(Submitted January 16, 1998)
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?
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 https://pantherfile.uwm.edu/kahl/www/Images/Weather/Other/analemma.html, which was taken by Dennis di Cicco.
I hope this helps,
Koji Mukai, David Palmer, and Tim Kallman
For the Ask an Astrophysicist Team
Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html
for help on other astronomy Q&A services.