Tutor. Well, let us proceed to the next page. The first three short columns, relating only to the duration of day-light and twilight, require no explanation: the fourth we shall pass over for the present; and the remaining five give the latitude of the planets.

James. What do you mean by the latitude, sir?

Tutor. The latitude of any heavenly body is its distance from the ecliptic north or south. The latitude of Venus, on new-year's day, 1803, is 4° north.

Charles. Then the latitude of heavenly bodies, has the same reference to the ecliptic, that declination has to the equator?

Tutor. It has.

James. But I do not see any table of the sun's latitude.

Tutor. I dare say your brother can give you a reason for this.

Charles. Since the latitude of a heavenly body is its distance from the ecliptic, and since the sun is always in the ecliptic, therefore he can have no latitude.

Tutor. The longitude of the sun and planets is the only thing in this page that remains to be explained. The longitude of a heavenly body is its distance from the first point of the sign Aries, and it is measured on the ecliptic. It is usual, however, as you observe in the Ephemeris, to express the longitude of a heavenly body by the

degree of the sign in which it is. In this way the sun's longitude on the 1st of January, 1809, is in Capricorn 10°. .45'. . 14"; that of the moon in Cancer 6°. .4'; that of Jupiter is in Pisces, 13°. .35'.

Charles. There are some short columns at the bottom of the former page that you have omitted.

Tutor. The use of these will be better understood when we come to converse respecting the planets. *

CONVERSATION XXVI.

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

For the explanation of Heliocentric Longitude, see Conversation XLI.

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, who supposed the earth perfectly at rest, and the sun, planets, and fixed stars to revolve about it every twenty-four 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 pro

bably 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 mind to form any adequate conceptions of such high numbers. Several methods have been adopted to assist the mind in comprehending the vastness 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-ball travels at the rate of 8 miles in one minute, and

* The young reader will, when he is able to manage the subject, see this clearly demonstrated by a series of propo. sitions in the 5th book of Dr. Enfield's Institutes of Natural Philosophy. Second Edition. See p. 346 to end of book V.

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.

Tutor. 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 seen in Aries, it advances to Taurus, then to Gemini, and so on.

James. How many planets are there belonging to the sun?

Tutor. There are seven, besides some smaller bodies of the same kind discovered within

these nine years. C (Plate v. Fig. 2.) represents the sun, the nearest to which Mercury revolves in the circle a; next to him is the beautiful planet Venus, who performs her revolution in the circle b; then comes the Earth in t; next to which is Mars in e; then Jupiter in the circle f; afterwards Saturn in g; and far beyond him the Herschel planet performs his revolution in the

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