This is the day when fall begins, when the autumnal equinox brings an end to summer and, soon, the northern United States is awash in the reds and yellows of turning leaves.
This year the beginning of fall is at precisely 5:18 p.m., Eastern Daylight Time today. At that moment, somewhere along the equator the center of the sun is directly overhead. (It doesn't matter where on the equator since it's only an instant, but this year the spot is over the eastern Pacific Ocean.)
Likewise, if you're at the north or south pole, at the appointed time, the sun will appear to be precisely on the horizon, circling around you as the earth circles.
The days, which are longest around June 21 when the solstice marks the start of summer, are getting shorter now at a rate of three minutes per day. They will keep getting shorter, though less dramatically, until the winter solstice comes on Dec. 21.
This is a day of small-scale mythology. It's widely believed, for instance, that every spot on earth gets precisely 12 hours of sunlight on this day, but given the slight irregularities of the planet and its orbit, it never quite works out that way.
It's close, but not exact. In Los Angeles, for instance, sunrise today is at 6:41 a.m., and sunset comes at 6:49 p.m.
From somewhere as well, the legend also began that on the equinox -- and only on the equinox -- you can balance an egg on end. It had something to do with everything being in balance -- the days 12 hours long, the sun's gravity coming directly from above. Whatever the roots of the story, it's no truer today than on any other day.
The BBC had some fun a few years ago at believers' expense, saying the idea dated to ancient China, but had gotten a boost from a 1945 story in Life Magazine, which told of people in Chunking, China, balancing eggs en masse. "Perhaps this piece of information is too egg-centric for you," the BBC wrote.
Of course, if you try it yourself on the equinox, you may well succeed -- it's just that you'll have the same rate of success on any other day of the year.
If you're a kid, you may be a little sorry (with school and all that) to see summer formally come to a close -- but polls tend to show that more of us prefer fall. The year seems to have a fresh beginning -- and, in fact, many older calendars had the new year coming now instead on January 1, a couple of weeks after the winter solstice on Dec. 21.
If you don't like shorter days and cooler weather, take comfort. The vernal equinox, which begins spring, is only six months away.