gated inftantaneously, though it moves with an incredible velocity; fo that light reaches from the fun to us in the space of eleven minutes of time, at more than the rate of 100,000 miles in a fecond. The orbits of all the fatellites of Saturn, except the fifth, are nearly in the fame plane, which plane makes an angle with that of Saturn's orbit, of about 31°; this inclination is fo great, that they cannot pafs either across Saturn or behind it, with respect to the earth, except when they are very near their nodes, fo that their eclipfes are not near fo fréquent as thofe of Jupiter. An occultation of the fourth behind the body of Saturn has been observed, and Caffini once faw a ftar covered by the fourth fatellite, so that for 13 minutes they appeared as one.

They are fo minute as not to be vifible unless the air is exceeding clear; Caffini obferved the fifth fatellite to diminish in fize, as it went through the eastern part of it's orbit, until it became quite invifible, while in the western part it increased in brightnefs, until it arrived at it's greatest splendor. In 1705 it was vifible in all parts of it's orbit, though the fane telefcopes were often used before to difcover it without fuccefs.

The Georgium Sidus is attended by two fatellites.

The following table fhews their general affections.

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SECONDARY

Ring of Saturn 21,000 Miles broad, and 21,000 Miles diftant from Body on all Sides. Thickness of the Ring unknown.

OF THE MOON'S MOTION.

You have feen that four of the primary planets are attended in their revolutions by fecondary planets; we are in like manner attended by the moon, the enlightens our nights, by reflecting the light the receives from the fun, as the other fatellites enlighten the planets to which they feverally belong.

If you imagine the plane of the moon's orbit to be extended to the fphere of the heaven, it would mark therein a great circle, which may be called the moon's apparent orbit; because the moon appears to the inhabitants of the earth to move in that circle, through the twelve figns of the zodiac, in a periodical month. This pofition is illuftrated by fig. 3, pl. 9; let EFGHI be the orbit of the earth, S the fun, a b c d the orbit of the moon, when the earth is at E: let ABCD be a great circle in the fphere of the heaven, in the fame plane with the moon's orbit. The moon, by going round her orbit according to the order of letters, appears to an inhabitant of the earth to go round in the great circle A B C D, according to the order of thofe letters for when the moon is at a, feen from the earth at E, fhe appears at A; when the moon is got to b, fhe appears at B; when to c, the will appear at C; when arrived at d, fhe will appear at D. It is true, when the moon is at b, the vifual line drawn from E, through the moon, terminates in L; as it does in M, when the moon is at d; but the lines L M and D B being parallel, and not farther diftant from each other than the distance of the earth's orbit, are as to fenfe coincident, their distance measured in the sphere of the heaven being infenfible; for the fame reafon, though the earth moves from E to F, in the time that the

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moon

moon goes round her orbit, fo that at the end of a periodical month the moon will be at a, and is feen from the earth at F, in the line F N; the moon will, notwithstanding, appear at A, the lines FN and E A being parallel, and as to sense coincident: in like manner, in whatever part of her orbit the earth is, as at H or I, the moon, by going round in her orbit, will appear to an inhabitant of the earth to go round in the great circle A B C D.

The plane of the moon's orbit produced till it cuts the plane of the ecliptic, makes an angle with it of about 5°: this angle is fometimes more, fometimes less then 5°. The points where the moon's orbit produced cuts the ecliptic, are called the moon's nodes; and her ascending the dragon's head, her descending node the dragon's tail. The moon's nodes have a flow motion of 19° 22' in a year, which carries them round the ecliptic, contrary to the order of the figns, in 19 years.

The line of the moon's node is a line drawn from one node to the other.

The extremities of the line of the nodes are not always directed towards the fame points of the ecliptic, but continually fhift their places from eaft to weft, or contrary to the order of the figns, performing an entire revolution about the earth, in the space of fomething less than nine

teen years.

The moon appears in the ecliptic only when fhe is in one of her nodes: in all other parts of her orbit fhe is either in north or fouth latitude, fometimes nearer to, fometimes further removed from the ecliptic, according as the happens to be more or lefs diftant from the nodes.

When the place, in which the moon appears to an inhabitant of the earth, is the fame with the fun's place, fhe is faid to be in conjunction. When the moon's place is oppofite to the fun's place, the

is faid to be in oppofition. When fhe is a quarter of a circle diftant from the fun, fhe is faid to be in quadrature. Both the conjunction and oppofition of the moon are termed fyzigies.

The common lunar month, or the time that paffes between any new moon and the next that follows it, is called a fynodical month, or a lunation. This month contains 29 days, 12 hours, 44 minutes, 3 feconds.

The moon's motion in her orbit is confidered abfolutely, or with relation to the fun. The moon's motion in her orbit, which is alfo her motion in longitude, is fometimes fwifter, fometimes flower; her mean motion is 13 degrees, 10 minutes, 35 feconds, in a day, which carries her round the zodiac in 27 days, 7 hours, 43 minutes. The time wherein the moon is carried round the zodiac is called a periodical month; or a periodic month is the time in which the moon performs one entire revolution about the earth, from any point in the zodiac to the fame again; and contains 27, days, 7 hours, 43 minutes.

The moon's motion confidered with relation to the fun is called her elongation from the fun. The moon's motion from the fun is the excess of the velocity of the moon's motion, above the velocity of the fun's apparent motion; in the ecliptic, this excefs is fometimes more, fometimes lefs. The moon's mean motion from the fun is 12 degrees, 11 minutes, 26 seconds, in a day, which carries the moon from one conjunction with the fun to another in 29 days, 12 hours, 44 minutes, 3 feconds. The time between any conjunction and the conjunction immedi- . ately following, as before obferved, is called a fynodical month, or a lunation, wherein the moon appears in all her phases.

If the earth had no revolution round the fun, or the fun no apparent motion in the ecliptic,

the