Equinox

The autumnal equinox approaches. It is only a couple hours away.

I’ve sometimes had arguments with people who think the seasons always begin on the same day every year. They don’t. Instead, they shift around a little bit. Autumn begins with the autumnal equinox, which sometimes falls on the September 22nd and sometimes on September 23rd, at least lately. I understand that if you look at a longer calendar the autumnal equinox may fall as late as the 24th.

I remember speculating in the past that day and night must be of (almost) equal length on the equinox, and that this must be true no matter where on the planet you are. But this was just based on some reasoning I was trying to do in my head, based on my extremely tenuous grasp of astronomy, and I wasn’t sure I had it right.

Thank heavens for Wikipedia.

On the equinoxes the sun rises exactly at East and sets exactly at West everywhere, and the length of the day equals the length of the night.

Indeed, it seems that the very meaning of the word “equinox” is Latin for “equal night.”

Update: Friend Anal pointed out the following:

Huh. So in Anchorage, the sunrise and sunset are closest to 12 hours apart on the 24th. In Austin, it will be the 26th. In New Orleans, it will also be the 26th. In Missoula, it will be the 25th. All this according to weather.com, anyway.

Crazy.

Note that none of those days is the actual day of the equinox. This confused me. My understanding was that day and night would be of equal length on this day, everywhere on the planet.

Wikipedia sez:

For terms of measuring the length of day on the equinox, sunrise starts when the sun is half way above the horizon and sunset is when the sun is halfway under the horizon. Using this definition, the length of the day is precisely 12 hours.

This led me to theorize that typical sunrise and sunset times (such as on weather.com) are calculated differently. And, indeed, the following passage from Stars and Planets by Peterson Field Guides seems to bear this out:

Although day and night are theoretically equal in length on the days of the equinoxes, that would be true only if the sun were a point, not a disk, and if the earth’s atmosphere did not bend sunlight. However, the top of the sun actually rises a few minutes before the center of the sun’s disk–the point used in calculations. Also, the earth’s atmosphere bends sunlight, so we can see the sun for several minutes before the time sunrise would occur and after the time sunset would occur if the earth had no atmosphere.

(I got this quotation from the handy Sunrise, Sunset website’s FAQ list.)