Trepidation: The Autumnal Equinox

Autumn will soon begin, and I must con­fess a cer­tain trep­i­da­tion. Soon the tele­phone will begin to ring, and the ques­tion will always be the same: “When does fall begin? And I mean exactly, to the minute.” (I never could under­stand why peo­ple need to know so exactly. What are they going to do, sac­ri­fice a chicken?)

The answer is not as easy as it sounds. (Pay atten­tion. This is com­pli­cated.) We deter­mine the sea­sons by the posi­tion of the sun in the sky. The sun’s posi­tion is com­pli­cated by two motions: the turn­ing of Earth on its axis to make day and night and the motion of Earth around the sun. It’s like try­ing to fig­ure out exactly where you are in space while you’re on the “Tilt-a-Whirl” at the fair. But let’s give it a go anyway.

As Earth moves around the sun, the sun seems to move slowly across the sky from day to day along a path called the ecliptic.

Of course, the sun’s motion is purely an illu­sion. We’re the ones who are moving.

The sun path car­ries it once a year in front of the fixed back­ground of stars along a great cir­cle called the eclip­tic. The eclip­tic passes through cer­tain con­stel­la­tions we have come to call the zodiac.

How­ever, Earth, bless its heart, also spins once a day around a line from the north to south poles. The cir­cle that the Earth moves around is its equa­tor. Thus, the starry back­ground that we are try­ing to mea­sure the sun’s motion against moves par­al­lel to a line right above Earth’s equa­tor. That imag­i­nary line in the sky is called the celes­tial equator.

Sadly, Earth’s axis is tilted 23 1/2 degrees away from its path around the sun. That means that the eclip­tic and the celes­tial equa­tor are tilted with respect to each other and are bound to cross one another in two places. We offi­cially mark the begin­ning of fall when the sun crosses the celes­tial equator.

Or to put it another way, for one mag­nif­i­cent moment, the sun is at that spot where the celes­tial equa­tor and the eclip­tic meet, a place we call the autum­nal equinox.

Or to put it yet another way, on that par­tic­u­lar day day­time and night­time are equal, which is what “equinox” really means.

The prob­lem is, that moment doesn’t hap­pen at the same time every year. The num­ber of days in a year is not even. Earth takes 365¼ days to travel around the sun, and its tough to put a frac­tion of a day on the cal­en­dar. So we add a leap day every four years to make the cal­en­dar come out right in the long run. In the mean­time, though, our clocks and cal­en­dars are always wrong if time is mea­sured pre­cisely by the posi­tions of sun and Earth.

Tak­ing all that into con­sid­er­a­tion, I am pleased to report that the cen­ter of the sun will be at the autum­nal equinox at 10:49 a.m. Sept. 22 (more or less, I think) and autumn will begin.

On March 20, 2013, when Earth is on the oppo­site side of its orbit around the sun, the sun will pass along the eclip­tic across the celes­tial equa­tor again, day and night will again be equal, and spring will spring forth.

Or will it? By con­ven­tion, we begin the sea­sons on these dates, but many of our ancient fore­bears con­sid­ered the equinoxes to be the mid­points of autumn and spring and, for that mat­ter, the win­ter and sum­mer sol­stices to be the mid­points of those sea­sons. There’s good rea­son to do so, but I’d bet­ter give your brains (and mine) a chance to cool down a lit­tle before I tell you why.

Plan­ets

The east­ern, morn­ing sky just before twi­light is filled with stars and plan­ets. Venus and Jupiter dom­i­nate, of course. (Venus is the brighter one.) The bright, red­dish stars Alde­baran in Tau­rus and Betel­geuse (below) in Orion add to the spec­ta­cle. But Orion has many bright stars, so wait until they rise as twi­light turns to dawn.

Tom Burns is the direc­tor of Perkins Obser­va­tory. He can be reached at tlburns@owu.edu.

Posted by Tom Burns on Sep 16 2012. You can follow any responses to this entry through the RSS Feed. Comments can be made below.