Yesterday was the equinox, but I couldn't help noticing that sunrise was at 7:07AM and sunset was at 7:16PM. That stretches the definition of "equi" a bit. Looking ahead, the day won't be within a minute of 12 hours until September 25 or 26. (One's a minute longer, one's a minute shorter.) So off to Google I went.
There are two things going on, it turns out. The first is that the equinox is relative to the center of the sun, but we count sunrise and sunset from when the top is visible. But that only accounts for 2.5-3 minutes at my latitude.
The bigger factor is atmospheric refraction: after the sun has actually set (all parts past the horizon), or the reverse in the morning, you can still see the sun. What? Yeah, apparently you can look westward at sunset and see "the sun" even though the sun is not in your line of sight; light bends. This effect varies with atmospheric conditions, but is usually good for about six extra minutes of day.
I said that I won't see a 12-hour day here for a few more days. Apparently that effect gets stronger as you move toward the equator; this site says at 5 degrees North that date isn't until October 17. It also says the day is never exactly 12 hours at the equator, when I thought the equator was the one place where you had reliable 12-hour days all year. Today I learned.
I wonder -- because I'm the sort of person who wonders about stuff like this -- what the effect is in halacha, Jewish law. The day starts at sunset, but when beginning Shabbat we add some extra time just to be safe -- 18 minutes in most communities. That's l'hatchila, what you should do from the outset, but b'dieved, after the fact, if you cut into the 18 minutes with your preparations, it's ok because it's not actually sunset yet. Except... maybe it is? If you have a bad week and light candles two minutes before (nominal) sunset -- when you can still see the sun in the sky, except it's not there -- have you kindled fire on Shabbat? Or do you go by what you can see anyway? I plan to ask this on Mi Yodeya if it's not already there, but first I have to finish Sukkot preparations.