It happens later on the west coast (broadcast delay?)

22 September 2009

Happy September equinox, everyone.

One of the cool things about this day is that it has a fairly high profile on the common, demi-Gregorian calendar, the equinoxes perhaps figuring even larger than solstices in my casual surveys. Another one is that it is only the autumnal equinox if you live north of the equator: otherwise, it is the vernal kind.


Meanwhile, though, while it is the closest we get to equal night and day (hence the name–nice of those Romans to use cognates so we’d recognize their mark on history) it is not, in fact, twelve hours of each. Due to a few things here and there, the total winds up being about twelve hours and eight minutes of daylight and eleven hours and fifty-two minutes of darkness, although I think the actual amounts end up varying by a few minutes depending on your latitude. Just goes to show that even recognizing one layer of complexity of function isn’t always enough.

Furthermore, as Dr. Tyson and Astronomy Picture of the Day have grandly demonstrated, the point on the horizon where the Sun rises and sets changes throughout the year, and today it rises pretty much due east and sets pretty much due west. On the December solstice it will reach its farthest south rising and setting, and farthest north on the June version. According to an astronomer I know who dabbles in such things, ancient sites like Stonehenge and Ahu a Umi still line up with these marks because even though the Earth’s axis wobbles on the ~28,000 year cycle of precession, the amount of tilt doesn’t change very much, and that is what matters for things like farthest north and south sunsets and -rises.

Since the new Moon on Friday marked the end of Ramadan and the beginning of Rosh Hashannah, we will have to talk about lunar calendars sometime soon, but not enough room in this post.

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