The annual September, fall, or autumnal equinox often stumps people, but this year it coincides with Jupiter's closest pass to Earth in a decade and accompanies a large harvest moon.
Chris Young/The State Journal-Register/AP
Need an excuse to celebrate tonight?
Try welcoming the fall equinox, when jugs of of iced tea yield to jugs of apple cider, pumpkin is a much a seasonal color as it is a squash, and landscapes carpeted with green begin to display splatters of gold, scarlet, orange, and russet.
At 11:09 Eastern Daylight Time tonight (which is 03:09 Universal Coordinated Time tomorrow), the autumnal equinox arrives.
That's in the northern hemisphere, of course. South of the equator, winter is yielding to spring. Since one hemisphere's vernal equinox and another hemisphere's autumnal, you'll be on top of your game in either hemisphere if you just call it the September equinox.
Now for the pop quiz. An equinox is:
A. A 24-hour period twice a year with precisely equal hours of daylight and darkness.
B. A condition of automotive trauma in which all cylinders in an engine – four, six, eight, take your pick – are knocking simultaneously and with equal volume level.
C. A brief moment in time twice a year when the center of the sun crosses the celestial equator (basically an extension of Earth's equator into space).
D. A uniquely equine combination of nitrogen and oxygen gas.
If you picked C, you've got it. The September and March equinoxes mark the two moments each year when the center of the sun crosses the celestial equator, or in Earthling terms, appears directly over the equator.
If you like to plan ahead, the March equinox next year will occur on March 20 at 23:21 UTC.