9.5 Planetographic Coordinates
Appearance returns the central longitude and latitude of the point at the apparent center of an object. The longitude and latitude are given relative to a specific coordinate system on the surface of the object. Similarly, the Features option, available to PlanetPlot, PlanetPlot3D, and certain other functions, uses longitude and latitude relative to the same coordinate system. A uniform coordinate system is used throughout Scientific Astronomer for specifying longitude and latitude on the surface of an object.
Geographic coordinates are used on the surface of the Earth. An equivalent system called planetographic coordinates is used for other planets. Alternative titles, such as areographic coordinates on Mars, selenographic coordinates on the Moon, and heliographic coordinates on the Sun, are sometimes used to refer to the equivalent systems.
The definition of planetographic coordinates requires a scheme for assigning longitude and latitude lines.
For each planet, a system of longitude lines is set up by first defining a prime meridian to act as the origin of longitude. A meridian is any line that begins at one pole of the planet and ends at the other pole. The prime meridian is an arbitrarily chosen meridian, typically passing through a prominent feature of the planet, such as a crater. In the case of the Earth, the prime meridian passes through the original Royal observatory at Greenwich, England. Once a zero of longitude is specified, by convention positive longitude is taken to be in the direction of rotation of the planet. That direction also corresponds to the direction east. There are 360 degrees of longitude lines wrapping around a planet.
The other component of planetographic coordinates is latitude. Latitude lines are circles centered on the axis of a planet. The zero of latitude is at the equator, and 90 degrees is at the north pole. The north pole is distinguished from the south by the fact that when viewed from above, the planet's rotation is counterclockwise.
In summary, planetographic coordinates are a system of coordinates created by the setting up of longitudinal and latitudinal lines on planets. A prime meridian is defined by the position of some prominent feature; positive longitude is in the direction of rotation, and positive latitude is in the direction of the north pole.
Specifics of Planetographic Coordinates
On the Moon, selenographic coordinates are used. The prime meridian passes through the mean center of the lunar disk as it faces the Earth. Positive longitude is toward Mare Crisium and positive latitude is toward Mare Serenitatis.
Heliographic coordinates are used on the Sun. The prime meridian passes through the center of the solar disk as seen from the Earth on the date 1853 November 9.9. This was the beginning of solar Synodic Rotation Number 1. By convention, the solar sidereal rotation period is taken to be exactly 25.38 days. Based on this value, the mean synodic period between rotation numbers is 27.2752316 days. The central latitude reaches a maximum of +7.25 degrees on September 9 and a minimum of -7.25 degrees on March 6 of each year. The central latitude is zero on June 6 and December 7 of each year.
On Mercury, the prime meridian is defined to be 20 degrees away from the crater Hun Kal (a name that means 20 in the Mayan language). Positive longitude is east of Hun Kal and positive latitude is toward Planitia Borealis.
The prime meridian on Venus passes through the crater Eve in Alpha Regio. Positive longitude is toward Thetis Regio and positive latitude is toward the Maxwell Mountains.
On the Earth, geographic coordinates are used. As stated earlier, the prime meridian passes through Greenwich. Positive longitude is toward Asia and positive latitude is toward the Arctic.
Areographic coordinates are used on Mars. The prime meridian passes through the crater Airy-O. Positive longitude is toward Syrtis Major and positive latitude is toward Acidalia.
Jupiter has several coordinate systems because different latitudes rotate at different rates. Scientific Astronomer uses System II coordinates, based on the mean atmospheric rotation of the north and south equatorial belts. Positive latitude is in the opposite hemisphere away from the Great Red Spot.
Saturn, Uranus, and Neptune also have several coordinate systems. This package uses the System III coordinates, based on the rotating magnetic field.
In the case of the Galilean moons, Io, Europa, Ganymede, and Callisto, a coordinate system similar to selenographic coordinates on the Moon is used. The prime meridian passes through the mean center of the moon's disk as it faces Jupiter.