The planets are divided into “primary” and “secondary.” By a “primary” planet we mean one which directly circulates round the Sun; by a “secondary” planet we mean one which in the first instance circulates round a primary planet, and therefore only in a secondary sense circulates round the Sun. The planets are also “major” or “minor”; this, however, is only a distinction of size.
The secondary planets are usually termed “satellites,” or, very often, in popular language, “moons,” because they own allegiance to their respective primaries just as our Moon—the Moon—does to the Earth. But the use of the term “moon” is inconvenient, and it is better to stick to “satellite.”
There is yet another method of classifying the planets which has its advantages. They are sometimes divided into “inferior” and “superior.” The “inferior” planets are those which travel round the Sun in orbits which are inside the Earth’s orbit; the “superior” planets are those whose orbits are outside the Earth.