Altitude (Observing)

In astronomy, altitude is the angle measured straight up from the horizon to an object in the sky, expressed in degrees from 0 degrees, which is exactly on the horizon, to 90 degrees, which is straight overhead at the zenith. Altitude is one half of the horizontal coordinate system, the most intuitive way to describe where something is in the sky for a specific observer at a specific place and time, with azimuth (the compass direction) making up the other half. Unlike right ascension and declination, which describe a star's fixed position relative to the celestial sphere regardless of when or where you're looking, altitude and azimuth constantly change as Earth rotates and as the observer's location changes, which is exactly why planetarium software and stargazing apps recalculate them continuously. Knowing an object's altitude is essential for practical observing: objects low on the horizon, at small altitudes, are viewed through much more of Earth's atmosphere, which increases atmospheric distortion, dims their light, and can shift their apparent color — one reason experienced astronomers wait for a star or planet to climb to a higher altitude before attempting detailed observation or astrophotography.