The angle of a celestial object measured upwards
from the observer's horizon. Thus, an object on the
horizon has an altitude of 0° and one directly overhead
has an altitude of 90°. Negative values for the
altitude mean that the object is below the horizon.
Altitude is usually used together with
azimuth to give the direction
of an object in the topocentric coordinate system.
Altitude is also frequently referred to as "elevation",
and this term was used until recently by Heavens-Above.
We now use the term "altitude" to be in line with most
other astronomy publications, and reserve "elevation"
to indicate the height of the observer above sea-level.