The inclinations of the orbits of the Planets to the plane of the ecliptic, are as follows, _viz._ the orbit of _Mercury_ makes an angle with it of almost 7 degrees; that of _Venus_ something above 3⅓ degrees; of _Mars_ a little less than 2 degrees; of _Jupiter_, 1⅓ degree; and of _Saturn_, about 2½ degrees. The orbits of the Planets are not circles, but ellipses or ovals. What an ellipsis is, may be easily understood from the following description. Imagine two small pegs fixed upright on any plane, and suppose them tied with the ends of a thread somewhat longer than their distance from one another: Now if a pin be placed in the double of the thread and turned quite round (always stretching the thread with the same force) the curved described by this motion is an _Ellipsis_. The two points where the pegs stood, (about which the thread was turned) are called the _foci_ of that ellipsis; and if, without changing the length of the thread, we alter the position of the pegs, we shall then have an ellipsis of a different kind from the former; and the nearer the _focus’s_ are together, the nearer will the curve described be to a circle; until at last, the two _focus’s_ coincide, and then the pin in the doubling of the thread will describe a perfect circle. The orbits of all the Planets have the Sun in one of their _focus’s_, and half the distance between the two _focus’s_ is called the _Excentricity_ of the orbits. This excentricity is different in all the planets, but in most of them so small, that in little schemes or instruments, made to represent the planetary orbits, it need not be considered.
The six Planets above-mentioned, are called _Primaries_, or _Primary Planets_; but besides these, there are ten other lesser Planets, which are called _Secondaries_, _Moons_, or _Satellites_. These moons always accompany their respective primaries, and perform their Revolutions round them, whilst both together are also carried round the Sun. Of the six Primary Planets, there are but three, as far as observation can assure us, that have these attendants, _viz._ the _Earth_, _Jupiter_, and _Saturn_.
The Earth is attended by the _Moon_, who performs her revolution in about 27⅓ Days, at the distance of about 30 Diameters of the Earth from it; and once a Year is carried round the Sun along with the Earth.