Familiarizing yourself with the night sky

Like our ancestors did thousands of years ago, it's quite natural for us today to view the night sky as an inverted bowl upon which the stars are attached and the Sun, Moon, and planets move. And it requires only a little imagination to further visualize that the inverted bowl of stars is actually the hemisphere of a globe — a celestial sphere — which we see from the inside out. Earth floats freely in space within this sphere, which lies at an immense but arbitrary distance. The sphere revolves slowly around our world, incrementally conveying the constellations from east to west until they return to their original positions a year later.

Of course, we recognize that the celestial sphere is an illusion: the Moon and Sun lie at very different distances, the stars are light-years away, and it is Earth's rotation and orbit that make the sky appear to move. But for purposes of tracking the motions and directions of celestial bodies, as well as learning the stars and constellations, a two-dimensional celestial sphere is an effective mental contrivance.

If we consider the sky as a sphere rotating about an axis, then it must possess directional bearings, a north and a south pole, and an equator. In the case of the celestial sphere, these are essentially projections into space of the Earth's cardinal directions, poles, and equator.