Altitude and azimuth are used in the horizon coordinate system to determine the location of objects such as stars in the sky from a specific location at a specific time. Altitude measures the angle between the object and the viewer's horizon and is between zero and 90 degrees. Azimuth measures the angular distance around the horizon to the object, from north to east, and is between 0 and 360 degrees.
For instance, if there is a star exactly halfway between the ground and the zenith, the highest point in the sky, it will have an altitude of 45 degrees. If this same object is directly east, its azimuth will measure 90 degrees.
The horizon coordinate system is used as a celestial coordinate system to assist in finding specific objects in the sky, no matter where on Earth the viewer is located. It divides the sky into two hemispheres, the upper hemisphere, which is everything the viewer can see, and the lower hemisphere, which is everything below the viewer's horizon that they can't see. As objects such as stars move across the night sky, their altitude and azimuth will change, although the viewers location stays the same.
The horizon coordinate system is the only observer-centered location system and is often used by stargazers to help locate constellations and other objects in the night sky, such as Venus or Mars.