Application of the Transit of Venus.

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dian. From A the planet Venus is seen crossing the sun in the line, Ca D; from в the apparent crossing is Fb G, farther from the centre, and therefore shorter in length and in duration. The business of the observers is to ascertain with the utmost accuracy the moment of time when Venus touches the edge of the sun, and also the moment of leaving the other edge, with a view to get the precise duration of the transit. This being known, enables us to say at what distances from the centre of the sun the two apparent transits took place; whence can be ascertained the distance (in angular measure) between Ca D and Fb G. That distance is the angle, b v a, at Venus, and is not the angle that we are in quest of, but is a stepping-stone to the real determination. It must be premised that the proportionate distance of the earth and Venus from the sun is known. Supposing Venus were exactly half the distance of the earth, the angle, a v b, would be found to be double the angle, A b B, the real parallax sought, namely, the angle at the sun, subtended by the distance of the two stations, A and B. As Venus is more than half the earth's distance from the sun, the known proportion of the two distances, A V and v b, will enable the angle of parallax, A b B, to be calculated.

The distance of the two stations being known, and the angle of parallax corresponding, the sun's distance is deduced. Or we may ascertain what would be the angle corresponding to the earth's semi-diameter, which is the proper angle of the sun's horizontal parallax.

1058. Another method, considered by some astronomers to be more exact than the Transit of Venus, is to reason from a certain inequality in the moon's motions, called the Parallactic Inequality. This method makes use of the ascertained distance of the moon; and as the inequality depends upon the proportion of the sun's distance to the moon's, it furnishes a means of computing the sun's distance, the moon's being known. By this method, a parallax was obtained amounting to 8"-9159.

The amount at present adopted, from which the sun's distance (91,430,000 miles) and all the elements of the solar system have been calculated, was agreed upon by Airy and Le Verrier, namely, 8"-94. It is thought that this may have to be reduced one-fiftieth of a second, to 8"-92. The sun's distance would then be about 21 millions of miles.

1059. Mars.-The nearest to the sun of the planets beyond the orbit of the earth, called the superior planets. Its distance (mean)

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The Small Planets.

is nearly one hundred and forty millions of miles. The heating power of the sun at this distance is less than half what it is on the earth; so that while the equatorial climate might be endurable by us, the poles would be cold in the extreme. The length of the day and the changes of the seasons are nearly the same as with us.

The appearance of the surface is reddish; but it is so plainly variegated, that inferences can be drawn respecting the constitution of the planet. At the poles there is a dazzling whiteness, always supposed to be snow; the extent of the white surface varying with the seasons. In other parts, the alternation of light and dark tracts is supposed to indicate land and water. If so, the proportions are the opposite to what we find on the earth; the land being four times the extent of the water.

The presence of an atmosphere is fully established. The spectroscope attests the existence of watery vapour.

1060. The small planets.—The length of the interval that separates Mars and Jupiter often suggested the idea that some planet circulated between the two. On the first day of the present century a planet was discovered, so small that it could never be seen by the naked eye. When its orbit was ascertained, it was found to occupy the blank. In fact, it curiously fell in with a regular law of progression that had been observed in the distances of the planets (called Bode's law). The only anomaly about it was its utterly insignificant size; for although the other planets show great differences of size, between the extremes of Mercury and Jupiter, yet, as compared with Mercury, Ceres is a mere fragment.

In two years from the discovery of Ceres, another small planet was discovered, at nearly the same distance; and very soon after a third and a fourth. The names of the three are Pallas, Juno, Vesta. There were thus four planets in the place where one should have been. No farther discovery was made till 1845, when a fifth was discovered. This was followed by a sixth in 1847, and, since then, the number has been continually growing. On the 25th of February of the present year (1876) was discovered the 160th.

They are all comparatively small in size. They are invisible to the naked eye; Vesta alone might be seen as a star of the sixth magnitude to a person that knows where to look for it.

Their orbits, while occupying the position of the supposed planet between Mars and Jupiter, are, as a rule, more eccentric, and more inclined to the ecliptic than the other planets. The first supposition respecting them was that they were fragments of an exploded

planet; but, not to mention that the explosion of a planet is unlikely, if they had arisen in that way, their orbits would have all passed through one point, which is not the case. Except their small size, nothing is known of their physical structure; their light is too faint for analysis by the spectroscope.

1061. Jupiter.-The giant of the planets. A man there would weigh nearly three times as heavy as on the earth. Eleven times the earth in diameter, it is thirteen hundred times the earth's volume, and three hundred times its mass. It is nearly five times as far from the sun as the earth is; whence the amount of light and heat received from the sun is very small. Yet it is a brilliant planet, notwithstanding.

The surface of Jupiter is crossed by a number of dark belts, which are constantly changing; but are nearly parallel to one another. There are also numerous spots of a more permanent character, although not known to be absolutely fixed. From these is deduced the enormously quick rotation of the planet about its axis; being only ten hours to a complete revolution. Accompanied with this rapidity of rotation, is the obvious flattening at the poles.

The belts are believed to be clouds in Jupiter's atmosphere. But the circumstances of the planet, in respect of the little influence of the sun, and of the greater force of gravity at the surface (nearly three times the earth's), greatly alter the atmospheric conditions. Putting all things together, Mr. Proctor supposes that the planet possesses a great internal heat, and, in this respect, has a greater resemblance to the sun than to the earth. It is subject to changes of colour, of which we do not know the cause.

Jupiter has four moons, or satellites, revolving round him at different distances, and observing all the laws of the planetary motions. Their periods of revolution are small: varying from two to sixteen days. Their orbits are so near the plane of Jupiter's orbit that they frequently pass his body either before or behind. Their passage in front is named a transit; when they pass behind they are eclipsed. Sometimes in passing on the far side they get into Jupiter's shadow without being behind his body: they are then occulted.

1062. Saturn.-Another huge planet, but less than Jupiter: the ring planet. His distance from the sun is nearly ten times the earth's ; to him, therefore, the sun's heat and light are almost as nothing. The planet has belts and spots like Jupiter, and from these he is

shown also to have a rapid whirl: the period of rotation is ten and a half hours. The density is about one-half of Jupiter's, little more than one-seventh of the earth's.

Saturn, like Jupiter, has a decided atmosphere; which cannot be maintained in the gaseous form by solar heat. By reasoning similar to that applied to Jupiter, he is considered by Mr. Proctor to possess a high degree of heat in his own body.

The ring of Saturn surrounds the planet, at a distance of 9760 miles. Its entire breadth is 37,570 miles, and its thickness about 138 miles. It is not one continuous ring, but a series of rings one within the other. At first was noticed one marked division into two rings, but other divisions have since been made out: an inside dusky and half-transparent ring being apparent. The rings revolve about the body of the planet in ten and a half hours. They are not coherent rigid masses, but streams of meteoric matter, or small satellites, probably mixed with vapour, which is the only constitution that would, under the laws of motion, possess stability. Saturn has eight satellites, exterior to the ring. The outermost of the eight is nearly four millions of miles distant from the planet, and is almost as large as the moon.

1063. Uranus.-The distance of this planet from the sun is nineteen times the earth's distance: the sun's influence being diminished to nearly the four hundredth part of his influence on the earth: so that as far as heat goes the planet would be just as well without the sun; although he would still appear a body of considerable luminosity. The diameter of the planet is four times the earth's; the density one-fifth of the earth's, or little more than the density of water. It has an atmosphere of marked character, and in all probability has a considerable heat of its own.

The planet has four satellites, whose motions are very exceptional. Not only are the orbits very much inclined (nearly at right angles to the ecliptic), but they move in a direction opposed to all the other bodies of the solar system.

1064. Neptune.-Remarkable for the history of his discovery. Some unaccounted-for disturbances of Uranus led to the suspicion that there might be an exterior planet yet undiscovered. The place of the planet was computed from the direction of the disturbances, and by means of this cue, the planet was actually detected on the 23rd September, 1846, within a very short distance of the computed place. His distance from the sun is enormous, thirty times the earth's his period of revolution being a hundred and ten days

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over 165 years.

He is somewhat larger than Uranus, but slightly

less dense. One satellite has been discovered.

People often speculate as to whether the other planets are inhabited. The only two that would, from their temperature, permit the existence of living beings such as belong to this earth, are Venus and Mars. The poles of the one planet and the equator of the other might support vegetable and animal life of the terrestrial types.

Comets.

1065. The word "comet " expresses the hairy, bearded, or brushlike appearance of the bodies so named. They must be seen to be conceived. Rare and capricious in their recurrence, their aspect is not only exceptional, but subject to great mutation during the time of their stay. Many of them can be distinguished into a head and a tail, the head being a rounded end, often with a bright point or nucleus, from which proceeds a vast brush or tail of thin luminous matter. So open is the texture, that the stars can be seen through every part, not excepting the head or nucleus. It is in the tailed form that they have awakened the greatest attention and surprise; but many show nothing but a nebulous disc, with a nucleus. On first appearing they are usually faint, but after a time become much lighter-an effect connected with their approach to the sun.

Since their motions have been studied and understood, they are seen to follow the laws of the planetary revolutions, but with considerable differences as to the shape and position of their orbits. These orbits are either very eccentric ellipses, or they are open curves, in which case a comet never revisits the glimpses of the sun. The inclination of the orbits is very various; and the motions often (like the satellites of Uranus) retrograde.

The greatest interest attaches to those that return within moderate periods. The one named Halley's has a period of seventy-six years. It was observed by him in 1682, and its orbit calculated. It has returned twice since, according to prediction-namely, in 1759 and 1835; and it has been traced backwards, and identified with a series of recorded comets.

An interesting comet of a short period named after the astronomer Encke, who discovered it, revolves in three years and four months, at a mean distance from the sun of little more than twice the earth's. This comet has undergone retardation, which, not being fully accounted for by the perturbing influence of the large planets, has