The longitude systems of most of those bodies with observable rigid surfaces have been defined by references to a surface feature such as a
crater. The north pole is that pole of rotation that lies on the north side of the
invariable plane of the Solar System (near the
ecliptic). The location of the prime meridian as well as the position of the body's north pole on the celestial sphere may vary with time due to precession of the axis of rotation of the planet (or satellite). If the position angle of the body's prime meridian increases with time, the body has a direct (or
prograde) rotation; otherwise the rotation is said to be
retrograde. In the absence of other information, the axis of rotation is assumed to be normal to the mean
orbital plane;
Mercury and most of the satellites are in this category. For many of the satellites, it is assumed that the rotation rate is equal to the mean
orbital period. In the case of the
giant planets, since their surface features are constantly changing and moving at various rates, the rotation of their
magnetic fields is used as a reference instead. In the case of the
Sun, even this criterion fails (because its magnetosphere is very complex and does not really rotate in a steady fashion), and an agreed-upon value for the rotation of its equator is used instead. For
planetographic longitude, west longitudes (i.e., longitudes measured positively to the west) are used when the rotation is prograde, and east longitudes (i.e., longitudes measured positively to the east) when the rotation is retrograde. In simpler terms, imagine a distant, non-orbiting observer viewing a planet as it rotates. Also suppose that this observer is within the plane of the planet's equator. A point on the Equator that passes directly in front of this observer later in time has a higher planetographic longitude than a point that did so earlier in time. However,
planetocentric longitude is always measured positively to the east, regardless of which way the planet rotates.
East is defined as the counterclockwise direction around the planet, as seen from above its north pole, and the north pole is whichever pole more closely aligns with the Earth's north pole. Longitudes traditionally have been written using "E" or "W" instead of "+" or "−" to indicate this polarity. For example, −91°, 91°W, +269° and 269°E all mean the same thing. For
Mercury, the only other planet with a solid surface visible from Earth, a thermocentric coordinate is used: the prime meridian runs through the point on the equator where the planet is hottest (due to the planet's rotation and orbit, the Sun briefly
retrogrades at noon at this point during
perihelion, giving it more sunlight). By convention, this meridian is defined as exactly twenty degrees of longitude east of
Hun Kal.
Tidally-locked bodies have a natural reference longitude passing through the point nearest to their parent body: 0° the center of the primary-facing hemisphere, 90° the center of the leading hemisphere, 180° the center of the anti-primary hemisphere, and 270° the center of the trailing hemisphere. However,
libration due to non-circular orbits or axial tilts causes this point to move around any fixed point on the celestial body like an
analemma. ==Latitude==