The Diurnal Motion of Sirius Centuries ago Greek astronomers imagined that there was another part to the sky below the astronomical horizon. The entire sky, they suggested, was a sphere with half of the sphere hidden by the Earth. The Celestial Sphere horizon, is the nadir . Circles passing through the zenith and nadir are known as vertical circles while the circles parallel to the horizon are termed parallels of altitude . Longitude and latitude locate a place on the surface of the spherical Earth.
Astronomers use similar coordinate systems to locate objects on the celestial sphere.
The coordinates in this system are called altitude and azimuth. Altitude is the angular distance measured from the horizon along the vertical circle through the object. For example, any object on the horizon has an altitude of 0 degrees, the zenith, directly overhead, has an altitude of 90 degrees, and a star midway between the horizon and zenith has an altitude of 45 degrees. Azimuth is the angular distance measured from the north point eastward around the horizon to the vertical circle through the object. The following table lists some altitudes and azimuths.