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Celestial Equator

October 15, 2012

There are lots of ways to split up the night sky. One of the most natural is to divide it into northern and southern halves. And in fact, astronomers have done just that with the celestial equator — the projection of Earth’s equator on the sky. You can trace the equator’s path this evening by following several constellations, which stretch from due west, to high in the south, to due east.

As darkness falls, Ophiuchus, the serpent bearer, is well up in the west, flanked by the divided halves of the serpent. None of their stars are all that bright. But Ophiuchus takes up a huge chunk of the sky, so if you look up from the western horizon, you’re looking right at it.

Fairly high in the south, there’s a more prominent constellation — Aquila, the eagle. Its brightest star, Altair, is a little above the equator. Altair is one of the points of the bright Summer Triangle, so it’s easy to pick out.

And over in the southeast there’s another faint constellation, but one that’s well known: Aquarius. A couple of its brighter stars lie almost atop the celestial equator.

And if you stay up until midnight, you’ll see the most famous equatorial denizens of all — the three stars of Orion’s Belt. They form a short line that’s almost due east. The Belt’s top star, Mintaka, is just a fraction of a degree from the equator.

Orion climbs across the sky during the night — with his top half in northern skies, and his bottom half in the south.


Script by Damond Benningfield, Copyright 2012

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