The earth is one of seventeen planets which revolve about the sun in elliptical orbits: of these, twelve have been discovered since the year 1787.[3] Mercury and Venus are nearer the sun than the earth, the others are more remote. The earth revolves at a mean distance of 95,000,000 miles from the sun’s centre, in a civil year of 365 days 5 hours 48 minutes 49·7 seconds, at the same time that it rotates in 24 hours about an axis which always remains parallel to itself, and inclined at an angle of 23° 27ʹ 34ʺ·69 to the plane of the ecliptic; consequently, the days and nights are of equal length at the equator, from whence their length progressively differs more and more as the latitude increases, till at each pole alternately there is perpetual day for six months, and a night of the same duration: thus the light and heat are very unequally distributed, and both are modified by the atmosphere by which the earth is encompassed to the height of about forty miles. With regard to magnitude, Mars, Jupiter, Saturn, Uranus, and Neptune are larger than the earth, the rest are smaller, but even the largest is incomparably inferior to the sun in size: his mass is 354,936 times greater than that of the earth, but the earth is nearly four times as dense. Though the planets disturb the earth in its motion, their form has no effect on account of their great distance; but it is otherwise with regard to the moon, which revolves about the earth at a mean distance of 240,000 miles, and is therefore so near that the form of both bodies causes mutual disturbances in their respective motions. The perturbations in the moon’s motions from that cause, compared with the same computed from theory, show that the earth is not a perfect sphere, but that it bulges at the equator, and is flattened at the poles: it even gives a value of the compression or flattening.[4] Again, theory shows that, if the earth were throughout of the same density, it would be much less flat at the poles than the moon’s motions show it to be, but that it would be very nearly the same were the earth to increase regularly in density from the surface to its centre; and thus the lunar motions not only make known the form, but reveal the internal structure of the globe. Actual measurement has proved the truth of these results. The courses of the great rivers, which are generally navigable to a considerable extent, show that the curvature of the land differs but little from that of the ocean; and as the heights of the mountains and continents are inconsiderable when compared with the magnitude of the earth, its figure is understood to be determined by a surface at every point perpendicular to the direction of gravitation, or of the plumb-line, and is the same which the sea would have if it were continued all round the earth beneath the continents. Such is the figure that has been measured in various parts of the globe.