longitude—The angle between the plane of a given meridian and the plane of an arbitrary initial meridian, generally the meridian of Greenwich. It may be measured as the angle at the poles between the two meridians, as the arc of the equator intercepted between the meridians, or as the arc of a parallel of latitude intercepted between the meridians.

longitude, astronomic—The angle between the plane of the celestial meridian and the plane of an initial meridian, arbitrarily chosen. Astronomic longitude is the longitude which results directly from observations on celestial bodies, uncorrected for deflection of the vertical. Astronomic longitude is measured by the angle at the celestial pole between the tangents to the local and initial meridians, or by the arc intercepted on the equator by those meridians. At an international convention held in Washington, D.C., in I884, the Meridian of Greenwich was adopted as the initial or prime meridian for all longitudes, and is generally so used. An expression of longitude is sometimes accompanied by a statement of the method used in its determination, often in adjective form, as “wireless longitude”; this may serve as an indication of its precision. Among the various methods used are the following: 1. Celestial signals—The local time at a new station of the occurrence of celestial phenomena is compared with the corresponding time of the occurrence at a base station. Among such phenomena are eclipses of the moon, eclipses of Jupiter’s satellites, and occultations of stars. 2. Chronometric—The local time at a new station is compared with the time at a base station by transporting chronometers from one station to the other. 3. Lunar-distance, moon-altitude, moon-culmination—The local time at a new station is compared with the time at a base station, both times being determined from the position of the moon relative to other celestial bodies. 4. Telegraphic—The local time at a new station is compared with the time at a base station by the electric-telegraph land-lines and sea cables. 5. Terrestrial signals—Local times noted at new and base stations of the occurrence of such signals as a flash of gunpowder. 6. Wireless—The local time at a new station is compared with the time at a base station by the use of radio. See also coordinates, astronomic; latitude, astronomic.

longitude, geodetic—The angle between the plane of the geodetic meridian and the plane of an initial meridian, arbitrarily chosen. A geodetic longitude can be measured by the angle at the pole of revolution of the ellipsoid between the local and initial meridians, or by the arc of the geodetic equator intercepted by those meridians. In the United States, before the introduction of the North American Datum of 1983, geodetic longitudes were numbered from the Meridian of Greenwich, but were computed from the meridian of station Meades Ranch as prescribed in the North American datum of 1927. In recording geodetic position it is essential that the geodetic datum on which it is based be stated. A geodetic longitude differs from the corresponding astronomic longitude by the amount of the prime vertical component of the local deflection of the vertical divided by the cosine of the latitude. In the United States this may amount to as much as 26″.

longitude equation—See equation, longitude.

longitudinal chromatic aberration—See aberration of light [OPTICS].

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

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