all other parts of it's orbit, it has latitude feen from it's primary.

Every circle, whofe plane produced paffes through the eye, appears a ftrait line: every circle viewed obliquely will appear an ellipfis, more or iefs wide, according as the eye is more or lefs elevated above the plane of the circle. The orbit of a fatellite feen from the earth, when it's primary's heliocentric place is in his fatellite's true node, and the earth in it's geocentric node, appears a ftrait line: when the primary is in any other part of his orbit, the fatellite's orbit will appear an ellipfis, whofe fhorteft axis increafes, the further the primary is from the node of the fatellite.

The earth's orbit is fo fmall in comparison of the orbits of Jupiter and Saturn, that whatever part of her orbit the earth is in, when thefe planets are in their fatellites true nodes, their fatellites will appear to defcribe lines very near to ftrait ones.

When a fatellite is in it's fuperior femicircle, i. e. that which is furtheft from the earth, it's geocentric motion is direct: when in it's inferior femicircle, i. e. that nearest our carth, it's geocentric motion is retrograde. Both thefe motions feem quickest, when the fatellite is nearest the center of the primary, and flower when they are more diftant; at the greateft diftance they appear ftationary for a fhort time.

Since the distance of Jupiter and Saturn from our earth is very great, and our eye is never elevated much above the planes of their orbits, every fatellite of Jupiter or Saturn feen from our earth, will appear always near it's primary, and to have an oscillatory motion, fometimes in a ftrait line, fometimes in an elliptic curve, going from it's primary, and returning to it again, first on one fide, and then on the other. The fatellites of Jupiter

or Saturn will fometimes be hid from us by their primary, fometimes pafs between us and their primary, fometimes a fatellite will pass between us and another fatellite.

The fatellites and their primaries mutually eclipfe each other, but there are three cafes in which the fatellites difappear to us.

The one is, when the fatellite is directly behind the body of it's primary, with respect to the earth; this is called an occultation of the planet.

Another is, when it is directly behind it's primary, with refpect to the fun, and fo falls into it's fhadow, and fuffers an eclipfe, as the moon, when the earth is interpofed between that and the fun.

The laft is, when it is interpofed between the earth and it's primary; for then it cannot be distinguished from the primary itself.

It is not often that a fatellite can be difcovered upon the difc of Jupiter, even by the beft telefcopes, excepting at it's first entrance, when by reafon of it's being more directly illuminated by the rays of the fun, than the planet itself, it appears like a lucid fpot upon it; fometimes however a fatellite is feen paffing over the difc like a dark fpot; this has been attributed to fpots on the furface of the fatellite, and that the more probably as the fatellite has been known to pafs over the difc at one time as a dark fpot, and at another time to be fo luminous as only to be diftinguished from the planet at it's ingrefs and egrefs. The beginnings and endings of thefe eclipfes are easily feen by a telescope, when the planet is in a proper fituation; but when it is in conjunction with the fun, the brightnefs of that luminary renders both the planet and the fatellite invifible.

By obferving the eclipfes of Jupiter's fatellites, it was difcovered that light is not propa

gated

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 pass either across Saturn or behind it, with refpect to the earth, except when they are very near their nodes, fo that their eclipfes are not near fo frequent as thofe of Jupiter. An occultation of the fourth behind the body of Saturn has been obferved, and Caffini once faw a ftar covered by the fourth fatellite, fo that for 13 minutes they appeared as one.

They are fo minute as not to be visible 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 brightness, until it arrived at it's greateft fplendor. In 1705 it was vifible in all parts of it's orbit, though the fame telescopes were often ufed 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 his Body on all Sides. Thickness of the Ring unknown.

OF THE MOON'S MOTION.

You have seen 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 she 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; becaufe 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, the appears at A; when the moon is got to b, the appears at B; when to c, fhe will appear at C; when arrived at d, fhe will appear at D. It is true, when the moon is at b, the visual 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 diftance of the earth's orbit, are as to fenfe coincident, their distance measured in the sphere of the heaven being infenfible; for the fame reason, though the earth moves from E to F, in the time that the

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moon