Planetographic Coordinates

numbers that define the position of a point on the surface of a planet. As on the earth, latitude and longitude can serve as planetographic coordinates.

Latitude is measured by the angle between the planet’s equatorial plane and the normal to the planet’s surface at the point in question. For planets with a small oblateness, this concept of latitude practically coincides with that of planetocentric latitude, which is measured by the angle between the equatorial plane and the line connecting the given point with the planet’s center. The northern hemisphere is considered to be the hemisphere containing the pole that is on the northern side of the Laplace invariant plane.

The longitude of a point is the dihedral angle between the meridian plane at the given point and the plane of the zero meridian, which passes through a point on the planet’s disk selected by international agreement. The longitude is reckoned from 0° to 360° in the direction opposite to the direction of the planet’s rotation for an observer located in an inertial, nonrotat-ing reference system. For planets devoid of clearly discernible features that could be used to provide a zero meridian, the latter is taken to be the meridian passing through the center of the planet’s disk, or the central meridian, at a certain fixed time. If we know the planet’s rotation period, we can determine the position of the zero meridian relative to the central meridian for any point in time. If the planet rotates with different angular velocities at different latitudes, a system of longitudes is established for each latitude zone; this is done, for example, with Jupiter, Saturn, and the sun.

Proper names are sometimes applied to the coordinates of specific planets. Thus we may speak of, for example, hermo-graphic coordinates for Mercury, or Hermes; Venerian coordinates for Venus; geographic coordinates for the earth; selenographic coordinates for the moon; areographic coordinates for Mars, or Ares; and jovigraphic coordinates for Jupiter.