The Description and Use of the Globes and the Orrery To Which is Prefix'd, by Way of Introduction, a Brief Account of the Solar System

Of the Order and Periods of the Primary Planets revolving about the Sun; and of the Secondary Planets round their respective Primaries. The Sun is placed in the midst of an immense space, wherein six opaque spherical bodies revolve about him as their center. These wandering globes are called the Planets, who, at different distances, and in different periods, perform their revolutions from West to East, in the following order:

1. ☿ Mercury is nearest to the Sun of all the planets, and performs its course in about three months. 2. ♀ Venus in about seven months and a half. 3. ♁ The Earth in a year. 4. ♂ Mars in about two years. 5. ♃ Jupiter in twelve. And lastly, ♄ Saturn, whose Orbit includes all the rest, spends almost 30 years in one revolution round the Sun. The distances of the Planets from the Sun are nearly in the same proportion. viz. Supposing the distance of the Earth from the Sun to be divided into 10 equal parts; that of Mercury will be about 4 of these parts; of Venus 7; of Mars 15; ofJupiter 52; and that of Saturn 95. 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.

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’sare 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.