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.
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 firstname.lastname@example.org.