about as ftrong as that with which a coal would be seen to glow in faint daylight.

These are the chief phenomena obfervable in the Moon. All the other fatellites are of a fimilar nature to this; but from their great distance from the Earth, we are unable to be fo particular in our defcription of them.

Though it be afferted, that the Moon and the other fecondary planets revolve round the primary planets as their centres, and the primary planets revolve round the Sun for their centre, yet it must be remembered, that this affertion is not the real mathematical truth. For the primary pla nets do not regard the Sun as their exact centre; but each primary planet, and the Sun, revolve round their com.non centre of gravity. Which common centre of gravity is that point where the two bodies, or the Sun and planet, will equiponderate each other. Thus, the centre of gravity in a common balance beam, or steel-yard, is the point of fufpenfion.

To discover the common centre of gravity of two bodies, is to find that point, whofe diftance from the greater body is lefs than its diftance from the leaft body, in the fame proportion as the gravity of the lefs body is lefs than that of the greater; and in two bodies of equal gravities, their common centre of gravity is equally distant from their two

Thus, if B be a body four times as great in magnitude as the body 1, and both be fuppofed to be connected by an inflexible wire B1, the common centre of gravity of the two bodies will be at the point g, which is four times nearer to B than to 1; or as B g is to g 1, fo is 1 to B.

Therefore, the common centre of gravity of the Earth and Sun is nearer to the centre of the Sun than to that

of

of the Earth, by as great a ratio as the quantity of matter in the Sun exceeds that in the Earth, which centre of gravity is in the body of the Sun. The common centre of gravity of Jupiter and the Sun is alfo within the body of the Sun, though very near its fuperficies.

The Sun is not acted upon by our Earth only, fo as to turn round the common centre of gravity of that and the Earth, without having regard to the other planets; but there is a common centre of gravity between the Sun and each of the primary planets; and each of these planets has its effect in caufing the Sun to turn round their centres of gravity.

As the centre of gravity of Jupiter and the Sun is the fartheft distant from the Sun's centre, owing to the great fize of Jupiter, and its diftance from the Sun, and as this centre is within the body of the Sun, it follows that the Sun is never removed above one of its own diameters out of its place.

Each of the fecondary planets, and its primary planet also, turn round their common centre of gravity.

The figures 1, 2, 3, 4, 5, 6, 7, and 8, fhew the true proportions of the planets Mercury, Venus, Mars, the Earth, Jupiter, Saturn, Georgium Sidus, and the Moon.

Jupiter has four fatellites; the times of whofe periods and distances have been noticed in the aft Section. Their periods were found from their conjunction with Jupiter, after the fame manner as the periods of the primary planets were discovered. Their diftances from the body of Jupiter are measured by a micrometer, and computed in femidiameters of Jupiter, and then reduced into miles.

The fatellites of Jupiter are of very great ufe in aftronomy; for, by an obfervation of the ecliptes of these fatel lites, we derive three great advantages :- - Ift, in determining the diftance of Jupiter from the Earth; adly, we find the progreffive motion of light: for by thefe eclipfes, it is evident that light does not come to us from Jupiter

Gg a

in

in an inftant; for if the motion of light were inftantaneous, it is evident we should fee the commencement of the eclipfes of the fatellites at the fame moment they really happen, whatever distance they might be from us; but, on the contrary, if light have a progreffive motion, it is evident, the farther we are from a planet, the later we should be in seeing the beginning of its eclipfe; and fo it is found to happen: the fatellites of Jupiter appear to be eclipfed later than the true computed time, and always proportionably later, as the Earth is removed farther from the planet. When the Earth and Jupiter are nearest to each other, that is, when they are both in conjunction, on the fame fide of the Sun, then the eclipfes of Jupiter's fatellites are feen to happen fooner than when the Sun is directly between Jupiter and the Earth; in which laft cafe, the distance of Jupiter from the Earth is greater than it is in the former cafe, by the whole diameter of the Earth's annual orbit, or by double the Earth's diftance from the Sun: in this laft cafe, we cannot obferve an eclipfe of Jupiter's fatellites, till near a quarter of an hour after the time we could have discovered it in the former cafe, that is, when Jupiter was at his least distance from the Earth. From hence it follows, that the light is near a quarter of an hour in pafling through a space equal to the diameter of the Earth's orbit, or near eight minutes in palling from the Sun to the Earth; which is at the rate of about twelve million of miles in a minute.

But the third and greatest advantage derived from the ob fervation of these eclipfes, is the difcovery of the longitude of different places on the Earth: for having the difference of time between two obfervations of the fame eclipfe, taken in two different places, we have the difference of longitude between the two places. For example, fuppofe there be two obfervers of an eclipfe, the one at London, the other at Barbadoes, the eclipfe will appear at the fame moment of real time to each perfon; but being under different meridians, the hour afdhe day will be different at each place; thus, if it be 12

o'clock at noon at London, it will be 8 o'clock in the morning at Barbadoes, by which the obfervers find the difference of longitude between the two places to be fixty degrees, or four hours in time.

The planet Saturn has feven fatellites, the fixth and feventh of which were difcovered by Dr. Herfchel, in the years 1787 and 1788. Their periods, diftances, &c. have been described in the last Section.

The Georgian planét, or Herschel, is alfo found to have fix fatellites revolving round him, like thofe of Jupiter and Saturn. Thefe fatellites were discovered by Dr. Herschel, for which fee the laft Section.

Thefe are the only primary planets which we are certain are attended by fatellites. Some aftronomers have imagined they discovered a fatellite belonging to Venus; but the many repeated obfervations which have been made by others to obferve it, and without effect, leave us room to fufpe&t they were deceived.

SECT. III.

OF THE FIXED STARS.

THE fixed stars, generally denominated fars by way of eminence, are thofe heavenly bodies which ufually keep the fame distance with regard to each other. All the heavenly bodies, except the primary and fecondary planets, and comets, are of this class.

The

The distances of the fixed stars are so great, that we have no distance in the planetary system with which we can coinpare them; for the diameter of the Earth's orbit, which is nearly 190,000,000 miles, bears no fenfible proportion to the distance of the nearest fixed ftar from the Earth.

The diftances of the fixed ftars have been the fubje&t of investigation to several astronomers. Various methods have been devised for the purpose of discovering the diftances of thefe heavenly bodies, but without fuccefs, on account of their almost infinite distance; the most accurate obfervations only give us a distant approximation; but by the best obfervations, however, we can fafely conclude that the nearest fixed star is upwards of forty thousand diameters of the Earth's orbit diftant from us, or eighty thousand times farther diftant from us than the Sun is.

The magnitudes of the fixed stars appear very different in different ftars, owing, in fome fort, to their real magnitudes, which are different, but principally owing to their different distances from us.

The stars are generally divided, according to their apparent magnitudes, into fix, and by fome into eight claffes. The first class, called ftars of the first magnitude, are those that appear largeft: ftars of the fecond magnitude appear fomewhat lefs; and thus every following clafs comprehends thofe ftars next in fize to the former clafs: the ftars of the fixth magnitude containing the fmalleft ftars vifible to the naked eye. All others that cannot be perceived, but by the help of telescopes, are called telescopic stars. It is not to be inferred from hence that Nature has divided the stars into thofe claffes; for there are almost as many claffes as there are ftars, fo great is their variety of magnitude and brightnefs!

The number of stars is also very great, and appears to be almost infinite; but aftronomers have deduced all that are visible to the naked eye into catalogues. Mr. Flamstead reduced 3000 stars into a catalogue, which contains all that are visible to the naked eye at any time of the year, and a

great