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The Question

(Submitted April 08, 1997)

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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