exhibits the same appearance to the inhabitants of the moon, as the moon does to us. The moon turns about its own axis in the same time that it moves round the earth; it is on this account that she always presents nearly the same face to us: for, by this motion round her axis, she turns just so much of her surface constantly towards us, as by her motion about the earth would be turned from us. This motion about her axis is equable and uniform, but that about the earth is unequal and irregular, as being performed in an ellipses; consequently, the same precise part of the moon's surface can never be shewn constantly to the earth; which is confirmed by a telescope, by which we often ob→ serve a little segment on the eastern and western limb, appear and disappear by turns as if her body librated to and fro; this phenomenon is called the moon's libration. The lunar motions are subject to several other irrégularities, which are fully discussed in the larger works on astronomy. OF THE SATELLITES OF JUPITER, SATURN, AND The existence of all the satellites, except the moon, must have remained unknown, without the assistance of the telescope. By the assistance of this instrument, Jupiter is found to be attended by four, Saturn by seven, and the Georgium Sidus by six. The satellites are distinguished according to their places; into first, second, &c.; the first being that which is nearest the planet. They revolve round their respective primaries in elliptic orbits, the primary planets being in the focus. The planes of the orbits of the secondary planets produced, intersect the heliocentric orbits of their primaries in two opposite points; which are called their nodes. Again, the planes of the orbits of the satellites produced, intersect the ecliptic in two opposite points, these are called the geocentric nodes of the satellites. The orbits of Jupiter's satellites are nearly, but not exactly, in the same plane. This plane produced makes an angle of about 3 with Jupiter's orbit. The second deviates a little from the rest. The orbits of Saturn's satellites, except the 5th, which deviates from the rest several degrees, are nearly in the same plane. They are nearly parallel to the plane of the equator. The orbit of the 5th satellite makes an angle with the orbits of its primary of 13' 8'. The system of Jupiter and his satellites is very large in itself; yet, on account of its immense distance from us, it appears to occupy but a small space in the sphere of the starry heavens; and, consequently, every satellite of Jupiter appears to us always near its primary, and to have an oscillatory motion, like that of a pendulum; going alternately from its. greatest digression on one side the planet, to its greatest on the other; sometimes in a straight line, at others in an elliptic curve. When a satellite is in its superior semicircle, or that half of its orbit that is more distant from the earth, its motion appears direct to us; when a satellite is in its inferior semicircle, nearest to the earth, the apparent motion of it is retrograde. Both these motions seem quickest, when the satellite is nearest the centre of the primary, and slower when they are more distant; at the greatest distance they appear stationary, for a short time. The satellites, and their primaries, mutually eclipse each other, in the same manner in which it has been shewn that the earth and the moon do. But there are three cases in which the satellites disappear to us. The one is, when the satellite is directly beyond the body of its primary, with respect to the earth; this is called an occultation of the planet. Another is, when it is directly behind its primary, with respect to the sun, and so falls into its shadow, and suffers an eclipse, as the moon, when the earth is interposed between that and the sun. The last is, when it is interposed between the earth and its primary; for then it cannot be distinguished from the primary itself. It is not often that a satellite can be discovered upon the disc of Jupiter, even by the best telescopes; excepting at its first entrance, when, by reason of its being more directly illuminated by the rays of the sun, than the planet itself, it appears like a lucid spot upon it; sometimes, however, a satellite is seen passing over the disc like a dark spot; this has been attributed to spots on the surface of the satellite; and, that the more probably, as the satellitehas been known to pass over the disc, at one time as a dark spot, and at another time to be so luminous, as only to be distinguished from the planet at its ingress and egress. The beginning and endings of these eclipses are easily seen by a telescope, when the planet is in a proper situation; but when it is in conjunction with the sun, the brightness of that luminary renders both the planet and satellite invisible. By observing the eclipses of Jupiter's satellites, it was discovered, that light is not propagated instantaneously, though it moves with an incredible velocity; so that light reaches from the sun to us in the space of eleven minutes of time, at more than the rate of 100,000 miles in a second. The orbits of all the satellites of Saturn, except the fifth, are nearly in the same plane, which plane makes an angle with that of Saturn's orbit, of about 31°; this inclination is so great, that they cannot pass either across Saturn or behind it, with respect to the earth, except when they are very near their nodes; so that their eclipses are not near so frequent as those of Jupiter. An occultation of the fourth, behind the body of Saturn, has been observed; and Cassini once saw a star covered by the fourth satellite, so that for 13 minutes they appeared as one. OF ECLIPSES. Those phenomena, that are termed eclipses, were, in former ages, beheld with terror and amazement, and looked upon as prodigies that portended calamity and misery to mankind. These fears, and the erronious opinions which produced them, had their source in the hieroglyphical language of the first inhabitants of the earth. We do not, however, imagine that even the most ancient of these knew any more of the laws and motions of the heavenly bodies, than what could be discovered from immediate sight; or, that they knew enough of the lunar system to calculate an eclipse, or even that they ever attempted it. The word eclipse is derived from the Greek, and signifies dereliction, a fainting away or swooning. Now, as the moon falls into the shadow of the earth, and is deprived of the sun's enlivening rays, at the time of her greatest brightness, and even appears pale and languid before her obscuration; lunar eclipses were called lunæ labores, the struggles or labours of the moon; to relieve her from these imagined distresses, superstition adopted methods as impotent as they were absurd. When the moon, by passing between us and the sun, deprived the earth of its light and heat, the sun was thought to turn away his face, as if in abhorrence of the crimes of mankind, and to threaten everlasting |