If you watch the stars on a dark night, it’s easy to think of the sky as a great dome. But as the night goes on, the dome rotates. New stars rise in the east, while others disappear in the west. So ancient skywatchers thought of the sky not as a dome, but a sphere that completely encircles us – the celestial sphere.

To the Greeks, the sphere was real – a perfect crystalline surface, with the stars hanging from it like lanterns. Earth stood still at the middle of the sphere, which turned around it.

Today, of course, we know that Earth is turning, and the stars are so far away that they appear to be fixed in place. Yet astronomers still use the celestial sphere. Their coordinate system is based on it. The system has lines of latitude and longitude, an equator, and north and south poles – all of which are projections of Earth’s coordinates.

The celestial poles, for example, are based on the projection of Earth’s poles – the directions in which our planet’s axis is pointing.

There’s also a celestial equator – an extension of Earth’s equator. As darkness falls tonight, it arcs from Aquarius, in the east; through Aquila, in the south; and down to Virgo, in the west.

Only those who live near the equator can see the entire celestial sphere. For everyone else, it’s clipped. And at the poles, only half of the sphere is ever visible – a great dome showing the same stars all year long.

Script by Damond Benningfield