28, 1997)

    The Question

    How fast does the Earth move around the Sun? Why, when the Earth moves at such a high rate of speed, don't we feel it?

    The Answer

    Earth's average distance to the Sun is 150,000,000 km (93 million miles), therefore the distance it travels as it circles the Sun in one year is that radius x 2 x pi, or 942,000,000 million kilometers in a year of 24 hours/day x 365 1/4 = 8,766 hours so you divide to get 107,000 km/h or about 67,000 mph.

    You could also say the Earth moves around the Sun at 30 km/s. The Sun circles the center of our Galaxy at about 250 km/s. Our Galaxy is moving relative to the 'average velocity of the Universe' at 600 km/second ( http://antwrp.gsfc.nasa.gov/apod/ap960205.html)

    As to why you can't feel this speed: it's because you have no 'speed organs' which can sense absolute speed, you can only tell how fast you are going relative to something else, and you can sense changes in velocity (accelerations). Scientists have no instruments which can sense absolute speed either, and so they deny that the concept of absolute speed has any meaning.

    Suppose you are in a car traveling down the road. How can you tell how fast you are going? The speedometer tells you how fast your wheels are turning, but you could be standing dead still, spinning your wheels trying to get off a patch of ice, so put black tape over the speedometer. The car vibrates because it's working so hard, not necessarily because it's moving, so get a solidly built car that doesn't vibrate, and use a vibration absorbing seat cushion so you can't feel anything. The air whips noisily past your window as you drive through it, or maybe you're sitting still in a windstorm (was that a cow flying by?), use earplugs so that doesn't distract you. Outside you can see the scenery whizzing by, but it's actually a rear-projection screen that 'they' are showing moving images on to confuse you. Don't believe it--paint the windows black.

    OK, now, how fast are you going? You have no way to tell. You don't feel like you're moving. You feel just as you would if you were standing still!

    Now scrape off the paint off your windows before you run into a tree (or a tree runs into you).

    David Palmer
    for Ask an Astrophysicist

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    Question ID: 971028e

    What effect do the tides have on the Earth's rotation rate? (Submitted April 21, 1998)

    The Question

    I am reading Sobel's Longitude , and she speaks of the Earth's rotation rate slowing as a result of tidal forces. At what rate is the Earth's rotation slowing down? What will be the result of this slowing, and how long will it take?

    The Answer

    The interaction of the Moon and the tides is pumping angular momentum out of Earth's spin and into the Moon's orbit.

    Currently the day is lengthening by about 1.5-2 milliseconds per century. http://tycho.usno.navy.mil/leapsec.html

    This is thought to be higher than normal due to resonance frequencies in the slosh time of the current configuration of oceans (which changes with continental drift). If this were to continue forever, the Earth and Moon would end up tidally locked so that they kept the same faces towards each other throughout each day=month which would be about 50 of our current days long. However, the Sun will expand and incinerate the Earth well before that happens.

    David Palmer
    for Ask an Astrophysicist

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    Question ID: 980421b

    Why is Earth the only planet with an ozone layer? And why is it the only one with life and water? (Submitted November 07, 1996)

    The Question

    Why is Earth the only planet that has a ozone layer? How come Earth is the only planet with life and water?

    The Answer

    First of all, keep in mind that ours may not be the only solar system out there, so who knows if the Earth is the only planet with life (or an ozone layer). Ozone is a molecule with just three Oxygen atoms. It is created when UV light reacts with oxygen gas, which has two Oxygen atoms. UV light also destroys ozone, so you need just the right mix of UV light and oxygen to get an ozone layer. Again, maybe there are some planets in other solar systems that have the right mix.

    The Earth is not the only planet with water--Mars and Venus both have water in their atmosphere (in fact, there is a lot of water in our solar system, for example comets are basically dusty ice balls) but it has the most liquid water because the temperature on Earth is mostly above freezing and below water's boiling point. To get life, you need just the right mix of materials, like carbon, oxygen, water, etc. You also need the right temperatures for life... most other planets are too hot or too cold. Finally, you need an atmosphere that would protect any life from radiation. In the case of the Earth, the ozone layer protects us from the Sun's UV light. Life is pretty improbable because all of these things have to come together.

    Andy Ptak and the Ask an Astrophysicist team

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    Question ID: 961107a3

    What if the hole in the ozone layer of Earth got really big? (Submitted April 03, 1998)

    The Question

    What if our Ozone Layer had a very large hole in it, enough to cover Australia?

    The Answer

    The ozone layer blocks out harmful ultraviolet (UV) rays from the Sun. Where there is less ozone, more UV gets through. UV rays affect different plants and animals in different ways--just as some people tan while others burn, some species are more susceptible to damage from ultraviolet rays than others.

    An ozone 'hole', a place where there the ozone layer is thinner than normal, was discovered a few years ago over Antarctica. Even at the ozone levels found in the hole, most of the UV light is blocked before it reaches the ground. But enough gets through to worry scientists.

    The ozone hole is centered on Antarctica, so for it to stretch all the way up to Australia, it would have to be quite large. That means that much of the Antarctic Ocean and the Southern parts of other oceans would be exposed, and these regions are very important to the world ecology. This might harm some species of plankton and other ocean plants and animals. Nobody really knows which species are most susceptible, and which will thrive as the UV reduces the competition. The result might be worse (from our perspective) than the current situation, because the current situation is what we are used to--when something good changes, it usually becomes worse.

    As for effects on individual people--an enlarged ozone hole would cause an increase in tanning, burning, and skin cancer. However, the amount of UV that gets through even 'normal' ozone layers is enough that it is wise to wear a hat and sunscreen if you want to reduce your risk of skin cancer and other effects.

    David Palmer
    for Ask an Astrophysicist

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    Question ID: 980403a

    What are meteorites? (Submitted April 09, 1997)

    The Question

    What are meteorites? Any neat facts about meteorites? I am in 3rd grade. I am looking for information about meteorites. Can you tell me a great site or lesson my teacher can use in my class?

    The Answer

    Rocks in space are called meteoroids, and there are many of these in our solar system -- especially between the planets Mars and Jupiter where there are thousands of big rocks (Asteroids). In addition as comets come near the Sun, (as Hale-Bopp is doing now) chunks of ice and rocks come are blown off by the sun.

    These rocks usually they burn up on impact. We see this on Earth as a "shooting star" -- also called a meteor. Sometimes the Earth passes through a bunch of these rocks (perhaps from a long-gone comet), so we see many shooting stars. This is referred to as a meteor shower.

    When a large rock hits the earth, it sometime does not completely burn up as it enters the atmosphere. These meteors eventually hit the ground, where they can be found by people on Earth. This is a meteorite.

    I searched the World Wide Web for information on meteorites, and I found a nice home page. It is:

    Meteors, Meteorites and Impacts
    http://www.seds.org/billa/tnp/meteorites.html

    Thank you very much for your interest.

    Sincerely,

    Jonathan Keohane
    -- for Imagine the Universe!

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    Question ID: 970409b

    How many meteorites enter our atmosphere a day? (Submitted April 11, 1998)

    The Question

    How many asteroids and meteoroids enter our atmosphere a day.

    The Answer

    The nominal flux of meteoroids with diameter 1 cm or larger is 10^-6 per square meter per year.

    Using 6500 km as the radius of the earth + atmosphere, the area is about 5 x 10^14 m^2.

    This means that, averaged over the earth's surface, there are about 1,400,000 meteoroids with diameter 1 cm or larger hitting each day.

    Needless to say, the vast majority of these burn up in the top parts of the atmosphere, and the number of meteoroids drops rapidly with increasing size.

    Dave Thompson and Damian Audley
    for Ask an Astrophysicist

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    Question ID: 980411b

    Did an asteroid ever hit Earth? (Submitted April 19, 1998)

    The Question

    Did an asteroid ever hit Earth?

    The Answer

    Yes, it is virtually certain that asteroids have hit earth. First of all, earlier in the history of the solar system there were probably many more asteroids than there are now, so the odds of a collision were greater. More recently a large asteroid impact is credited with having led to the extinction of the dinosaurs. Other planets and their moons show many craters, which provide evidence for asteroid impacts. On earth these are destroyed by weathering. The June 1997 issue of Sky and Telescope has an article on this subject in which they estimate that a very large asteroid impact occurs once every 100 million - 1 billion years (the earth is 4 billion years old), and a large asteroid impacts once every 1 - 100 million years.

    Tim Kallman
    for "Ask an Astrophysicist"

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    Question ID: 980419c

    Are we going to be hit by an asteroid? (Submitted September, 1998)

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    Due to the high number of questions we've received about the threat of an asteroid striking the Earth, we've decided to dedicate a web page to this issue. Most of these concerns about asteroids are probably due to the recent summer movies "Armageddon" and "Deep Impact".

    First of all, the most important thing to remember, is that these movies are works of fiction intended to entertain audiences. They are not real! Oftentimes, movies distort reality (especially the reality of science) to make it more exciting.

    But, sometimes there is a kernel of truth behind a movie, though it is often hard to discern what that is. So here, we will attempt to bring you the truth about asteroids and what they mean for Earth, as well as give you some references to check out.

    First and foremost, we know of nothing currently on a collision course with Earth! It is true that Earth has been hit by meteors or asteroids before and will be hit again someday. However, currently, we are not in any immediate danger. In particular, we now know that the asteroid which was originally announced to possibly pose a small risk of an impact in 2028 will miss us completely. http://impact.arc.nasa.gov/news_detail.cfm?ID=60


  • What are the chances of us being hit and when would it happen?

    The first thing to remember is that space is big and empty. Which makes the chance that we will be hit by anything from space very small. In much of space, for example, large-sized objects are hundreds or thousands of light years apart. Even the asteroid belt has so much space in it, that we can send space probes through it without any problems. The asteroids in the belt are spread over a ring that is more than a billion kilometers in circumference, more than 100 million kilometers wide, and millions of kilometers thick.

    Here's what JPL's Near Earth Asteroid Tracking team has to say:

    "The most dangerous asteroids, capable of a global disaster, are extremely rare. The threshold size is believed to be 1/2 to 1 km. These bodies impact the Earth only once every 1,000 centuries on average. Comets in this size range are thought to impact even less frequently, perhaps once every 5,000 centuries or so."

    The Asteroid and Impact Hazard page says:

    "The threshold for an impact that causes widespread global mortality and threatens civilization almost certainly lies between about 0.5 and 5 km diameter, perhaps near 2 km. Impacts of objects this large occur from one to several times per million years.

    "Because the risk of such an impact happening in the near future is very low, the nature of the impact hazard is unique in our experience. Nearly all hazards we face in life actually happen to someone we know, or we learn about them from the media, whereas no large impact has taken place within the total span of human history... It is this juxtaposition of the small probability of occurrence balanced against the enormous consequences if it does happen that makes the impact hazard such a difficult and controversial topic."

    This is a difficult issue because an impact would pose enormous risk, yet because the odds of it occurring within our lifetimes is so low, it is unnecessary to run around believing that the sky is falling. There are two things to consider: one is that there are many organizations with telescopes trained to the sky, watching and tracking asteroids and comets, compiling a list of potentially hazardous objects to keep an eye on. Many of these objects are decades away from approaching the Earth which gives us a lot of time to track them in order to accurately predict their orbits.


  • If tomorrow, we discovered an object that will intersect Earth's orbit, what would happen?

    The Near Earth Asteroid Tracking Team replies:

    "Actually, some 100 bodies have already been discovered on orbits which take them so close to the Earth's orbit, that they could hit in the far distant future. This is because the orbits of these bodies change slowly with time. Although their orbits do not intersect Earth's orbit at present, they could hit in a few thousand years or more.

    "The scenario you have in mind is most likely to unfold as follows. In the course of our search for Earth-crossing asteroids, we could find one that will hit not in the next year, or even in the next ten years, but might hit in the next hundred years. We believe that the chance that we will find such an object is only 1 in 1,000, even after a complete search. If we do find such an object, we will have plenty of time to track it, measure its orbit more precisely, and plan a system for deflecting it from its current orbit (hopefully away from the Earth's). There will be no great hurry, and no great panic. It would be a project for all the world's nations to take part in. It could be a globally unifying event. Because we will have found it long before it actually hits the Earth, it probably would take only a small impulse (chemical rockets, or perhaps mass drivers) to divert it from a threatening path.

    "There is a much smaller chance that we would find one that could impact in the next 10 years. The chance of that happening is 1 in 10,000. If this were to happen, we would probably still have time to launch a crash program of scientific and technological research, with the goal of characterizing both the structure of the menacing asteroid, and the best means for diverting its orbit."


  • Risk Analysis

    Now would be a good time to look at some of the risks we face in our daily lives - risks that we don't think twice about taking. For example, according to the Independent Insurance Agents of America:

    "Today a motor vehicle accident occurs every second. Auto accidents cause an injury every 14 seconds, and every 13 minutes a car accident results in a fatality. More than 31 million accidents occur per year, at an annual cost of almost $100 billion."

    Yet most people continue to drive their automobiles regardless. For the same reason, that we can't live our lives paralyzed by the fear that something bad may happen, we shouldn't let the remote possibility of being struck by a meteor or asteroid rule our lives.

    Risk analysis is the process of determining the risks associated with an act (driving a car to Florida for a spring break vacation, for example) or a product (such as a food additive), etc. When considering risks, and determining whether something is "worth" the risks it has, many factors must be considered. For example, an event could be fairly likely to happen, but have only mild consequences (such as the risk of getting a ticket if you park for 2 hours and 15 minutes in a two hour zone). Or, it could be somewhat common and moderately serious (such as the chances that you will get in a car accident of some sort during your driving lifetime). Or, it could be very, VERY unlikely, but have devastating consequences if it did (such as the chances that a 5 km comet will hit the Earth next year). Sometimes the chances are so small that something will happen that we are willing to accept the risk, the consequences if it does happen.

    Many factors enter into a person's (or a society's) decision of what are "acceptable risks". Some people are unwilling to consider any risks acceptable, but it is hard to live life like that, since breathing and eating and living on the Earth all have a certain amount of risk. The important thing is to keep it all in a balanced perspective, and to realize that risk is a part of life. It is important to minimize it as much as possible, of course, but carrying it to an extreme can do more harm than good, too.


  • Who is watching the skies?

    There are agencies, NASA among them, that realize that although we are in no immediate danger, asteroid impacts are something that could pose a problem somewhere in the future. Panicking about something that could happen some day is not constructive at all. However, it is in everyone's best interest not to ignore this, but to be watchful in order to provide the maximum warning time should a possible threatening situation occur. The following organizations are doing just this:

  • The Near Earth Asteroid Tracking Team

  • SpaceWatch

  • The Asteroid and Comet Impact Hazard Group


  • Facts about asteroids

  • SEDS Asteroid Page
  • Asteroid Page
  • Asteroid Fact Sheet
  • What are asteroids?


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    Question ID: danger

    • Radiation and Particles Around the Earth

    What are the effects of the solar cycle on the earth? (Submitted December 05, 1996)

    The Question

    Based on media info, I'm trying to find out something about the "new" solar cycle we are going into. What possible affect would result from the flaring that is supposed to take place. I'm wondering about affects and potential MAGNITUDE in areas in Electromagnetism/daily activities, possible biological affects, and elemental. I realize that all of these may not be in your field of expertise, but would welcome any info you could pass on.

    The Answer

    Your questions are well beyond the HEASARC area of expertise. I tried a bit of WWW surfing and didn't find anything very satisfying, however you should try some as well. The thing to remember is that the solar cycle is about 11 years so there won't be anything happening that hasn't happened before. Predictions of the strength of the maximum haven't been terribly accurate in the past (the last one about 1990 was supposed to be much stronger than it turned out to be). Strong flares will affect some communication systems and the astronauts need to be careful, but for most of us it will be business as usual. Other space-related effects include increased drag on satellites. As Earth receives more energy from the Sun, the atmosphere puffs up increasing the density of the residual atmosphere in the satellite's path. Aurora also become more spectacular.

    Cheers,
    Steve Snowden
    for Imagine the Universe!

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    Question ID: 961205b

    How do sunspots affect the Earth? (Submitted May 06, 1998)

    The Question

    How do sunspots affect the earth? Why?

    The Answer

    The primary effect on the Earth is on our ionosphere. This is the very upper part of the atmosphere. Increased sunspot activity frequently accompanies an increase in the outflow of matter from the Sun in the form of a "solar wind". Charged particles in this wind can interact with atoms in the upper atmosphere and sometimes wreak havoc with our communications systems. It can interfere with the operation of satellites by introducing background static. During periods of heightened solar activity, the Earth's upper atmosphere swells up slightly in response to the extra heating, which in turn increases the rate of decay of satellites in low Earth orbit.

    You can learn more about sunspots on our Solar System archive page, under "The Sun":

    J.K. Cannizzo
    (for "Ask an Astrophysicist")

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    Question ID: 980506a

    Are aurorae caused by solar flares? (Submitted December 06, 1996)

    The Question

    I am a Junior at new trier high school and I am doing a research project on the Aurorae. I was curious if you had any useful info on any type of patterns on the aurorae also data on them. I was also curious about the connection of the Aurorae to the 11 year cycle. I have an idea on what the connection is. Is it that the 11 year cycle causes an unusual amount of sunspots witch in return cause the sunflares that send the particles into the air. If that is correct how do the Sun spots cause the Sun flares. If you can not help me I understand. but if you can I would appreciate your help. Thank you very much, see ya.

    The Answer

    Basically you've got it right. The main issue is the effect of convection and magnetic fields. The Sun has strong magnetic fields, and magnetic fields can exert a pressure on plasmas (a plasma is a gas in which the atoms are at least partially ionized) such as are in the atmosphere of the Sun. Convection stirs up the atmosphere and creates regions with stronger magnetic fields than is usual. Because of magnetic pressure, the plasma in these regions can be cooler than the rest of the atmosphere, making them look dark Anyway, occasionally the magnetic fields between two nearby spots will "connect". Plasma can flow along the "connection", known as a "flux tube", with magnetic pressure keeping the plasma in the tube. If the flux tube stretches too far above the surface of the Sun, it will break and some of the plasma will be ejected. This is a flare, and some of the ejected plasma may hit the Earth, making the aurora brighter, which is also a case of magnetic pressure affecting particles. So the cycle of sunspots reflects the amount of magnetic activity, which then affects the number of flares.

    The charged particles which are released in the flare interact with Earth's magnetic field which sends the particles down near the poles where the particle belts dip into the atmosphere. The particles interact and excite atoms in the atmosphere which then produce the observed light of the aurora.

    The University of Alaska has an interesting WWW page on aurora that you should check out. It is at:

    Cheers,
    Steve Snowden, Andy Ptak, and Karen Smale
    for Imagine the Universe!

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    Question ID: 961206

    Is there any great danger to Earth from Solar Flares? (Submitted November 24, 1997)

    The Question

    I am a freshman at the University of Arizona. I am writing a paper on solar flares for astronomy and was wondering if there in any chance that a solar flare could be large enough to wipe out all communication and bring mass destruction to our planet. Thanks.

    The Answer

    Dear freshman at the U. of Arizona

    You can sleep soundly tonight--all we know about solar flares suggests they can never pose a life hazard at the Earth's surface*. And as far as I know, no one ever suggested that one of them caused the demise of dinosaurs. Asteroid impacts, giant volcanoes--yes; flares--no.

    Very few flares produce charged particles, and the energies involved are generally too low to penetrate the entire atmosphere, which is equivalent to about 10 feet of concrete. In such a thick layer, both ions and electrons dissipate their energy among a growing number of secondary fragments, whose number grows but the energy of each one drops, until it is so low that other processes stop them.

    The Earth's magnetic field also helps deflect particles, especially near the equator. I don't remember numbers, but I would guess that in the last 40 years, the most flare particles did was double for a few hours the cosmic ray intensity to which all life is exposed continually, day in and day out.

    For your project on flares, you may want to look up on the web:

    http://www.phy6.org/Education/wsun.html
    http://www.phy6.org/Education/wsolpart.html

    and other files reachable from those.

    Dr. David P. Stern
    for 'Ask an Astrophysicist'

    *note added:

    However, you may be interested that in August 72 (between the missions of Apollo 16 and 17) there was a solar flare intense enough to have delivered a fatal radiation dose to any crew caught in an Apollo capsule in Earth-moon transit!

    Dr. Paul Butterworth

    Addendum (Dec 29, 2009): Although the above answer remains appropriate to the original question, which appears to envision a Hollywood disater movie like scenario, solar flares can disrupt satellite based communications and cause some damages on the ground. For more, see: http://science.nasa.gov/headlines/y2008/06may_carringtonflare.htm

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    Question ID: 971124a

    What is the cause of Geomagnetic Storms? (Submitted March 10, 1998)

    The Question

    I'm from White Bear Lake, Minnesota. I was flipping through the encyclopedia and I found an entry called "magnetic storms." I wanted to learn more about them. I've tried to find these answers using the school library, by searching your site and doing an Alta Vista search, but I'm not finding much, and the stuff I'm finding in encyclopedia is too confusing to understand. I hope you can help me answer these questions.

    The Answer

    A magnetic storm (or more correctly geomagnetic storm, or geomagnetic disturbance) happens when a pulse of particles and magnetic field from the Sun hits the earth (and it's magnetic field). This pulse is called a Coronal Mass Ejection (CME) and they are associated with solar flares, although they are not the same thing. When these CMEs hit the earth, they compress the Earth's magnetic field, and the changing magnetic field can create electricity in long metal objects such as oil pipelines (which makes them decay faster) and electric power lines. A 1989 power blackout in Ontario was caused by one of these geomagnetic storms. Magnetic storms also can effect radio communications, and the high energy particles that come along with them can damage satellites and even astronauts. They occur randomly, but are most common when the Sun has a lot of sunspots (it's "active" phase) every eleven years. They start at the Sun, but when they hit the earth, they span the whole globe, although they are worse close to the earth's magnetic poles. But people (other than astronauts) are completely protected by the earth's atmosphere, except for secondary problems such as blackouts.

    The National Oceanic and Atmospheric Administration (NOAA) predicts magnetic storms, just as they predict hurricanes and the like, using data from spacecraft such as the Advanced Composition Explorer (ACE), which can see the CME before it hits the Earth.

    For more information, you can look at Cosmicopia (http://helios.gsfc.nasa.gov/).

    Also, the physics department at the U of Minnesota has a very good space physics/Aurora group.

    They have an observatory very close to you. It is out by "Marine On St. Croix" and shares a site with the O'Brien astronomical observatory.

    And they also have an active program where they give talks to schools, like your own. They might direct you to the current experts on this subject.

    Thanks for your questions.

    Eric Christian and Jonathan Keohane
    for Ask an Astrophysicist

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    Question ID: 980310b

    Can you tell me about the radiation in the Van Allen Belt? (Submitted February 28, 1997)

    The Question

    I wonder if you could tell me exactly what the VAN ALLEN BELT is and how much radiation does it contain, ie how many rems of radiation are there out there? Plus, what protection would organic life need to be protected from this radiation?

    The Answer

    David Stern, a researcher in another lab here at Goddard, has graciously supplied an answer to your question, given below:

    "The radiation belts are regions of high-energy particles, mainly protons and electrons, held captive by the magnetic influence of the Earth. They have two main sources. A small but very intense "inner belt" (some call it "The Van Allen Belt" because it was discovered in 1958 by James Van Allen of the University of Iowa) is trapped within 4000 miles or or so of the Earth's surface. It consists mainly a high-energy protons (10-50 MeV) and is a by-product of the cosmic radiation, a thin drizzle of very fast protons and nuclei which apparently fill all our galaxy.

    " In addition there exist electrons and protons (and also oxygen particles from the upper atmosphere) given moderate energies (say 1-100 keV; 1 MeV = 1000 keV) by processes inside the domain of the Earth's magnetic field. Some of these electrons produce the polar aurora ("northern lights") when they hit the upper atmosphere, but many get trapped, and among those, protons and positive particles have most of the energy .

    "I looked up a typical satellite passing the radiation belts (elliptic orbit, 200 miles to 20000 miles) and the radiation dosage per year is about 2500 rem, assuming one is shielded by 1 gr/cm-square of aluminum (about 1/8" thick plate) almost all of it while passing the inner belt. But there is no danger. The way the particles move in the magnetic field prevents them from hitting the atmosphere, and even if they are scattered so their orbit does intersect the ground, the atmosphere absorbs them long before they get very far. Even the space station would be safe, because the orbits usually stop above it--any particles dipping deeper down are lost much faster than they can be replenished.

    "If all this sounds too technical but you still want to find out-- what ions and magnetic fields and cosmic rays are, etc.--you will find a long detailed exposition (both without math) on the World Wide Web at: http://www.phy6.org/Education/Intro.html

    Good luck!

    David Stern

    Note:

    Another point of particular interest to us in high-energy astrophysics is the South Atlantic Anomaly. This is a region of very high particle flux about 250 km above the Atlantic Ocean off the coast of Brazil and is a result of the fact that the Earth's rotational and magnetic axes are not aligned (see http://heasarc.nasa.gov/docs/rosat/gallery/display/saa.html). The particle flux is so high in this region that often the detectors on our satellites must be shut off (or at least placed in a "safe" mode) to protect them from the radiation.

    Andy Ptak
    for Ask an Astrophysicist

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    Question ID: 970228a

    What are the boundaries of the South Atlantic Anomaly? (Submitted October 04, 1996)

    The Question

    I would like to know more about the South Atlantic Anomaly. What are its boundaries?

    The Answer

    The boundaries of the SAA vary with altitude above the Earth. At an altitude of 500 km, the SAA ranges between -90 and +40 in geographic longitude and -50 to 0 in geographic latitude. In this region, the extent of the SAA increases with increasing altitude.

    Below is a map of the SAA at an altitude of ~ 560 km. The map was produced ROSAT by monitoring the presence of charged particles. The dark red area shows the extent of the SAA. The green to yellow to orange areas show Earth's particle belts.

    South Atlantic Anomaly

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    Question ID: 961004

    Does the South Atlantic Anomaly rotate? (Submitted February 10, 1997)

    The Question

    I would like to know if there is any evidence for counter clock wise rotation of the South Atlantic Anomaly (SAA). Tried a few other sources, some asked what's the SAA? Never heard of it. Assumed if anyone would know it would be your outfit.

    The Answer

    This question is a good one for the Imagine the Universe! Ask an Astrophysicist service. Part of our job is to create observation schedules for the Rossi X-ray Timing Explorer. To do this, we have to model the location of the SAA. So I must say that you've come to the right place !

    The South Atlantic Anomaly does not rotate. It changes its size (both in height above the earth and extent in latitude and longitude) due to a number of factors, with the primary one being solar activity. But there is no rotation to it.

    Jim Lochner
    for the Ask an Astrophysicist team

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    Question ID: 970210d

    What causes the South Atlantic Anomaly? (Submitted March 07, 1997)

    The Question

    In a previous question you have told us what the boundaries of the South Atlantic Anomaly are. Can you please tell me what causes the SAA?

    The Answer

    The South Atlantic Anomaly is of particular interest to us in high-energy astrophysics. This is a region of very dense radiation ("high particle flux") above the Atlantic Ocean off the coast of Brazil. The particle flux is so high in this region that often the detectors on our satellites must be shut off (or at least placed in a "safe" mode) to protect them from the radiation.

    The South Atlantic Anomaly comes about because the Earth's field is not completely symmetric. If we were to represent it by a compact magnet (which reproduces the main effect, not the local wiggles), that magnet would not be at the center of the Earth but a few hundred miles away, in the direction away from the "Anomaly." Thus the anomaly is the region most distant from the "source magnet" and its magnetic field (at any given height) is thus relatively weak. The reason trapped particles don't reach the atmosphere is that they are repelled (sort of) by strong magnetic fields, and the weak field in the anomaly allows them to reach further down than elsewhere (see also https://wiki.oulu.fi/display/SpaceWiki/Radiation+belts).

    Also, Dr. Steve Snowden has a brief description with a cool graphic at http://heasarc.gsfc.nasa.gov/Images/rosat/misc_saad.html

    Hope this answers your question.

    Regards,
    Laura Whitlock
    for Ask an Astrophysicist

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    Question ID: 970307a

      The Moon

    Why does the moon appear during the day? (Submitted March 14, 1997)

    The Question

    Why does the Moon sometimes come out during the day?

    The Answer

    You can see the Moon in the daytime because it is big and brightly lit by the Sun. The surface of the Moon is about as reflective as an asphalt road--rather dark but not totally black. When you look at the Moon, you are seeing the light which reflects off it. This is not nearly as bright as the Sun, but it is up to 100,000 times as bright as the brightest nighttime star.

    During the day, the brightness of the sky washes out the light from the stars: a region of the sky including a bright star is only very slightly brighter than a region of the sky without a bright star, so your eye cannot notice the difference. However, the region of the sky containing the Moon is much brighter, so you can see it. You can also sometimes see Venus during the day if the conditions are right and you know exactly where to look, but anything dimmer is lost.

    It might be useful to think of the Sun as a large light bulb, and the moon as a large mirror. There are situations where we can't see the light bulb, but we can see the light from the bulb reflected in the mirror. This is the situation when the moon is out at night. We can't see the Sun directly because the earth is blocking our view of it, but we can see its light reflected from the moon. However, there are also situations where we can see both the light bulb and the mirror, and this is what is happening when we see the moon during the day. You can explore this for yourself with a light and a hand mirror. Depending on which way you face (away from the light or sideways to the light) you can see either just the mirror, or both the light and the mirror.

    I hope this helps!

    David Palmer and Tim Kallman
    for the Ask an Astrophysicist team

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    Question ID: 970314b2

    Is there any night time on the Moon? (Submitted March 02, 1998)

    The Question

    Is there any night time on the Moon? Is the Moon's daylight the same as the Earth's daylight or is it brighter?

    The Answer

    The moon orbits the earth with a period of four weeks ( a month) and during the orbit it always has the same side facing the earth. So this means that on the moon there is day and night, but they are both two weeks long instead of 24 hours.

    The Moon's daylight is brighter and harsher than the Earth's. There is no atmosphere to scatter the light, no clouds to shade it, and no ozone layer to block the sunburning ultraviolet light.

    The nights are also brighter, at least on the side of the Moon near to us. The night is lit up by sunlight reflected from Earth, while the night on Earth is lit up by sunlight reflected from the Moon. Earth is much bigger than the Moon, and Earth is also more reflective (with its clouds and oceans, it reflects more light than the dark Moon rocks). Earthlight on the Moon is much brighter than Moonlight on the Earth.

    Hope this helps,
    Jeff Silvis and David Palmer
    For Ask an Astrophysicist

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    Question ID: 980302b

    Is there a dark side of the Moon? (Submitted March 09, 1998)

    The Question

    Is the dark side of the moon ever exposed to sunlight?

    The Answer

    The term "dark side of the Moon" is really a misnomer, because the side that we are familiar with is dark just as frequently. A better term might be "far side of the Moon."The orbit of the Moon is such that one rotation is just about as long as one revolution in its orbit around the Earth. Because of this, one side is facing us during its orbit. However, this side is often dark. During a "new moon", the moon is between the Earth and the Sun, and so the side we know is totally dark. When the moon is opposite from the Sun, the side we know will be totally bright, and the far side will be dark.

    Steve Bloom
    for Ask an Astrophysicist

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    Question ID: 980309b

    Why is the Moon's revolution and rotation period the same? (Submitted May 19, 1998)

    The Question

    Is there a scientific reason why the Earth's moon has the same rotation and orbital periods? (As in, why does the same face of the moon always faces us Earthlings?).

    Also, is the moon the only object in the solar system that does this sort of thing?

    The Answer

    The Moon is by no means the only object which is phase-locked; a large number of the satellites of other planets also share this property. There is a nice compilation of orbital and rotation periods of the solar system here:

    http://www.solarviews.com/eng/data1.htm#orb

    so you can see for yourself just how many.

    The basic reason for this phenomenon is the tidal force. The tide of the ocean is well known; less well-known but equally real is the tidal distortion of the entire planet --- the continents (and everything underneath) are deformed daily by the tidal force (of the Sun and the Moon), roughly by the same height as the tide of the ocean. A moon close to a planet is (relatively speaking) subject to a much greater tidal force than the Earth.

    If a moon has two, diametrically opposed, permanent tidal bulges, then it's dynamically most stable if one is always pointed towards its parent planet.

    This explanation is incomplete in that there is a lot of geophysics (structures of the solar system bodies) involved, something I don't know a whole lot on, but I think it gives you a broad overview.

    Best wishes,

    Koji Mukai
    for Ask an Astrophysicist

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    Question ID: 980519a

    How often do lunar eclipses happen? (Submitted September 30, 1996)

    The Question

    I live down in Southeast Texas. I saw the lunar eclipse on September 26, 1996 I just wanted to know how many years does that certain eclipse happen.

    The Answer

    Lunar eclipses happen every year or two. One of the astronomy textbooks that we have has a list of 16 in 13 years. Of those, 11 were total eclipses. There will be another lunar eclipse next year but it will not be total. The next total lunar eclipse visible from the US will occur in the year 2000.

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    Question ID: 960930a

    What do the Moon rocks Apollo brought back tell us? (Submitted April 21, 1998)

    The Question

    How have the moon rocks from Apollo helped scientists learn about the composition and history of the moon?

    The Answer

    Moon rocks have told scientists many things.

    For example, Moon rocks lack both iron and easily vaporized 'volatile' elements. This suggested the theory that the Moon was produced when a huge planetesimal, perhaps as big as Mars, slammed into the still-forming Earth, ripping material out of its crust. The crustal material formed a ring around Earth which then coalesced into the Moon. The iron of Earth had already sunk down to the core before the collision, so there is not much iron on the Moon. The extreme heat of the impact vaporized the volatile elements. So the composition of the Moon rocks tells us about how it was formed. Such a violent event also had a substantial effect on Earth, so by studying the Moon, we now understand more about our own planet.

    As another example; you can tell when a rock was formed by various methods (including looking at the decay of radioactive elements in the material). The ages of the Moon rocks, many of which were formed by later meteor collisions, tells us the history of how often big meteors fall. This reveals an epoch of heavy bombardment which ended about 4 billion years ago. Almost immediately after the end of this epoch (as close as we can tell from the fossil record) life appeared on Earth. The fact that life appeared so quickly might mean that life is easy to make. This, in turn, would mean that most hospitable planets in the universe have life, and we might not be alone.

    David Palmer
    for Ask an Astrophysicist

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    Question ID: 980421c

    Besides water, what else is in the ice found on the moon? (Submitted December 27, 1998)

    The Question

    Lunar Prospector has confirmed the existence of ice on the Moon. Ancient impacts are the likely source of this ice. We have at our doorstep a possible repository of our primordial solar system. Is there any evidence of other chemicals present within the ice? Are there any plans or discussions to send a probe to return a sample of this ice to Earth for analysis?

    The Answer

    I agree, the discovery of ice on the moon was a major breakthrough.

    You can find information on the Lunar Prospector mission and results at: http://nssdc.gsfc.nasa.gov/planetary/lunarprosp.html

    NASA has a web site about future lunar missions which can be found at: http://www.lpi.usra.edu/expmoon/future/future.html

    Hope this helps,
    Mike Arida
    for Ask an Astrophysicist

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    Question ID: 981227a

    What are some of the substances found on the Moon? (Submitted March 29, 1998)

    The Question

    What are some of the substances found on the moon ?

    The Answer

    There is a good, readable account of what the Apollo astronauts found on the moon in the July 1994 issue of Scientific American. I would recommend that article to you. Here is a list of some of the substances that are mentioned in the article:

    plagioclase feldspar, olivine, pyroxene, anorthosite

    The article does a good job of describing how the presence and/or absence of certain elements on the moon led to what is now considered by many scientists as the correct theory of its formation.

    J.K. Cannizzo
    for Ask an Astrophysicist

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    Question ID: 980329c

    Does the Moon have a molten core? (Submitted March 10, 1998)

    The Question

    I am in third grade. I would like to know what the core of the moon is like. Is it magma like the Earth?

    The Answer

    We know that the Moon has a very weak magnetic field. Oddly enough, this is fairly strong evidence that it does not have a molten core. Rotating planets or moons with molten cores will produce magnetic fields through "the dynamo effect." In a planet like the Earth, the molten core can flow freely in a process called convection. In addition, the Earth rotates, adding to the movement of the molten core. The flowing molten iron-nickel material can produce electrical current, which, in turn produces a magnetic field that surrounds the Earth. If the Moons core were molten , then it would have a field too, though it would be weaker. We only a detect a very weak field, much weaker than that expected from the dynamo effect.

    Since small "moonquakes" have been measured, which probably originate in the core of the Moon, it could be partially molten. But for the reason described above, it can not be totally molten.

    Much of the information in this reply can be found in Michael Seeds' _Foundations of Astronomy_, and probably several other astronomy and geology books.

    Steve Bloom
    for Ask an Astrophysicist

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    Question ID: 980310c

    Can you provide me with the latest theories on the origin of the Moon? (Submitted June 26, 1997)

    The Question

    I am a first year science teacher and I having been looking for the latest theories on the origin of the moon. Can you provide me with some new information?

    The Answer

    We are X-ray and gamma-ray astronomers in our group...not really active in the "solar system" fields of studies. Please keep that in mind.

    What we know (from tests on lunar samples brought back by the Apollo astronauts) is that the Moon is about 4.6 billion years old. It formed at about the same time as the rest of the solar system did.

    There are 3 possibilities for its creation:
    1. it formed near the Earth as a separate body
    2. it formed as part of the Earth and separated from it
    3. it formed somewhere else and was captured by the Earth

    Given that we know the rough ages of the Earth and Moon are the same, we can conclude the following: if it formed as part of Earth and separated, it must have done so right at the beginning of the solar system. Also, the Moon has a different chemical composition that the Earth...which some scientists believe points to it having formed as a separate body, either near or far. Last I heard, the 3 theories all had their strengths and weaknesses...but none were definitive.

    The latest theory we have heard given serious consideration of is that a Mars-sized asteroid knocked a lot of the surface off the just-formed Earth. A long string of rocky fragments would be blown out from the Earth like a tail. All of the iron falls back onto the Earth and settles in the core. Part of the rocky tail accretes to make the Moon. That may be why the Moon doesn't have an iron core and is somewhat short on certain other elements when compared to Earth.

    There is also a Scientific American article by G. Jeffrey Taylor in the July 1994 issue (pp 40-47). The article clearly states that the giant impact theory is the current favorite; if there is a bias, I think it's only a matter of degree (it may be less overwhelming a favorite than this article makes it out to be).

    The reasons why giant impact theory has become such a favorite is listed on p43 of this article --- for one, such a collision is a natural consequence of planet formation. For another, "it simply explains too many observations", including the similar oxygen isotope ratios between the Moon and the Earth, and the angular momentum of the Earth-Moon system. The capture theory has difficulty with the former, while the fission theory cannot explain the angular momentum.

    You might also try contacting the folks at the Lunar and Planetary Institute to see if they have strong opinions (and why!) about any particular theory. The LPI web site will be found by any Search Engine.

    Regards,
    Laura Whitlock and Koji Mukai
    for the Ask an Astrophysicist Team

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    Question ID: 970626

    Does the Moon's phase affect the crime rate? (Submitted January 03, 1997)

    The Question

    I'm wondering if you have an opinion about whether or not the Moon's phases affect the crime rate. Please write back as soon as you possibly can. If you have no opinion or knowledge on the subject could you please refer me to someone who does.

    The Answer

    This question is outside of our area of expertise, but we can help you out. Cecil Adams (a popular columnist of the "why does this happen/work?" variety) covers topics such as this in his books The Straight Dope or More of the Straight Dope by Cecil Adams. The gist of his answer (from More of the Straight Dope) is as follows:

    Scientists have looked for a correlation between phase of the Moon and such things as murders, violent crime, or births. In particular, many police officers or emergency room personnel have noted a seeming rise in activity in their line of work during full Moons. Scientific studies done to isolate this have, however, shown *no* correlation, contrary to the beliefs of those involved. In other words, the Moon's phase doesn't seem to have any affect on the number of crimes committed and babies born.

    So why do people seem to notice an increase in these things around the time of the full Moon? Social scientists speculate it's because people are more likely to notice, and remember, a full Moon, rather than the Moon at other phases. Thus, if a strange murder is committed when the Moon's a crescent, people covering the crime may not remember the phase of the Moon that night. If, however, the Moon is full, a police officer might be more likely to remember the phase of the Moon that night, since the full Moon is bright and very obvious. Thus, crimes, births, and strange occurrences happen all month long, but only those on the full Moon are associated with the Moon's phase when people talk about them.

    Andy Ptak and Gail Rohrbach
    for the Ask an Astrophysicist team

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    Question ID: 970103b



     

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