Happy equinox!

The September equinox arrives today at one minute before noon, Eastern Daylight Time. One technical definition of the equinox (from the U.S. Naval Observatory) is that day when the geometric center of the Sun’s disk passing through the equator, with that point appearing above the horizon everywhere on Earth for 12 hours. Another definition, one that I like a little bit better, is “the date when the Sun crosses the celestial equator moving southward in the Northern hemisphere.”

One thing many people (including geographers, who really ought to know better) fail to understand is that the astronomical equinox does not mean that all regions on Earth have exactly 12 hours of daylight and 12 hours of dusk/dark. The reason for this, according to the U.S. Naval Observatory, is twofold: one, the Sun is not a simple point source of illumination: it has discernible width, and it takes a little while for it to rise or set. Now, because sunrise is defined as first contact (the moment the leading edge of the sun appears above the horizon) and similarly, sunset is defined as last contact (the moment the trailing edge of the sun disappears below the horizon), and because the Sun is not a point source of illumination (it has discernible width: a full half-degree across it measures, same as the full moon–the reason we can have total solar eclipses), it takes a measurable amount of time for the entire disk to appear and disappear. Therefore, there are usually about 12 hours and 6 minutes of daylight on the equinox at the equator, and even longer at the poles, where it takes the sun longer to rise and set, because it’s rising and setting at an angle.

The second reason day length is greater than 12 hours at the astronomical equinox is atmospheric distortion: when the Sun is low on the horizon, Earth’s atmosphere bends the light of the sun a little bit. Just enough, in fact, so that the actual moment of astronomical sunrise at any given location is AFTER the Sun is visible from that location. The same thing happens when the sun sets: the sun disappears from view AFTER it has actually set, because the light rays traveling through Earth’s atmosphere bend them just enough.

Nevertheless, the equinoxes mark the astronomical ends of summer and winter, respectively, and the beginnings of the transitional seasons. But we shouldn’t call them vernal and autumnal equinoxes, because, even though there is twice as much of Earth’s land mass in the Northern Hemisphere as in the Southern, it’s not fair to those who live in the oceanic hemisphere to call their autumnal equinox a vernal one, and vice-versa.

There seems to be a much smaller tradition of partying at the equinoxes than there is at the solstices. Midwinter and Midsummer (Shakespeare’s fantasy play takes place on the solstice, actually, not the “true” middle of summer, which would be in late July or early August) have a much wilder history than Michaelmas and the Ides of March (Walpurgisnacht doesn’t count, because it comes about midway between equinox and solstice). One obvious exception to this, of course, is the hebraic tradition of the high holy days at the end of September. (Unlike some other lunar-based calendars, the Jewish calendar keeps the new year at about the same time of the astronomical year, although it’s rarely on the equinox.)

And this lack of partying at the equinox only makes sense; it’s far better to party on the shortest and longest nights of the year, when there’s little else to do. In the spring, people are too busy getting ready for the planting season to think about partying. And in the fall, people are too busy getting the harvest in to have much more than a cursory celebration. Now, after the harvest’s in, that’s a different story…

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