Earth; diminutive as it appears before you in this inftrument, it's real diameter is near 8000 miles; it revolves round the fun in the fpace of 365 days, into which number the brazen ecliptic is divided; this revolution conftitutes our year, while it's revolution round it's axis forms day and night.

The little ball clofe and annexed to the earth, reprefents the Moon, of which I fhall fay nothing at prefent, as there is a part of the inftrument for explaining more particularly her phenomena.

The planet Mars is next in order, being the firft above the earth's orbit; he revolves round the fun in about 686 days; fo that our earth, as you will obferve by the inftrument, goes nearly twice round, while he is performing his revolution; he is fuppofed to move at the rate of 55,783 miles an hour. To this planet our earth and moon will appear like two moons, fometimes half or three quarters illuminated, but never full.

Jupiter, the largest of all the planets, is next beyond Mars; and our earth muft have gone nearly 12 times round the ecliptic for one revoJution of Jupiter; yet fo far is it's path removed from the fun, that to go round it in this fpace of time, it moves at the rate of 30,193 miles an hour. Though larger than the earth, it appears but fmall in the heavens, becaufe, as you know, obje&is decrease in their apparent magnitude in proportion to their real diftance. It is attended with 4 fatellites, here reprefented by thefe 4 balls; they are invifible to the naked eye, but appear beautiful through a telescope.

Saturn, the next planet, is ftill higher in the fyftem, performing it's circuit in about thirty years of our time; fo that in this inftrument it's motion is fcarcely fenfible, while in the heavens it goes at the rate of 22,298 miles an hour. It is accompanied by five fatellites, and a large lumi

nous ring, here reprefented by this ivory circle, and which is one of the moft curious phenomena of

nature.

The Georgium Sidus, or Georgian planet, fo called in compliment to his Majefty King George the III. (the Royal patron and promoter of the arts and fciences) is the feventh planet in our fyftem; it is near twice the diflance of Saturn from the fun, round which it revolves in about 80 years.

To explain, by the planetarium, why the fun, being a fixed body, appears to pass through all the figns of the zodiac in twelve months, or one year. It will few that this phenomenon is occafioned by the annual motion of the earth.

As the general phenomena of the planetary fyftem will be the best understood by an induction of particulars, I fhall remove all the planets but thofe whofe motion I am going to explain; for inftance, I fhall leave only the earth and fun, and place the earth over Libra, and it is plain that the fun will then be transferred by the eye of a fpectator on the earth to Aries, in which fign it will appear at the latter end of March: move the earth on in it's orbit to Capricornus, and the fun will appear at Cancer in June, feeming to have moved from Y to 5, though it has not stirred, the real motion of the earth having caufed the fpectator to transfer the fun to all the intermediate points in the heavens, and thus given it an apparent motion. Continue to move the earth till it arrives at Aries, and the fun will be feen in Libra in the month of September: moving the earth on to Cancer, the vifual ray of the fpectator refers the fun to Capricorn, as it appears in the month of December. Lafily, continue moving the earth,

and

and it will arrive at Aries, where we fet out. Thus I have fhewn, that it is the motion of the earth which caufes the fun to appear in all the different figns of the zodiac. Cuftom, indeed, has taught us to fay the fun is in Aries, when it is between us and Aries, and fo of any other fign; whereas it would have been more proper to fay, that the earth is in Libra.

To fhew why at different times of the year we fee the heavens decorated with an entire different collection of fars.

This phenomenon is occafioned by the earth's progreffive or annual motion: while the earth is traverfing it's courfe under the vaft concave of fixed flars, we are gradually carried under the different conftellations. From hence it is evident, that at night when the earth is turned from the fun, we fhall in fucceffion have the opportunity of viewing from time to time all the ftars in the zodiac, and confequently a different conftellation. will prefent itfelf every month.

Thus, the Pleiades are not vifible in the fummer; but in the winter the carth is got between the fun and them. Thefe flars are obfervable at night, becaufe they are not intercepted from our fight by the fun's rays; and in this manner they appear during the whole winter, only they feem to get more wefterly every night, as the carth moves gradually by them to the caft. To make this fill more clear, place the earth in the planetarium between the fun and any of the figns, that fide towards the fun will be day, and that towards the figa night it follows, that at night we are turned towards the flars, which in that fign (fuppofe, as before, the Pleiades in Taurus) will then be confpicuous to us; but as the fpring approaches, the

earth

earth withdraws itself from between the fun and the Pleiades, till at length the earth, by it's progreffive motion, gets the fun between it and the ftars, which then lie hid behind the folar rays: after the fame manner, while the earth performs it's annual tract, the fun, which always feems to move the contrary way, darkens, by 'his fplendor, the other conftellations, fucceffively; but the stars oppofite to thofe hid by the fun, are at night prefented to our view.

GENERAL PHENOMENA OF THE PLANETS.

I fhall now place the earth, Mars, and Venus, on the planetarium, and as cach planet moves with a different degree of velocity, they are continually changing their relative pofitions. Thus on turning the handle of the machine, you find, 1ft, that the earth moves twice as faft as Mars, making two revolutions while he makes one; and Venus, on the other hand, moves much fafter than the earth. Secondly, that in each revolution of the earth thefe planets continually change their relative pofitions, correfponding fometimes with the fame point of the ecliptic, but much oftener with different points.

To explain the conjunction, oppofition, elongation, and alber phenomena of the inferior planets.

We may now proceed to make fome obfervations on the motions of Venus, as obferved in the planetarium. If confidered as viewed from the fun, we fhall find that Venus would appear at one time nearer to the earth than at another; that fometimes fhe would appear in the fame part

of the heavens, and at others in oppofite parts thereof.

As the planets, when feen from the fun, change their pofition with refpect to the earth, fo do they alfo, when feen from the earth, change their pofition with refpect to the fun, being fometimes nearer to, at others farther from, and at times in conjunction with him.

But the conjunctions of Venus or Mercury, feen from the earth, not only happen when they are feen together from the fun, but alfo when they appear to be in oppofition to the folar fpectator. To illuftrate this, bring the earth and Venus to the first point of Capricon; then by applying a ftring from the fun over Venus and the earth, you will find them to be in conjunction, or on the fame point of the ecliptic.

Whereas if you turn the handle till the fun is between Venus and the earth, a fpectator in the fun will fee Venus and the earth in oppofition; but an inhabitant of the earth will fee Venus not in oppofition to the fun, but in conjunction with him.

In the first conjunction Venus is between the fun and earth; this is called the inferior conjunction. In the fecond, the fun is fituated between the earth and Venus; this is called the fuperior conjunction.

After either of thefe conjunctions, Venus will be feen to recede daily from the fun, but never departing beyond certain bounds, never appearing oppofite to the fun; and when the is feen at the greateft diftance from him, a line joining her center with the center of the earth, will be a tangent to the orbit of Venus.

To illuftrate this, I take off the fun from it's fupport, and the ball of Venus from it's fupporting ftem, and place this wire, (fig. 2, pl. 11, so

that