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orbits, combined with the annual motion of the Earth. We have already alluded to this phenomenon in our second chapter, page 16.

When a planet appears to move from the neighbourhood of any fixed stars, towards others which lie to the east, its motion is called direct, because the planets move round the Sun from west to east. When a planet seems to move towards the stars which lie westward, its motion is said to be retrograde; and at other times it appears to be stationary.

Now as the planet is, in fact, constantly moving round the Sun, as its centre, it is said to have the three foregoing motions, which will be easily apprehended by the accompanying diagram (fig. 34).

Suppose a planet to revolve round the Sun from west by south to east, according to the letters a, b, c, &c. When the planet is so situated, that a line from the centre of the Earth will graze that part of the orbit where the planet is situated, the planet's motion is said to be stationary, as at b c and ƒg. The planet is then approaching to, or receding from, the Earth. When the planet is furthest from the Earth, it has its natural or direct motion, as from d to e. But when the planet is nearest to the Earth, as at a, its motion is termed retrograde, because, owing to the motion of the Earth in the same direction, it seems to go back in its course.

MARS.

See Mars, alone, runs his appointed race,
And measures out exact the destined space;
Nor nearer does he wind, nor further stray,
But finds the point whence first he rolled away.

This is the nearest planet which is exterior to the Earth, and is remarkable for having a more ruddy colour than most of the other planets. This ruddiness is believed to be due to a peculiar constitution of its atmosphere; whereby it absorbs all the component parts of the solar rays but the red, which it reflects.

The mean diameter of this planet is about 4189 miles, being rather more than half the diameter of the Earth. His form is that of an oblate spheroid, which is occasioned by his rotation on his axis, and is much more flattened than that of the Earth, his greater diameter being to his smaller as 16 to 15. He revolves round the Sun in an orbit which is, at the mean distance, about 144 millions of miles from the Sun: this orbit he travels round in about 687 of our days with a velocity of 53000 miles in an hour. We say our days, in order not to confound them with the day of Mars; for, as he rotates on his own axis in about 24 hours 39 minutes, his day is rather longer than ours, so that the period of his revolution is about 668 of his days. His rotation on his axis is estimated by permanent spots on his disk, which appear to be mountains as shown in figure 35.

There are some circumstances in the appearance of Mars which confirm the statement, that his orbit is exterior to that of the Earth. The appearance which he commonly presents is gibbous. This phase may be

illustrated by figure 36, A a в b.

Here we see that

A a B is a semicircle, and that A & B is part of an ellipse; these joined together form the gibbous phase. Thus Mars sometimes appears gibbous, (as shown in fig. 35,) but never crescent-formed; as he would be sometimes, if he revolved within the Earth's orbit: he appears nearly five times as large at one time as he does at another, on account of his being so much nearer to the Earth when the Earth is between Mars and the Sun, than when the Sun is between Mars and the Earth. When at his furthest distance from the Earth he subtends an angle of 4", and when at his greatest distance an angle of 18′′, and the inclination of his orbit to the plane of the ecliptic gives an angle of 1° 51'.

This planet is about one-seventh of the size of the Earth, and is supposed to enjoy about half the light and heat that we have, but in a similar proportion throughout his year, from the circumstance that his axis is inclined to the plane of the ecliptic, nearly at a like angle with the Earth's, though he is much further off from the Sun.

The matter, of which this planet is composed, is about one-fourth lighter than that of the Earth. His rotation on his axis, as determined by the motion of certain spots on his surface, is found out in the same way as has been already explained concerning the Sun.

No moon has ever been found to accompany this planet; though, being further from the Sun than the Earth, it would seem to stand more in need of a luminous auxiliary. But, by means of a good telescope, the poles, or extremities of the rotating parts of this planet, are observed to present a white appearance which becomes fainter when the pole, in the course of the

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