ORBIT

, is the path of a planet or comet; being the curve line described by its centre, in its proper motion in the heavens. So the earth's Orbit, is the ecliptic, or the curve it describes in its annual revolution about the sun.

The ancient astronomers made the planets describe circular Orbits, with an uniform velocity. Copernicus himself could not believe they should do otherwise; being unable to disentangle himself entirely from the excentrics and epicycles to which they had recourse, to account for the inequalities in their motions.

But Kepler found, from observations, that the Orbit of the earth, and that of every primary planet, is an ellipsis, having the sun in one of its foci; and that they all move in these ellipses by this law, that a radius drawn from the centre of the sun to the centre of the planet, always describes equal areas in equal times; or, which is the same thing, in unequal times, it describes areas that are proportional to those times. And Newton has since demonstrated, from the nature of universal gravitation, and projectile motion, that the Orbits must of necessity be ellipses, and the motions observe that| law, both of the primary and secondary planets; excepting in so far as their motions and paths are disturbed by their mutual actions upon one another; as the Orbit of the earth by that of the moon; or that of Saturn by the action of Jupiter; &c.

Of these elliptic Orbits, there have been two kinds assigned: the first that of Kepler and Newton, which is the common or conical ellipse; for which Seth Ward, though he himself keeps to it, thinks we might venture to substitute circular Orbits, by using two points, taken at equal distances from the centre, on one of the diameters, as is done in the foci of the ellipsis, and which is called his Circular Hypothesis. The second is that of Cassini, of this nature, viz, that the products of the two lines drawn from the two foci, to any point in the circumference, are everywhere equal to the same constant quantity; whereas, in the common ellipse, it is the sum of those two lines that is always a constant quantity.

The Orbits of the planets are not all in the same plane with the ecliptic, which is the earth's Orbit round the sun, but are variously inclined to it, and to each other: but still the plane of the ecliptic, or carth's Orbit, intersects the plane of the Orbit of every other planet, in a right line which passes through the sun, called the line of the nodes, and the points of intersection of the Orbits themselves are called the nodes.

The mean semidiameters of the several Orbits, or the mean distances of the planets from the sun, with the excentricities of the Orbits, their inclination to the ecliptic, and the places of their nodes, are as in the following table; where the 2d column contains the proportions of semidiameters of the Orbits, the true semidiameter of that of the earth being 95 millions of miles; and the 3d column shews what part of the semidiameters the excentricities are equal to.

The Orbits of the comets are also very excentric ellipses.