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Of the Solar System.

TUTOR. We will now proceed to the description of the Solar System. James. Of what does that consist, sir?

Tutor. It consists of the sun, and planets, with their satellites, or moons. It is called the Solar System, from Sol, the sun, because the sun is supposed to be fixed in the centre, while the planets, and our earth among them, revolve round him at different distances.

Charles. But are there not some people who believe that the sun goes round the earth?

Tutor. Yes, it is an opinion embraced by the generality of persons

not accustomed to reason on these subjects. It was adopted by Ptolemy, a celebrated astronomer of antiquity, who supposed the earth perfectly at rest, and the sun, planets, and fixed stars to revolve about it every twentyfour hours.

James: And is not that the most natural supposition?

Tutor. If the sun and stars were small bodies in comparison of the earth, and were situated at no very great distance from it, then the system maintained by Ptolemy and his followers might appear the most probable.

James. Are the sun and stars very large bodies then?

Tutor. The sun is more than a million of times larger than the earth which we inhabit, and many of the fixed stars are probably much larger than he is.

Charles. What is the reason, then, that they appear so small?

· Tutor. This appearance is caused by the immense distance there is between us and these bodies. It is known with certainty, that the sun is more than 95 millions of miles distant from the earth, and the nearest fixed star is probably more than two hundred thousand times farther from us than even the sun himself.*

Charles. But we can form no conception of such distances.

Tutor. We talk of millions with as much ease as of hundreds or tens. but it is not, perhaps, possible for the

* The young reader will, when he is able to manage the subject, see this clearly demonstrated by a series of propositions in the 5th book of Dr. Enfield's Institutes of Natural Philosophy, second edition. See p. 346 to the end of book V. See also the Treatises on Astronomy, by Brinkley, Vince, and Woodhouse.

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mind to form any adequate concep


tions of such high numbers. Several methods have been adopted to assist the mind in comprehending the vast ness of these distances. You have some idea of the swiftness with which a cannon-ball proceeds from the mouth of the gun?

James. I have heard at the rate of eight miles in a minute.

Tutor. And you know how many minutes there are in a year?

James. I can easily find that out by multiplying 365 days by 24 for the number of hours, and that product by 60, and I shall have the number of minutes in a year, which number is 525,600.

Tutor. Now if you divide the distance of the sun from the earth by the number of minutes in a year, multiplied by 8, because the cannon


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ball travels at the rate of 8 miles in one minute, you will know how long any body issuing from the sun, with the velocity of a cannon-ball, would employ in reaching the earth.

Charles. If I divide 95,000,000 by 525,600, multiplied by 8, or 4,204,800, the answer will be more than 22, the number of years taken

for the journey.


Is it then probable that bodies so large, and at such distances from the earth, should revolve round it every day?

Charles, I do not think it is.Will you, sir, go on with the description of the Solar System?

Tutor. According to this system, the sun is in the centre, about which the planets revolve from west to east, according to the order of the signs in the ecliptic; that is, if a planet is

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