PAR T II.

The SOLAR SYSTEM, and the Ufe of the ORRERY.

CHAPTER I.

A general View of the Universe.

HE heavenly Bodies, which are vifible from the Earth; are Sun,

T Moon, and Stars.

The Stars are of two Sorts. One Sort (of which there is by far the greater Number) are called fixed Stars; because they always appear in the fame Places, or at the fame Distance, from one another. The other Sort of Stars are called Planets, or wandering Stars; because they are perpetually changing their Places and Distances, both with regard to the fixed Stars, and one another.

་

The

The Planets may eafily be diftinguished from the fixed Stars, by their not twinkling, as the fixed Stars do, excepting those of them which are vertical, or nearly fo*. And generally the Planets are the Stars which appear fooneft in the Evening, and are latest ere they disappear in the Morning; therefore it is one of the Planets that is the Morning or Evening Star.

The fixed Stars are at fuch immeasurable Distances from the Earth, that we know but very little of them. The Sun, Moon, and Planets, are much nearer to us, and are better known.

The Sun is a great burning Globe, or fiery Ball, whose Diameter is computed to be 763,000 Miles. It always remains immovable in the vaft Expanfe, except that it is obferved to turn round its own Axis in about twenty-five Days and a half.

Many dark Spots, of various irregular Figures, may commonly be feen, with Telescopes, in the Surface of the Sun; by means of which his Rotation on his Axis has been difcovered, and the Time of it ascertained. Thefe Spots are not very permanent, but fome of them disappear again in a few Days after their firft Appearance. Others of them have been obferved to continue during four or five Rotations of the Sun's Body,

* Which the Planets cannot be in our Latitude.

The

The Planets are round opake Bodies, which have no Light of their own, but reflect the Light of the Sun.

The Planets, and the Earth (which is truly a Planet) are continually moving round the Sun in Circles, or rather in Ellipfes, or Ovals, of different Bigneffes, in the following Order.

1. Mercury is nearest the Sun, and performs his Revolution in about three Months.

Fig. II.

N. B. This Planet is always fo near the Sun that it is fellom feen.

2. Venus is next, and revolves in about feven Months and a half.

3.

4.

The Earth in a Year.

Mars in about two Years.
5. 4 Jupiter in about twelve Years.
6. Saturn in about thirty Years.

The Space of Time in which each Planet revolves round the Sun, is also called that Planet's Year. Thus Saturn's Year is equal to about Thirty of ours.

These Six are the primary Planets. Some of them have secondary Planets, or Satellites, or Moons, moving continually round them. As the Moon round the Earth. Jupiter has four Moons, Saturn has five.

The Planets are of different Magnitudes; fome are bigger than the Earth, others are lefs. Mercury is feven Times lefs than the Earth; Venus nearly equal to the Earth

Mars

Mars fix times lefs; Jupiter one thousand seven hundred and twenty-eight times bigger; Saturn fix hundred times bigger; the Moon is fixty times lefs than the Earth *.

A Machine which represents the Motions of the Planets, is called an ORRERY. And it will very well answer the purpose, if it reprefents only the Motions of two primary Planets, for Inftance, the Earth and Venus; and of one secondary Planet, viz. the Moon: for as the others move in the fame Manner, a fufficient Notion may be formed by these, of the Motions of all the reft.

The proportionable Magnitudes, and respective Distances of thefe Bodies to one another,

*The Syftem of the World here described, is called the Pythagorean or Copernican Syftem; as having been anciently taught by the wife Samian Philofopher Pythagoras, and revived, after it had been in a manner loft, by the famous Polish Philofopher, Nicholas Copernicus, who was born at Thorn, in the Year 1473.

The most famous of the antiquated, and now juftly exploded Syftems, are the Ptolemean and the Tychonian.

The PTOLEMEAN Syftem (held long before by Ariftotle and Hipparchus) was taught by Ptolomy an Egyptian Astronomer, who is faid to have lived 138 Years before Christ. He fuppofed the Earth to be immovably fixed in the Center of the Universe, and the seven Planets, viz. the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn; and, above them all, the Firmament of fixed Stars, to be conftantly revolving round the Earth in twenty-four Hours from East to Weft.

The TYCHONIAN Syftem was taught by Tycho Brahe, a noble Dane, who was born A. D. 1546. It fuppofes the Earth to be fixed and immovable, as the Ptolemean Syftem does; and that all the Stars and Planets revolve

round

another, are not to be conveniently expreffed in an Orrery. For fuppofe a Ball of one Inch Diameter to reprefent the Earth, then the Ball that reprefents the Sun fhould, in true Proportion, be very near eight Feet Diameter, (the Sun being 885,736 times bigger than the Earth) and the Distance of the Earth from the Sun fhould be about 284 Yards, which is more than half a Quarter of a Mile.

round the Earth in twenty-four Hours: but it differs from the Ptolemean in that it not only allows a menftrual Motion to the Moon round the Earth, as the Center of its Orbit, but it makes the Sun to be the Center of the Orbits of Mercury, Venus, Mars, Jupiter, and Saturn, in which they are carried round the Sun in their refpective Years, as the Sun is round the Earth in a Solar Year: and all these Planets, together with the Sun, are fuppofed to be whirled round the Earth in twenty-four Hours.

This Hypothefis is fo embarraffed and perplexed, that but few Perfons embraced it. It was afterwards refined upon by Longomontanus, and fome others; who allowed the diurnal Motion of the Earth on its own Axis, but denied its annual Motion round the Sun. This Hypothesis, partly true and partly falfe, is called the Semi-Tychonian System. However, the Pythagorean Syftem has generally been received by the greatest Mathematicians and Philofophers, ever fince the Revival of it by Copernicus: and it has been at length established on fuch a folid Foundation of mathematical and phyfical Demonstration, by the great Sir Isaac Newton, as puts it out of all Danger of being ever overthrown by any new contrived Syftem, fo long as the Sun and Moon fhall endure.

СНАР.