Coordinates are a set of numbers that uniquely locate a particular place. Everyone who uses a coordinate system must agree on its zero point and the way the measurements are made.
We can locate ourselves on the Earth using the terrestrial coordinate system. Latitude describes angular distance north or south of the equator and longitude describes angular distance east or west from the prime meridian.
Positions and sizes of stars and other celestial objects in the sky are conveniently described using angles. Angles are often measured in degrees, minutes, and seconds of arc.
A convenient coordinate system for locating stars and other celestial objects is the horizon system. Altitude is the angular distance of the star above the celestial horizon. Azimuth locates the point on the horizon below the star.
The stars, planets, and Moon appear to move westward during the night. This westward motion can be detected in about a minute.
Most stars rise and set, but stars in the circumpolar zone centered near Polaris always remain above the horizon. These stars appear to wheel counterclockwise about Polaris. Stars not located in the circumpolar zone move on circles that carry them below the horizon.
