The orbits of the Planets are not all in the same plane, but variously inclined to one another; so that supposing one of them to coincide with the above scheme, the others will have one half above, and the other half below it; intersecting one another in a line passing through the Sun. The plane of the Earth’s orbit is called the _Ecliptic_; and this the astronomers make the standard to which the planes of the other orbits are judged to incline. The right line passing thro’ the Sun, and the common intersection of the plane of the orbit of any planet and the Ecliptic, is called the _Line of the Nodes_ of that planet; and the points themselves, wherein the orbit cuts the Ecliptic are called the _Nodes_. [Sidenote: _Excentricity._] 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. [Sidenote: _Primary Planets._] [Sidenote: _Secondary Planets._] 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_.