EQUATOR

In Astronomy, the great circle of the celestial sphere, of which the plane is perpendicular to the axis of the earth's diurnal motion. It is called the equator, because when the sun is in its plane the days and nights are exactly equal all over the world. The equator divides the sphere into the northern and southern hemispheres and the apparent diurnal motions of all the celestial bodies are performed in circles which are parallel to it. The right ascensions are measured on the equator; and the declinations on circles which intersect it at right angles. The equator, in the heavens, is often styed the equinoctial. In Geography, the equator is the great circle of the terrestrial sphere which is every where equally distant from the two poles, and divides the earth into the northern and southern hemispheres. Terrestrial longitudes are measured on the equator, or some one of its parallel circles; commencing from some arbitrary point, which different nations a ume variously, most of them adopting the meridian which pa es through their capital city or principal observatory. Latitudes are counted from the equator along the meridian. [s. 428]