One way to describe the location of an object in the sky
is by using *altazimuth
cooordinates*, also know as *horizon coordinates*. These coordinates are
always relative to a specific observer's location.

It takes two numbers to describe a location in horizon coordinates.

The

*altitude*or*elevation*is the height of the object above the idealized horizon (or where the horizon would be if the local terrain were flat). An elevation of π/2 radians is straight up, also known as the*zenith*.In practice, some objects at negative altitudes are visible: from South Baldy at 10,000' in the Magdalenas, it is possible to see a bit below the horizon in some directions.

The

*azimuth*is the equivalent of the compass direction. An object due north of the observer is considered azimuth zero, and values increase clockwise in the order N→E→S→W→N. In radians, the values are in the range [0,2π).