equator-¹In a system of polar or spherical coordinates, the great circle of a sphere cut by a plane which is perpendicular to the polar axis through the center of the sphere. ² (astronomy) The great circle on the celestial sphere whose plane is perpendicular to the axis of rotation of the earth. In astronomic work, because parallel lines meet at infinity, the plane of the celestial equator is sometimes assumed to pass through the point of observation. ³ (geodesy) The circle on the ellipsoid midway between its poles of revolution.

equator, magnetic—The imaginary line, on the Earth’s surface, at which the magnetic inclination is zero degrees, i.e., the magnetic field is horizontal.

equator, true—The actual celestial equator at a specified instant. This term distinguishes the actual equator from an equator which is an average or otherwise mathematically defined.

equatorial radius—The radius assigned to the great circle comprising the terrestrial equator.

equatorial stars—Stars having declinations dose to zero and whose diurnal path is a parallel of declination close to the equator. Equatorial stars, because of their apparently greater speed of travel, are preferred for time and longitude determinations. For observations to determine time, latitude, or azimuth, at stations within the continental United States, and to give a selection of the brighter stars suitably spaced for any season of the year the equatorial belt is regarded as extending from 10° to 15° south declination to from 25° to 30° north declination, all to be suitably consistent in position for the type of observation that is to be made.

equatorial tide—Tides that occur approximately every two weeks when the moon is over the equator. At these times, the moon produces minimum inequality between two successive high waters and two successive low waters.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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