LETTER II.

Of the Solar System in general.

MY DEAR BOY,

As the earth which we inhabit is a planet,

and revolves, with the heavenly bodies, round the fun, it will be proper for me first to give you a general idea of the Solar System, that is, the Syftem of the World.

The aftronomer confiders the azure Sky, or Firmament, as nothing more than the apparent boundary of fight, which must therefore be of a spherically concave figure; in this all the celestial bodies are feen. Those which are far diftant from us appear as luminous points, which we call STARS.

Of these STARS, fome feem to be fixed, that is, they never change their places with respect to one another, and some are moveable; these latter are called PLANETS, and are found to revolve about one large central body, which the ancients fuppofed to be the Earth, but which, by the obfervations of modern aftronomers, is now discovered to be the Sun; whence the true Syftem of the World comes to be called the Solar System. This was, in the early ages of the world, known to, and taught by Pythagoras, and from whence is was

called the Pythagorean Syftem. But, in procefs of time, this part of fcience was loft; of late years, however, it has been revived and taught by Copernicus, and from thence called the Copernican Syftem. Alfo because Sir Ifaac Newton demonftrated the caufe of, and laws that regulated the motions of the heavenly bodies, it is very properly by fome denominated the Newtonian Syftem.

This SYSTEM consists of

The SUN, which is a prodigiously large body, fixed in the centre; and, by its great power of attraction, governs the motions of the planets revolving round it, and illuminates, warms, and animates them with light and heat. It is upwards of one million of times larger than our earth.

1. MERCURY, the firft planet in the Syftem, revolves about the fun in eighty-eight days, at the distance of 36 millions of miles.

II. VENUS, at the diftance of 68 millions of miles, completes her revolution round the fun in 224 days.

III. The EARTH, on which we live, at the diftance of 95 millions of miles, performs its period in 365 days and fix hours.

IV. MARS, at the diftance of 145 millions of miles, in little less than two of our years.

V. JUPITER, at the distance of 390 millions of miles, in 12 years.

VI. SATURN, at the immenfe diftance of 903 millions of miles, creeps round its orbit in about 30 years.

VII. GEORGIUM SIDUS, or, as it is called by men of science, the Herfchel, at the ftill farther diftance of 1813 millions of miles, performs his revolution in little more than 83 years.*

VIII. The COMETS, which in various and vaftly eccentric orbits, revolve about the Sun, as the centre of their motions.

Among the planets there are four which are found to have their fecondary Planets, Satellites, or Moons, revolving constantly about them, as the centres of their motions; viz.

The EARTH, which has only one Moon, revolving about it in 27 days, 7 hours, 43 minutes, at the distance of about 240,000 miles.

* Besides these, there have lately been discovered four very finall planetary bodies: The firft was observed January 1, 1801, by M. Piazzi, who has given it the name of Ceres; the fecond was firft feen by Dr. Olbers, on the 28th of March, 1802, and by him called the Pallas. A third was discovered by M. Harding, and a fourth by Dr. Olbers, during the year 1807. Dr. Herschel would diftinguish these from the other planets by the name of Afireoids. In their fize and motion they refemble comets; in the clearness of their light, they are fimilar to the planets.

JUPITER is obferved with a telescope to have four Moons revolving about him, and to have zones or belts on the surface of his body. SATURN has feven Satellites, two of which have been lately difcovered by Dr. Herfchel, within the orbits of thofe formerly known; and, befides thefe, a ftupendous Ring furrounds his body.

The GEORGIUM SIDUS, or, as it is more generally called, the Herschel planet, has fix Satellites, which, as well as the planet itself, have been discovered by Dr. Herfchel.

These are the constituent parts of the Solar Syftem, which is now received and approved as the only true Syftem of the World.

As to the FIXED STARS, we know nothing of their distance, only that it is immensely great. They are supposed to be vastly large bodies, and fhine by their own light, and are most probably Suns, each of them defigned to be the centre of a fyftem, and to have planets revolving round it, in the fame manner as the earth revolves about our sun.*

*To give you fome idea of the immenfe diftance of the nearest fixed ftar, fuppofe a cannon ball to be discharged from a twenty-four pounder, and to move at the rate of about 19 miles in a minute, it would be 760,000 years paffing from the nearest fixed star to our earth.-Sound,

We will now, my young friend, proceed to a particular defcription of the planet, which is the place of our prefent refidence; in the mean time, be affured that I am,

Your's most affectionately.

Quefiions formed for the Exercife of the Pupil.

HOW many kinds of flars are there'?

Why are fome called fixed ftars?

What are the planets ?

Where was the true fyftem of the world first taught? By whom was it revived?

Of what does the folar fyftem confist?

How large is the fun?

Which is the first planet, or that which is nearest the fun?

How long is MERCURY in revolving about the fun, and at what distance?

which travels at the rate of about thirteen miles in a minute, would be 1,120,000 years traversing the same diftance. The exact diftance in miles has been found by Dr. Bradley to be not lefs than 7,600,000,000,000. Dr. Herschel carries the matter ftill farther: he says he has feen ftars by his telescope, the light of which, though travelling at the rate of 12 millions of miles in a minute, has been nearly two millions of years in travelling to the earth. See Philos. Trans. for the year

1802.