That these important dates don't fall on the same day every year is unacceptable. Call your Senators and Congresscats! Tell them you want a well-regulated calendar Americans can count on. Lets bring order back to our lives. Regular seasons for regular folks!
About perturbation of the equinoxes from Wikipedia*:
- The actual equinox is a single moment in time — it does not take the whole day.
- Because the Sun is a sphere and not a point source of light, the actual crossing of the Sun over the equator takes approximately 2 and 1/2 days. The equinox occurs halfway through the transit when the center of the Sun is directly over the equator.
- Disregarding atmospheric effect, that the Sun is not a point source of light and that the Earth's orbit is not perfectly circular, the equinox day will have 12 hours of daylight and 12 hours of nighttime.
- At the Equinoxes, the rate of change for the length of daylight and nightime is the greatest. At the poles, the Equinox marks the transition from 24 hours of nightime to 24 hours of daylight. High in the Arctic Circle, Longyearbyen, Svalbard, Norway has an additional 15 minutes more daylight everyday around the time of the Spring equinox. Whereas, in Singapore, which lies virtually on the equator, the amount of daylight each day varies by just seconds.
- It is 94 days from the June solstice to the September equinox, but only 89 days from the December solstice to the March equinox. The seasons are not of equal length because of the variable speed the Earth has in its orbit around the Sun.
- The instances of the equinoxes are not fixed but fall about six hours later every year, amounting to one full day in four years, but then they are reset by the occurrence of a leap year. The Gregorian calendar is designed to follow the seasons as accurately as is practical. It is good, but not perfect. Also see: Gregorian calendar#Calendar seasonal error.
- Smaller irregularities in the times are caused by perturbations of the Moon and the other planets.
- Currently the most common equinox and solstice dates are 20 March, 21 June, 22 September and 21 December, the four year average will slowly shift to earlier times in the years to come. This shift is a full day in about 70 years (largely to be compensated by the century leap year rules of the Gregorian calendar). But that also means that as many years ago the dates of 21 March, 22 June, 23 September and 22 December were much more common, as older books teach and older people still remember.
** I don't think the Wiki weasels have twisted this entry. Unless they are from ESU#3 and think George Bush is responsible for sowing the seeds of confusion with his equinoctal perturbations.