Online Astronomy eText: The Sky
The Changing Motion of the Sun Link for sharing this page on Facebook
      It has been known since ancient times that the motion of the Sun along the Ecliptic is not uniform. Although it moves about a degree to the East (relative to the stars) each day, its motion gradually changes during the year, being faster in December and January, and slower in June and July. The actual change from day to day is very small and not easily noticeable with the timekeeping methods available in ancient times, but during that part of the year when the Sun moves faster than normal on one day, it moves faster than normal every day, and over a month or so the difference adds up in a very noticeable way. Namely, the length of the seasons is not the same as it would be if the Sun followed an absolutely uniform circular motion centered on the Earth.
      If the Sun had a uniform motion, each season would be exactly the same length as the others, or a quarter of the 365 1/4 day seasonal year, which is about 91 days, 7 1/2 hours. But the seasons are not the same. To show this, the table below gives the dates of the Equinoxes and Solstices and the difference in days from each to the next over a period of three years. (Note that each year the times are about 6 hours later, because of the 1/4 day difference between the calendar year, and the seasonal year. Also, to do the calculation for winter 2007/08, the 2008 Vernal Equinox -- March 20, 0548 UT -- was used, even though it isn't shown in the table.) There are small differences in the length of the seasons from year to year, due to perturbations of our orbit by other planets and our motion relative to the barycenter of the Earth-Moon system, but the overall difference is very clear. Spring is a day and a half longer than it should be, and Summer is nearly two and a half days longer; while Autumn is nearly one and a half days shorter than it should be, and Winter is nearly two and a half days shorter, making Winter nearly five days shorter than Summer. And this is a large enough difference to have been very easily noticed, even in ancient times.

The Length of the Seasons
Year200520062007
Vernal EquinoxMarch 20, 1233 UTMarch 20, 1826 UTMarch 21, 0007 UT
Spring92 d 18 h 13 m92 d 18 h 00 m92 d 17 h 59 m
Summer SolsticeJune 21, 0646 UTJune 21, 1226 UTJune 21, 1806 UT
Summer93 d 15 h 37 m93 d 15 h 37 m93 d 15 h 45 m
Autumnal EquinoxSeptember 22, 2223 UTSeptember 23, 0403 UTSeptember 23, 0951 UT
Autumn89 d 20 h 12 m89 d 20 h 19 m89 d 20 h 17 m
Winter SolsticeDecember 21, 1835 UTDecember 22, 0022 UTDecember 22, 0608 UT
Winter88 d 23 h 51 m88 d 23 h 45 m88 d 23 h 40 m
(Dates & times of equinoxes and solstices from USNO)

Interpreting the Sun's Changing Motion
      What are we, or more to the point, what were the ancients to make of the non-uniformity of the Sun's motion? They could have supposed that it didn't move uniformly along its path around the Earth. But since the stars have uniform circular motions, it was presumed that all celestial bodies should move in some combination of such motions; so how could the Sun appear to move non-uniformly, if it was supposed to move uniformly?
     Ptolemy suggested that although the Sun's orbit was a uniform circular motion around the Earth, the Earth was not at the center of the orbit, but at a point slightly off-center, called the equant, which made it look like the Sun was moving non-uniformly, because when it was on the near side of its orbit around the Earth its motion looked larger and faster than it really was, and when it was on the far side of its orbit its motion looked smaller and slower than it really was. The Earth only had to be 3.4% off-center to cause a non-uniformity in the Sun's apparent motion sufficient to create the several days' difference in the seasons; and that was a small enough difference not to have noticeable effects save for the Sun's non-uniform motion. Still, it should be noted that this means that even in ancient times it was known that we are furthest from the Sun in Summer, and closest to it in Winter, so that the seasons have absolutely nothing to do with our distance from the Sun. It is rather ironic that nowadays many people erroneously think that summer and winter are caused by the changing distance between the Earth and Sun, while two thousand years ago educated people already knew that was wrong. (For a further discussion of this topic, see Cassini Measures the Orbit of the Sun.)

A More Detailed Look
     Students in my lab class are encouraged to calculate the changing motion of the Sun to see how it changes during the year as an extra-credit part of my Mercury Orbit Project. To do this yourself, refer to Mercury Project Part 3x. (Note: I created the Mercury Orbit Project in 1983, and therefore hold its copyright; but anyone is welcome to use the project as-is or to modify it as they desire, per Creative Commons copyright rules.)