ff. tellite has been known to pafs over the difk at ne time as a dark spot, and at another fo lumious that it could not be diftinguished from Juiter himself, except at its coming on and going (115.) To account for this phænomenon, we uft fay that either the fpots are fubject to change; r, if they be permanent like thofe of our moon, hat the fatellites at different times turn different arts of their globes towards us. Poffibly both hefe caufes may contribute to produce the phæomena juft mentioned. For thefe reafons alfo oth the light and apparent magnitude of the faellites are variable: for the fewer fpots there re upon that fide which is turned towards us, he brighter it will appear; and as the bright fide mly can be feen, a fatellite muft appear larger he more of its bright fide it turns towards the earth, and the lefs fo the more it happens to be covered with fpots. The fourth fatellite, though generally the fmalleft, fometimes appears bigger than any of the reft: the third fometimes feems leaft, though ufually the largeft; nay, a fatellite may be fo covered with fpots as to appear lefs than its fhadow paffing over the difk of the primary, though we are certain that the fhadow muft be fmaller than the body which cafts it. To a fpectator placed on the furface of Jupiter, each of thefe fatellites would put on the various appearances of the moon; but they appear to us always round, having conftantly their enlightened half turned towards the earth. (116) When thefe moons pafs through their inferior femicircles, they caft a fhadow upon Jupiter and thus caufe an eclipfe of the fun to his inhabitants; and in fome fituations this fhadow may be obferved going before or following the fatellite. On the other hand, in paffing through their fuperior femicircles, the fatellites may be eclipfed in the fame manner as our moon is to us, by paffing through the fhadow of Jupiter; and this is actually the cafe with the firft, fecond, and third of thefe bodies; but the fourth, by reafon of the largenefs of its orbit, paffes fometimes above or below the fhadow, as is the cafe with our moon. The beginnings and endings of thefe eclipfes are eafily feen by a telescope when the earth is in a proper fituation with regard to Jupiter and the fun; but when this or any other planet is in conjunction with the fun, the fuperior brightnefs of that luminary renders both it and the fatellites invilible. From the time of its first appearing after a conjunction until near the oppofition, only the immerfions of the fatellites into his fhadow, or the beginnings of the eclipfes are visible; at the oppofition, only the occultations of the fatellites, by going behind or coming before their primary, are obfervable; and from the oppofition to the conjunction, only the immerfions, or end of the eclipfes are to be feen. (117) This is exactly true in the firft fatellite, of which we can never fee an immersion with its immediately fubfequent emersion: and it is but rarely that they can be both feen in the fecond; as in order to their being fo, that fatellite muit be near one of its limits, at the fame time that the planet is near its perihelion and quadrature with the fun. With regard to the third, when Jupiter is more than 46° from conjunction with, or oppofition to, the fun, both its, immerfions and im mediately fubfequent emerfions are visible; ap they likewife are in the fourth, when the distance of Jupiter from conjunction or oppofition is 24°. (118.) When Jupiter is in quadrature with the fun, the earth is fartheft out of the line that paffes through the centres of the fun and Jupiter, and therefore the fhadow of the planet is then moft expofed to our view: but even then the body of the planet will hide from us one fide of that part of the fhadow which is neareft to it, through which the first fatellite pafies; which is the reafon that, though we fee the entrance of that fatellite into the fhadow, or its coming out from thence, as the earth is fituate on the east or weft fide thereof, we cannot fee them both; whereas the other fatellites going through the fhadow at a greater diftance from Jupiter, their ingrefs and egrefs are both vifible. The relative diftances of thefe moons from their primary, are fhewn in Plate XV. fig. 11. (119) SATURN, when viewed through a good telescope, makes a more remarkable appearance than any of the other planets. Galileo firft difcovered his uncommon thape, and from the difcoveries made by him and other aftronomers, it appears that this planet is furrounded by a broad thin ring, the edge of which reflects little or none of the fun's light to us, but the planes of the ring reflect the light in the fame manner that the planet itself does; and if we fuppofe the diameter of Saturn to be divided into 3 equal parts, the dia meter of the ring is about 7 of thefe parts. The ring is detached from the body of Saturn in fuch a manner, that the diflance between the innermoft part of the ring and the body is equal to its breadth. If we had a view of the planet and his ring with our eyes, perpendicular to one of the planes of the latter, we should fee them as in Plate XV. fig. A.; but our eye is never fo much elevated above either plane as to have the vifual ray ftand at right angles to it, nor indeed is it ever elevated more than about 30° above it; fo that the ring being commonly viewed at an oblique angle, appears of an oval form, and though very good telescopes double, as reprefented Plate XV. fig. 11. and fig. B. (120.) Both the outward and inward rim are projected into an ellipfes, more or lefs oblong according to the different degrees of obliquity with which it is viewed. Sometimes our eye is in the plane of the ring, and then it becomes invifible; either because the outward edge is not fitted to reflect the fun's light, or more probably because it is too thin to be feen at fuch a distance. As the plane of this ring keeps always parallel to itself, that is, its fituation in one part of the orbit is always parallel to that in any other part, it difappears twice in every evolution of the planet, that is about once in 15 years; and he fometimes appears quite round for months together. At other times, the distance betwixt the body of the pianet and the ring is very perceptible; and Mr Whitton tells us, that Dr Clarke's father faw a ftar through the opening. (21) When Saturn appears round, if our eye be in the plane of the ring, it will appear 28 I Distances in Periodic times Semi-t Miles. Angles of Orbs. Id 21h 18' 27" 170,000 2 2 17 41 22 217,000 I 3 PARTI. as a dark line acrofs the middle of the planet's of these fatellites expreffed in femidiameters of disk; and if our eye be elevated above the that planet, and in English miles are as follow: plane of the ring, a fhadowy belt will be vifible, caufed by the fhadow of the ring as well as by the interpofition of part of it betwixt the eye and the planet. The fhadow of the ring is broadeft when the fun is moft elevated, but its obfcure parts appear broadest when our eye is moft elevated above the plane of it. When it appears double, the ring next the body of the planet, appears brighteft; when the ring appears of an elliptical form, the parts about the ends of the largest axis are called the anfe. Thefe, a little before and after the disappearing of the ring, are of unequal magnitude: the largest anfa is longer vifible before the planet's round phafe, and appears again fooner than the other. In the diagram, Plate XV. fig. 1. are delineated the phases of the ring from its full appearance in 1782 to its difappearance in 1789, and its full re-appearance in 1796. The th (127.) The four first describe ellipfes like thole of the ring and are in the fame plane: their inci nation to the orbit is from 30° to 31°. defcribes an orbit inclined from 17° to 18 to the orbit of Saturn, his plane lying between the eclip tic and thofe of the other fatellites. Dr Herfche obferves that the 5th fatellite turns round its axis once, exactly in the time in which it rerolves round the planet Saturn. In this refpect it refenbles our moon, which does the fame thing. The proportional diftances of the 5 fatellites formerly known to aftronomers are fhewn in Plate XV. fg. (122.) Dr HERSCHEL has found that the ring is double or that there are two concentric rings; alfo that it has a motion of rotation in its own plane, its axis of motion being the fame as that of Saturn himself, and its periodical time rob 32′ 15," 4: But he thinks it probable that the concentric rings may not revolve in the fame period.-11. Their dimenfions, and the space between them.(128.) The GEORGIUM SIDUS or HERSCHEL, he ftates in the following proportion to each other: Inner diameter of the fame Ring Inner diam. of the larger Ring 1 Breadth of the outer ditto being more remote than Saturn, is alfo lef's known. Its apparent magnitude is fo fmall, that it ca miles. feldom be feen by the naked eye, and even when 146345 viewed by means of the telescope, it appears only 184393 a few feconds in diameter. This planet was di 190248 covered by Dr Herfchel in 1781; and in 1:37 be 204883 likewife difcovered two fatellites that revolve 20000 round it. They are probably not lefs than Jupter's moons, and their orbits are nearly perpenci cular to the ecliptic. This is a circumftance ra which they differ from the attendants of all the other planets. 200 2839 (123.) Dr Herfchel concludes, from his obfer vations on the Ring, that its ftructure is fuch, as to allow it to remain permanently in its prefent ftate; nor does he think it at all probable that the ring is of that changeable nature which fome perfons have imagined. (124.) The fame excellent aftronomer from a feries of obfervations on the belts of Saturn has concluded that he revolves upon his axis in rob 16' o," 4, that he has a denfe atmosphere, and that his polar diameter is to his equatorial one as 10 to II. (125.) Saturn has befides his ring feven little fecondary planets or fatellites revolving round him. One of them, which till lately was reckoned the 4th in order from Saturn, was difcovered by Huygens in 1655 by means of a telescope roo feet long; and the others, viz. the 1ft, 2d, 3d, and 5th, at different times by Caffini, between 1671 and 1684, by the help of glaffes of 100 and 136 feet. The 6th and 7th have lately been difcovered by Herfchel with his 40 feet reflecting telescope in 1787 and 1788. These he has called the 6th and 7th fatellites, though they are nearer to Saturn than the other 5; that the names may not be miftaken with regard to former obfervations of them. (126.) The periodical revolutions and diftances (130.) The COMETS, viewed through a telefcope, have a very different appearance from aj of the planets. The nucleus, or ftar, seems much dimmer. They are to appearance farrounded with atmospheres of a prodigious fize, often re fing ten times higher than the nucleus, and have often likewise different phases, like the moon. (131. The head of a comet, feen through good telescope, appears to confiíl of a fold globe, and an atmosphere, that furrounds it. The feed part is frequently called the sucleus; which through a telescope, is eafily diftinguished from the atmosphere or hairy appearance. (132.) A comet is generally attended with a blaze or tail, whereby it is diftinguiled fri” 1 ftar or planet; as it is alfo by its motion. times the tail only of a comet has been vilibu, at |