Circle of Right Ascension

Great circles passing through the poles of the equinoctial.

Declination Circles. The great circles upon which declinations are measured.

Diurnal Circle. A heavenly body is said to describe a diurnal circle owing to the apparent daily revolution of the celestial sphere.

Hour Circle. The great circle of the celestial sphere passing through the poles of the heavens and perpendicular to the equinoctial.

Latitude Circles. Great circles perpendicular to the plane of the ecliptic, upon which latitudes are measured.

Polar Circle. The small circles of the terrestrial sphere parallel to the equator.

They are 23 deg. 28 min. from the poles; the northern is termed the Arctic circle, and the southern the Antarctic circle.

Vertical Circle. A great circle passing through the zenith.