CONVERSATION XX.

TUTOR.

Of Mars.

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 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 supposes the

sun; a, b, d, 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 : 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 when the earth has advanced to g, Mars will appear stationary at a: 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 %, and this retrograde motion will be apparent till the earth has arrived 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 no less than 190 millions of miles, the whole length of the orbit of the earth. This will be at 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.