Term: celestial equator

The celestial sphere is something like the globe. It has poles that it rotates about, so it might as well have an equator that divides the celestial sphere exactly in half: the half nearest to the north celestial pole, and the half nearest the south celestial pole. Just as we locate cities on the globe with latitude and longitude, we'll soon introduce similar coordinates to locate stars on the celestial sphere. The celestial equator always stretches from the East point on the horizon to the West point, and hence must cross the meridian (which runs North to South). Here at CSB/SJU the celestial equator crosses the meridian about 45° South of zenith.

If we wanted, we could orient the globe so that Minnesota was "on top" and up from the globe corresponded to up here, but most commonly globes are oriented so the north pole is up. Similarly it often makes more sense to orient the celestial sphere so the north celestial pole is up; but then the stick figure standing straight up in Minnesota (the blue ball is Earth), seems to be tilted. Note that the celestial poles are extensions of the Earth's poles, and the celestial equator can be found by expanding out the Earth's equator.