Today is the September Equinox. You’ve probably already heard it a few times today; people running around proclaiming with utmost exuberance how today is the first day of Fall. In the Northern Hemisphere, the announcement is the harbinger of shorter days and dropping temperatures. But what is really going on today?
When someone says today is the first day of Fall, what they really mean (whether they know it or not) is that today represents an equinox; specifically, the September Equinox.1 On Earth, an equinox is the point in its orbit around the Sun when both hemispheres are equally illuminated; our tilted Earth lines up to a point in which the Sun passes directly over the equator. This happens twice a year, on the March and September equinoctes (that’s the plural form of equinox, use this information smugly).
Contrary to popular belief, the day of the equinox does not represent the day where daylight and darkness are equal. You can thank geometry, the atmosphere, and the Sun’s angular diameter to cause that equality to happen at different times geographically. What today does mean though, is that the equinoctes are the only two days in which the Sun rises due-East and sets due-West, and which the Sun would pass directly overhead from an observer on the equator.
One other very important thing that you must know if you don’t learn anything else today: Way too many people believe that the equinoctes are the only day of the year that an egg can be balanced on its end. While it is true that on the equinox an egg can be balanced, it’s also true of every other day of the year; it makes no difference!
There are other times during the year (read: our orbit around the Sun) that we recognize Earth residing at a special place. There’s Perihelion and Aphelion, and then the widely-celebrated solstices, but I’ll save those for another time.
Happy September Equinox!
- What about them being called the Spring and Fall (or their Latin names, Vernal and Autumnal) equinoctes? Well, that wasn’t exactly fair to those in the Southern Hemisphere, whose seasons are opposite those in the Northern Hemisphere. ↩