Astronomy Answers
From the Astronomical Dictionary

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the coordinate system

co- = [Latin] together, ordinatio = [Latin] ordering, arrangement

A coordinate system is a tool that allows fixing of positions by measuring distances in different directions. Those distances are the coordinates. There are two often-used classes of coordinate systems: those that use rectangular (or cartesian) coordinates, and those that use polar coordinates. Polar coordinates use angles (for the direction) and one distance, and usually have a base plane that the angles are tied to. Rectangular coordinates use only distances, measured in mutually orthogonal directions from a common origin. Coordinate systems are often named for the thing in or on which they measure positions, or for the base plane or origin with which they are associated.

Here are a number of polar coordinate systems that are used in astronomy:

Table 1: Coordinate Systems

system	object	base plane	longitude	latitude
geographical	Earth	equator	longitude	latitude
equatorial	sky	equator	right ascension	declination
ecliptic	sky	ecliptic	longitude	latitude
galactic	sky	milky way	longitude	latitude
horizontal	sky	horizon	azimuth	altitude
selenographical	Moon	equator	longitude	latitude
heliographical	Sun	equator	longitude	latitude
arographical	Mars	equator	longitude	latitude
jovigraphical	Jupiter	equator	longitude	latitude

The column marked "system" indicates the name of the system; "object" lists the object to which the coordinates apply; "base plane" shows the plane that has a latitude of 0; "longitude" and "latitude" provide the names of the coordinates that correspond to longitude and latitude.

The names of polar coordinate systems that fix positions on a celestial body are often made up of the Latin or Greek name of the body, followed by graphical, for example geographical for the Earth. The names of the corresponding rectangular coordinate systems (with the center of the body as origin) usually have the same first part, but followed by centric, for example geocentric for the Earth, planetocentric for planets in general, or heliocentric for the Sun.