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The planets all move round the sun in the order of the signs, that is, from the west towards the east; but their apparent motion, as seen from the earth, is very irregular : (hence they are termed planets, or wandering stars:) sometimes it is from west to east, called direct; and sometimes from east to west, called retrograde: at other periods they appear for a while stationary. But this apparent irregularity is occasioned by their being viewed from a body which is itself in motion.

Suppose a person placed at the centre of a large circular area, and that several bodies are moving round that centre, all the same way, but at different distances from it, and with different velocities; these bodies, to the person placed at the centre, will all appear to be moving in the same direction ; but if the person were placed at a distance from the centre, and carried round in one of those bodies, he would no longer see the rest moving in the same orderly manner as before. Such is the case with the planets seen from the sun, the centre of the system; they all pursue their regular courses from west to east :-but viewed from the earth, one of the moving bodies, their apparent motion is very different from the real.

The motion of the inferior planets may be very familiarly illustrated by carrying a small ball, or globe, round a circular wire, having placed a candle at a distance on one side, and a screen on the other, to receive the shadow of the ball. Whilst the ball is carried round the circle, without ever changing its course, the shadow against the screen moves backward and forward, something like the vibrations of a pendulum.

Such is the case with Mercury and Venus ; they keep moving backward and forward in that part of the heavens in which the sun is; Mercury never going farther from the sun than 29o and Venus than 47o.

To account for the retrograde motion of the superior planets, it may be observed, that when two bodies are moving the same way, with different velocities, that body which moves more slowly, seems to recede from the other, with a motion equal to the difference of their velocities: thus, whilst the earth and one of the superior planets, suppose Jupiter, are both moving eastward, (the planet being in opposition,) the earth moving faster than Jupiter, the latter is left behind: and, though his real motion is eastward, he appears to be moving from east to west.

The planets can only be seen when some part of their surface on which the sun shines is turned towards the ob server. Hence the inferior planets present all the phases of the moon.

When Mercury or Venus is between the earth and the sun it is said to be in its inferior conjunction. In this situation the whole of the enlightened portion of the planet is turned away from the earth, and it is invisible to us; but as it proceeds on its course a thin crescent of light is presented to our view, which gradually passes into the halfmooned and gibbous form. When arrived at its superior conjunction, that is, when the sun is interposed between the earth and it, the whole of its enlightened side is turned towards us, but in this situation its brilliancy is lost in the blaze of the sun's rays.

Mars is sometimes slightly gibbous, (i. e. something less than a circle, like the moon between the first and second quarter,) but with this exception, all the superior planets shine with a full disc. The reason of this is that they are so much further removed from the sun than the earth is, that in whatever part of its orbit the earth may be we see them nearly as we would view them from the sun itself.

The planets do not move in a circle, as the ancients and even as Copernicus supposed, but in an ellipse or oval, and they do not proceed in their course with a uniform velocity, but always move fastest when nearest the great source of attraction, the sun.

The laws of planetary motion, as originally discovered by Kepler, and usually called Kepler's three laws, are the following:

Ist. A line drawn from the centre of a planet to the centre of the sun describes equal areas round him in equal times.

2nd. The orbits of the planets are ellipses, having the sun in one of their foci.

3rd. The squares of the periodic times are proportional to the cubes of their mean distances from the sun.

The secondary planets or satellites obey the same laws as the primary. They all move from west to east except the satellites of Uranus, whose motions are supposed to be from east to west; and they all, so far as is known, occupy the same time in making a rotation upon their axis as in completing a revolution in their orbit; they consequently always present the same phase to their primary.

Mercury. This planet is nearer to the sun than any that has yet been discovered : it is seldom visible, being generally

lost in the sun's rays: its diameter is about 3,200 miles, and distance from the sun 37 millions of miles. It rotates on its axis in 24 hrs. 5 m. It turns round the sun in 88 days, moving at the rate of about 110,000 miles per hour, or 30 miles per second; it receives its name from the extreme rapidity of its flight.

Mercury being so much nearer the sun than the earth is, the heat of the sun there will be 7 times greater than our summer heat. This being greater than the heat of boiling water, all the water on our globe would be evaporated, and every thing on its surface burnt to atoms, were it similarly situated. Hence any beings inhabiting it must be very different from us. The density of Mercury is very great, being rather greater than that of quicksilver. A cannon ball, passing at the rate of 20 miles in a minute, would take 34 years in going from the sun to Mercury.

Venus. $ This planet receives its name from its extreme beauty; it may easily be known, from its being the most brilliant of all the planets, and from its rising before the sun in the morning, when it is a morning star, or setting after the sun in the evening, when it is an evening star. Venus and Mercury, when viewed through a telescope, have all the phases of the moon.

The diameter of Venus is nearly equal to that of the earth, being 7,800 miles: the length of its day is also nearly the same as that of ours, being 23 hours, 21 minutes, the time that it takes to turn on its axis. This is known by the spots on its surface. Its distance from the sun is 69 millions of miles; and it finishes its journey round the sun in 224 days, 16 hours, moving at the rate of 80,000 miles per hour, or, 23 m. per second. The light and heat of the sun at Venus are double that which is enjoyed by the inhabitants of this globe. A cannon

ball would require more than 6 years in passing from the sun to Venus. The density of Venus is a little greater than that of the earth, and like the earth it possesses an atmosphere. Venus, as well as Mercury, is sometimes seen to pass over the sun's disc: this is called a transit, . and it furnishes astronomers with the means of determining the distances of all the planets. Captain Cook's

first voyage to the South Sea was undertaken for the purpose of observing, at Otaheite, the last transit of Venus, in 1769. The next transit will be in 1874.

The reason that a transit of Venus does not take place at every revolution is, that its orbit does not coincide with that of the earth. A transit can only take place when the sun, earth, and Venus are in the same straight line, and this can never be excepting when the earth and Venus happen to be both in the line of intersection of the planes of their orbits, that is, when Venus is in her nodes.

The same observations are applicable to the transits of Mercury, which take place at intervals of 6, 7, 13, 46, and 263 years.

It is obvious that no transits of the superior planets can occur, but the earth being an inferior planet to Mars, Jupiter, &c., will perform occasional transits to them.

The Earth. The next planet in the order of distance from the sun is that which we inhabit. The equatorial diameter of the earth is 7,925, its polar 7,899 miles, and the number of square miles upon its surface 197 millions. It turns upon its axis in 23 hrs. 56 m. as indicated by its return to the same star.

This period is called a siderial day. The mean solar day consists of 24 hrs., the additional 4 m. being required, as already explained, to bring any point on the earth's surface to the same position in regard to the sun. The length of the solar day is not uniform; this arises from the inequality in the rate of the earth's motion round the sun, and the inclination of the earth's axis to the plane of its orbit. (See Prob. XXX.)

Its mean distance from the sun is 95 millions of miles. It performs a revolution in its orbit in 365 d. 6 h. 9 m.

10 sec., moving at the rate of 68,000 miles per hour, which is more than 1,100 miles per minute, or 181 m. per sec.

The equinoctial, or tropical year, which is the interval between two successive returns of the sun to the same equinox, consists of 365 d. 5 brs. 48 m. 49 sec. The difference between the equinoctial and the siderial, or true year, is owing to a regression of the equinoctial points, which is caused by the attraction of the sun and moon upon the projecting matter at the equator. The precession of the equinoxes, as this motion is called, causes a slow change in the apparent position of the pole star. In about 12,000 years, Vega, the principal star in Lyra, will be very near the pole, and will consequently be the pole star. The pole of the heavens will make a complete revolution in about 26,000 years.

The earth is in its perihelion, or that part of its orbit nearest the sun, on Dec. 31st, and in its aphelion, or furthest from the sun, on July 1st. The sun is about 3 millions of miles nearer to us in the depth of winter than in the middle of summer.

It seems strange that we should have the coldest season when we are nearest to the source of heat, but it must be remembered that during winter the sun continues only a short time above the horizon, and his rays fall very obliquely upon the earth.

The earth is surrounded with a thin fluid called air, the whole body of which forms the atmosphere.

The lower parts of the atmosphere are denser than the higher; and the density diminishes, the greater the altitude ; this it does so rapidly that at an elevation of 18,000 ft., which is nearly that of Cotopaxi, we have ascended through one-half the body of air incumbent on the earth's surface. It is owing to the atmosphere that the rays of light coming from the sun are dispersed in all directions, and thus the whole heavens become illuminated. Without an atmosphere, we should derive no benefit from the light of the sun, except when our sight was directed to him ; all the other parts of the heavens would appear dark, and the stars would be visible at noon-day. It is also the atmosphere, that produces twilight, as already explained. By refracting the rays of light, it causes the sun to appear in the morning before he is above the horizon, and in the evening after he is set.

A cannon ball would take 9 years in passing from the sun to the earth.

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