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 brigh