| MadSci Network: Astronomy |
Hi Michael,
This is not an easy question to answer in a text message. I'll do my best, but your best bet is to look up an introductory astronomy textbook. I find that a good one for this is "Exploration of the Universe," by Abell, Morrison, and Wolff. Chapter 6 of the Fifth Edition discusses the matter of timekeeping.
In essence, the differences that you found result from the fact that the Earth is not a perfect 24 hour clock. There are two important effects that make the length of the apparent day differ from 24 hours.
1) The Earth spins on an axis that is tilted with respect to its orbit around the Sun. Because of this, the Sun follows a tilted path through the sky over the course of a year, known as the ecliptic. It moves from west to east along the ecliptic. Near either one of the solstices, the motion of the Sun along the ecliptic is parallel to the equator. At these times the Sun makes the most rapid progress east-to-west over the course of a day. The length of the apparent solar day (from one noon to the next) is longest at these times, because the Earth has to turn a little bit further than average each day between noons. BUT, our clocks only measure constant, 24 hour days. As a result, even if the Sun were to rise at constant intervals, it would rise later each day according to our clocks.
2) For the other effect, let's look at what happens around the Summer Solstice, in the Northern hemisphere. At that time, the Earth happens to be at the point in its orbit when it is furthest from the Sun. At that time, it moves more slowly than average in its orbit. This tends to reduce the length of the solar day, and partially offsets the above effect in Summer. The opposite happens during our winter. Then, the Earth is closer to the Sun, moves faster in its orbit, making the solar day even longer.
Primarily because of 1) we see the Sun rising later each day near the solstice. You will see, as a result, that the Sun rises earlier *before* the summer solstice than after it. Similarly, sunrise occurs later each day for a couple of weeks *after* the winter solstice. These effects last for that time when the actual interval between one sunrise and the next doesn't change much (it changes slowly near the solstices). The difference that you saw between the extremes of sunrise and sunset occurs because the above two effects do change noticeably over the course of a day.
--Stephen Murray