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planetocentric and planetographic coordinates





Two systems in which the positions of features on the surface of a near-spherical body such as a planet or satellite may be recorded. Specific terms are used when referring to familiar bodies: heliocentric and heliographic coordinates for the Sun; selenocentric and selenographic for the Moon; geocentric and geographic for the Earth; areocentric and areographic for Mars; zenocentric and zenographic for Jupiter; saturnicentric and saturnigraphic for Saturn; and so forth.

Planetocentric coordinates are referred to the equatorial plane of the body concerned and are much used in the calculations of celestial mechanics. Planetocentric longitude is measured around the equator of the body from a prime meridian defined and adopted by international agreement. (The prime meridian may be referred to a visible feature in the case of a solid-surfaced body such as Mars, but in the case of a gaseous planet such as Jupiter it is a purely hypothetical concept.) Planetocentric latitude is measured in an arc above or below the equator of the object in the usual way.

Planetographic coordinates are used for observations of the surface features of those planets whose figures are not truly spherical, but oblate. They are referred to the mean surface of the planet, and are the coordinates actually determined by observation. They can readily be converted to planetocentric coordinates if required. As the oblate planets are symmetrical about their axes of rotation, there is little difference in practice between planetocentric and planetographic longitudes. However, the differences between planetocentric and planetographic latitudes are quite significant for very oblate bodies such as Jupiter and Saturn.


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   • CELESTIAL MECHANICS