Ms. Science and the Autumnal Equinox

Because she rocks, Megan wrote this piece for me (and you) to celebrate the Autumnal Equinox, which is today.

This year, the Autumnal Equinox occurs at 12:03 am EDT, September 23rd. Most of us know that Autumnal Equinox marks the official (read: astronomical) end of the Summer and beginning of Autumn, and that on that day we are supposed to have the same amount of daytime and nighttime (hence the whole equi-nox bit). But what else does it mean? What is an equinox, anyway? And why did you say it happens at three minutes after midnight, Ms. Science? Well, Timmy and Janey, I’m glad you asked.

During the course of a single day, the Earth’s rotation causes celestial objects–the Sun, Moon, planets, stars–to rise in the East and set in the West. So far so good. However, if you watch celestial objects over longer periods of time–days to weeks–you’d see something very different. The Sun, for example, appears to move eastward one degree per day with respect to the background stars, due to the Earth’s orbit about it. That is, if you could see the Sun in front of the much more distant stars, the Sun would appear, over the course of a year, to trace out a path eastward. Astronomers call this imaginary line across the sky the ecliptic.

There are other imaginary lines and points in the sky as well. The North Celestial Pole (NCP) is a projection of the Earth’s North Pole onto the sky; Polaris, the North Star, is quite close to the NCP, which is why it appears not to move during the course of the night–the Earth silently spins underneath this fixed point. Ninety degrees away is a curve called the Celestial Equator (CE)–the projection of the Earth’s own equator onto the sky (imagine running around the Earth’s equator and drawing a line upwards as you did it. That’s the CE). Now, the CE isn’t aligned with the ecliptic; they are tilted by an angle of 23.5 degrees, thanks to the tilt of the Earth’s rotation axis. Imagine holding two hula hoops, with one parallel to the ground (that’s the CE) and one tilted 23.5 degrees (that’s the ecliptic).

This all means that the Sun is sometimes north of the CE (and so quite high in the sky for the Northern Hemisphere) and sometimes south of the CE (and so quite low in the sky for the Northern Hemisphere). The point at which the Sun is highest north is called the Summer Solstice, the farthest south, the Winter Solstice, and they mark the beginning of (Northern) Summer and Winter, respectively.

But what about the equinoxes? The equinoxes are the two points in the sky that the ecliptic and the CE touch (or, in the hula hoop model, they’re where your hands are, probably). The Autumnal Equinox is the point that the Sun crosses the CE moving from the Summer Solstice eastward (and southward) to the Winter Solstice. The Vernal Equinox is the point that the Sun crosses the CE moving east and northward toward the Summer Solstice.

And now you know why astronomers and meteorologists just love telling you *exactly* when an equinox occurs. The Autumnal Equinox occurs this year at 12:03 am EDT September 23rd because it is at that moment that the Earth’s orbit brings the Sun across the Celestial Equator. How cool is that?

That egg thing, though? You know, where you are supposed to be able to balance eggs on the equinox more easily for some reason? That’s a myth. My friend, Phil Plait, the Bad Astronomer shows this pretty clearly here, in a picture taken in his Bad Kitchen in late Bad October.

Incidentally, on the morning of an equinox, the Sun rises directly from the East and sets directly in the West. If you live near a street that runs directly east-west, you can see this yourself. Here’s a rather famous example. That’s the basic principle behind Stonehenge, which I think is pretty cool: our roads and buildings are, in their own way, modern Stonehenges.

Any more questions?

Helen Boyd

is the author of My Husband Betty and She's Not the Man I Married.


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