equator

/i kway"teuhr/, n.

1. the great circle on a sphere or heavenly body whose plane is perpendicular to the axis, equidistant everywhere from the two poles of the sphere or heavenly body.

2. the great circle of the earth that is equidistant from the North Pole and South Pole.

3. a circle separating a surface into two congruent parts.

4. See celestial equator.

[1350-1400; ME < ML aequator, L: equalizer (of day and night, as when the sun crosses the equator). See EQUATE, -TOR]

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Great circle around the Earth that is everywhere equidistant from the geographic poles and lies in a plane perpendicular to the Earth's axis.

This geographic, or terrestrial, Equator divides the Earth into the Northern and Southern Hemispheres and forms the imaginary reference line on the Earth's surface from which latitude is reckoned (i.e., 0° latitude). In astronomy, the celestial equator is the great circle in which the plane of the terrestrial Equator intersects the celestial sphere; it is thus equidistant from the celestial poles. When the Sun lies in its plane, day and night are everywhere of equal length; this happens at the equinoxes.

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▪ geography

      great circle around the Earth that is everywhere equidistant from the geographic poles and lies in a plane perpendicular to the Earth's axis. This geographic, or terrestrial, Equator divides the Earth into the Northern and Southern hemispheres and forms the imaginary reference line on the Earth's surface from which latitude is reckoned; in other words, it is the line with 0° latitude.

      In astronomy, the celestial equator is the great circle in which the plane of the terrestrial Equator intersects the celestial sphere; it consequently is equidistant from the celestial poles. When the Sun lies in its plane, day and night are everywhere of equal length, a twice-per-year occurrence known as equinox.

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