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take in revolving round the sun; thus the periodical time of the earth is 365 days; that of Venus 2244 days; that of Mercury 88 days. Charles. How then would you find the distance of Mercury from the sun?

Tutor. By the rule of three: I would say as the square of 365 days (the time which the earth. takes in revolving about the sun) is to the square of 88 days, (the time in which Mercury revolves about the sun,) so is the cube of 95 millions, (the distance in miles of the earth from the sun) to a fourth number.

James. And is that fourth number the distance in miles of Mercury from the sun?

Tutor. No; you must extract the cube root of that number, and then you will have about thirty-seven millions of miles for the answer, which is the true distance at which Mercury revolves about the sun.

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Tutor. Next to Venus is the earth and her satellite the moon; but of these sufficient notice has already been taken, and therefore we shall pass on to the planet Mars, which is known in the heavens by a dusky red appearance. Mars,

together with Jupiter, Saturn, and the Herschel, are called superior planets, because the orbit of the earth is enclosed by their orbits.

Charles. At what distance is Mars from the sun?

Tutor. About 144 millions of miles; the length of his year is equal to 687 of our days, and therefore he travels at the rate of more than 53 thousand miles in an hour; his diurnal rotation on his axis is performed in 24 hours and 39 minutes, which makes his figure that of an oblate spheroid.

James. How is the diurnal motion of this planet discovered?

Tutor. By means of a very large spot which is seen distinctly on his face, when he is in that part of his orbit which is opposite to the sun and earth.

Charles. Is Mars as large as the earth?

Tutor. No; his diameter is only 4189 miles in length, which is but little more than half the length of the earth's diameter. And owing to his distance from the sun he will not enjoy one half of the light and heat which we enjoy.

James. And yet, I believe, he has not the benefit of a moon.

Tutor. No moon has ever been discovered belonging either to Mercury, Venus, or Mars. Charles. Do the superior planets exhibit similar appearances of direct and retrograde motion to those of the inferior planets?

Tutor. They do suppose s (Plate VIII. Fig. 18.) the sun: a, b, c, d, e, f, g, h, the earth, in different parts of its orbit, and m Mars in his orbit. When the earth is at a, Mars will appear among the fixed stars at x: when by its annual motion the earth has arrived at b, d, and f, respectively, the planet Mars will appear in the heavens at y, z, and w: when the earth has advanced to g, Mars will appear stationary at : to the earth in its journey from g to h the planet will seem to go backwards or retrograde in the heavens from o to z, and this retrograde motion will be apparent till the earth has arriv ed at a, when the planet will again appear stationary.

James. I perceive that Mars is retrograde when in opposition, and the same is, I suppose, applicable to the other superior planets; but the retrograde motion of Mercury and Venus is when those planets are in conjunction.

Tutor. You are right: and you see the reason, I dare say, why the superior planets may be in the west in the morning when the sun rises in the east, and the reverse.

Charles. For when the earth is at d, Mars may be at n, in which case the earth is between the sun and the planet : I observe also that the planet Mars, and consequently the other superior planets, are much nearer the earth at one time than at others.

Tutor. The difference with respect to Mars is

VOL. TU

no less than 190 millions of miles, the whole length of the orbit of the earth. This will be a proper time to explain what is meant by the Heliocentric longitude of the planets referred to in the Ephemeris.

James. Yes, I remember you promised to explain this when you came to speak of the planets; I do not know the meaning of the word heliocentric.

Tutor. It is a term used to express the place of any heavenly body as seen from the sun; whereas the geocentric place of a planet, is the position which it has when seen from the earth. Charles. Will you show us by a figure in what' this difference consists?

Tutor. I will let s (Plate VIII. Fig. 19.) represent the place of the sun, b Venus in its orbit, a the earth in her's, and c Mars in his orbit, and the outermost circle will represent the sphere of fixed stars. Now to a spectator on the earth a, Venus will appear among the fixed stars in the beginning of Scorpio, but as viewed from the sun, she will be seen beyond the middle of Leo. Therefore the Geocentric longitude of Venus will be in Scorpio, but her Heliocentric longitude will be in Leo. Again, to a spectator at a, the planet Mars at c, will appear among the fixed stars, towards the end of the sign of Pisces; but as viewed from the sun he will be seen at the beginning of the sign Aries; consequently the geocentric longitude of Mars is in Pisces; but his heliocentric longitude is in Aries.

CONVERSATION XLII.

Of Jupiter.

Tutor. We now come to Jupiter, the largest of all the planets, which is easily known by his peculiar magnitude and brilliancy.

Charles. Is Jupiter larger than Venus?

Tutor. Though he does not appear so large, yet the magnitude of Venus bears but a very small proportion to that of Jupiter, whose diameter is 90,000 miles in length, consequently his bulk will exceed the bulk of Venus 1500 times: his distance from the sun is estimated at more than 490 millions of miles.

James. Then he is five times farther from the sun, than the earth, and consequently, as light and heat diminish in the same proportion as the squares of the distances from the illuminating body increase, the inhabitants of Jupiter enjoy but a twenty-fifth part of the light and heat of the sun that we enjoy.

Tutor. Another thing remarkable in this planet is, that it revolves on its axis, which is perpendicular to its orbit, in 10 hours, and in consequence of this swift diurnal rotation, his equatorial diameter is 6000 miles greater than his polar diameter.

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