The Altitude-Azimuth system measures the vertical and horizontal positions of objects in the sky. It is sometimes referred to as a horizontal system because the coordinates are relative to the horizon rather than fixed to the stars. This means that any measurements using this system are dependent on two variables:

• The time and date. This is because the Earth is rotating on its axis and orbiting the Sun.
• Where the observer is. Your view of the sky changes depending on where you are on the globe.

The azimuth (Az) tells you the horizontal position: where the object is in your 360° field of view. The Az is measured starting at north so an object to the north has an azimuth of 0°. The Az angle increases the further clockwise an object is from the north. So for an object to the east, Az = 90°. For an object to the south, Az = 180. Finally, for an object to the west, Az = 270°.

The altitude (Alt) tells you the vertical position: how far the object is above the horizon. So for an object on the horizon, Alt = 0°. In contrast, for an object directly above the observer, Alt = 90°. This point where Alt = 90° is called the zenith. The observer's meridian is the curved line running from north to south through the zenith.

The altitude is useful for finding the times when an object is visible in the night sky. You have the best chance of viewing an object when it has an altitude of at least 30°. This is when the astronomical seeing is best.

You can also use the altitude to work out when an object will rise and set in the sky. When an object's altitude is 0°, it is either rising or setting on the horizon. Objects rise in the east, pass through the meridian, and set in the west.

Stars above the north and south poles never set below the horizon. They just appear to circle the pole during the night. However, their altitude depends on where you are on Earth. If you were standing on the Earth's geographic North Pole, the north celestial pole is right above your head. If you were standing at the equator, it is on the horizon.