Prime Meridian - Other Planetary Bodies

Other Planetary Bodies

As on the Earth, prime meridians must be arbitrarily defined. Often a landmark such as a crater is used, other times a prime meridian is defined by reference to another celestial object, or by magnetic fields. The prime meridians of the following planetographic systems have been defined:

• The prime meridian of the Moon lies directly in the middle of the face of the moon visible from Earth and passes near the crater Bruce.
• The prime meridian of Mars is defined by the crater Airy-0.
• The prime meridian of Venus passes through the central peak in the crater Ariadne.
• Two different heliographic coordinate systems are used on the Sun. The first is the Carrington heliographic coordinate system. In this system, the prime meridian passes through the center of the solar disk as seen from the Earth on 9 November 1853, which is when Richard Christopher Carrington started his observations of sunspots. The second is the Stonyhurst heliographic coordinates system.
• Jupiter has several coordinate systems because its cloud tops—the only part of the planet visible from space—rotate at different rates depending on latitude. It is unknown whether Jupiter has any internal solid surface that would enable a more Earth-like coordinate system. Scientific Astronomer uses System II coordinates, based on the mean atmospheric rotation of the north and south Equatorial belts. System III coordinates use Jupiter's magnetic field.
• Titan, like the Earth's moon, always has the same face towards Saturn, and so that face is 0 longitude.