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tained; for the great law of Kepler applies :-As the square of the Earth's periodical time, a sidereal year, is to the cube of its distance; so is the square of any other planet's periodical time, to the cube of its distance from the Sun. A concise method of determining the distances of the other planets from the Sun by the distance of the Earth, may be to assume a proportional distance for the Earth, and say,-As the proportional distance of the Earth from the Sun is to its real distance; so is the proportional distance of any other planet from the Sun, to its real distance. The proportional distances of the planets was not difficult to obtain; and has been long known.

If the distance of the Earth from the Sun be assumed at 100,000, the distances of the other principal planets

would be,

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1,894,736.

It is amusing and gratifying to know what vast interest was felt in the transit of 1761, and what vast pains were taken to observe it with accuracy. Persons were sent into various parts of the world, to make observations on this important phenomenon.

Mr. Short, of London, made his observations at the Saville house in London, in the presence of several of the royal family. The transit was observed at the royal observatory at Greenwich, and other places in England; at Paris, by De la Lande; at Stockholm observatory, latitude 59° 20' N., longitude 180 E. from Greenwich, the whole transit being visible, was observed by Wargentin. It was also observed at Hernosand in Sweden; at Torneo in Lapland; at Tobolsk in Siberia; at Madras, at Calcutta, and at the Cape of Good Hope. Dr. Maskelyne's observations at St.

Helena, interrupted by the cloudy state of the weather, were not completely successful. The same happened in part at London, and at the royal observatory in Greenwich. "Early in the morning," June 6th, "when every astronomer was prepared for observing the transit, it unluckily happened, that, both at London and the royal observatory at Greenwich, the sky was so overcast with clouds, as to render it doubtful whether any part of the transit should be seen; and it was 38 minutes 21 seconds past seven o'clock, apparent time, at Greenwich, when the Rev. Mr. Bliss, astronomer royal, first saw Venus on the Sun."

Mr. Short took great pains in computing the Sun's parallax from the best observations, both in England and in other countries, on this transit of 1761, and found it to have been 8.52", on the day of the transit, when the Earth was very near its aphelion, or the Sun near its greatest distance from the Earth; consequently 8.65", when the Sun is at its mean distance. Prior, in his Lectures, considers this mean parallax 8.73", which does not materially differ from the above statement. In the projection of eclipses, the parallax of the Sun is usually considered 9".

The observations on the transit of 1769 did not materially differ in their result from those of 1761. They rather confirmed the parallax deduced from the prior observations.

What is parallax? To what is it equal? What is the difference between diurnal and annual parallax? Does diurnal parallax increase or decrease with the nearness of the heavenly body? Does parallax make a body appear above or below its true place? In what is the horizontal parallax of the Moon of great importance? How did Aristarchus propose to find the Sun's horizontal parallax? How did Hipparchus propose to obtain a triangle for finding the Sun's parallax? Who first suggested the use to be made of the transit of Venus over the Sun's disk? What did Kepler predict? How was Dr. Halley engaged when the thought occurred to him that transits might be used for finding the Sun's parallax? Who first viewed Venus passing over the Sun's disk? When was the first transit of Venus observed? Who gave directions for taking the

transits of 1761 and 1769? If the distance of the Earth from the Sun be known, how can the distance of the other planets be ascertained? Was there much interest felt in the transit of 1761 ?

Where

was it observed ? What prevented the transit from being fully

observed in some places? From the various observations on the parallax, what did Mr. Short make the parallax? From this, what would be the mean parallax? What effect had the observations on the transit of 1769 on the result of the preceding observations?

CHAPTER XII.

The Fixed Stars.

THE fixed stars are so denominated from their always retaining the same situation in relation to each other. We have seen, that the Earth is, at one season of the year, 190,000,000 miles distant from its situation at the opposite season; yet these stars have no sensible parallax. The star which is north at one time, is north at any other time. Most of the stars, indeed, appear to have a diurnal revolution round the Earth; but this arises from the rotation of the Earth on its axis, and is no more than is caused by that rotation.

That the stars always retain the same apparent situation, must be owing to their immense and inconceivable distance. Let two persons be placed one rod distant from each other, east and west. An object, ten rods. distant, which is due north from one, will easily be perceived not to be north of the other. But let the object' be ten miles distant from these observers, and if it be north of one, it will scarcely be perceived not to be north of the other; the angle can be ascertained only by nice observation. Let this principle be applied to the fixed stars, and the student will be sensible, that their

distance is truly immense. We form very inadequate ideas of the Earth's distance from the Sun ;* of course of twice that distance. But this immense distance, 190,000,000 miles, makes no perceptible difference in the situation of the fixed stars, even when viewed with the nicest instruments. “From what we know," says Mr. Ferguson," of the immense distance of the stars, the nearest may be computed at 32,000,000,000,000 of miles from us, which is farther than a cannon ball would fly in 7,000,000 of years."

From the distance of the stars it may be concluded, that they shine by their own native light; and not by the reflected rays of the Sun. For those rays, decreasing in number in any given space as the squares of the distances increase, cannot by reflected light make objects visible at a distance so inconceivably great.

The fixed stars are, without doubt, suns to other systems. Thus they are now considered by the unanimous consent of astronomers. They may be distinguished from the planets by the twinkling of their light. The diameter of a star appears much less viewed through a good telescope, than when seen without the aid of instruments.

Not more than 1000 stars are visible to the naked eye in either hemisphere. They seem, indeed, innumerable, when, in a clear evening, we turn our eyes towards the heavens. But, in attentive observation, most of those bright spots, which appeared to be stars, vanish. They are probably reflections from minute particles of various kinds continually floating in our atmosphere. The British catalogue contains not more than about 3,000 stars, in both hemispheres; though it includes many not visible to the naked eye. By improved reflecting telescopes the number is found to be great beyond all conception. "Dr. Herschel says that, in the most crowded part of the milky way, he has had fields of view, that contained no less than 588 stars, and

* See the time it would require a courier to pass from the Earth to the Sun, Chap. I., Sec. VII.

these were continued for many minutes; so that, in a quarter of an hour, he has seen 116,000 stars pass through the field of view of a telescope of only 15' aperture; and at another time, in 41 minutes, he saw 258,000 stars, pass through the field of his telescope."*

Many stars appear single to the naked eye, which, on being viewed with a good telescope, are found to consist of two, three, or more stars. Some are denominated by Dr. Herschel insulated stars, because they seem removed from the attractive force of other stars. Such are our Sun, Arcturus, Capella, Sirius, and many others.

"A binary sidereal system, or double star, properly so called, is formed by two stars situated so near each other as to be kept together by their mutual attraction." It is, however, evident that stars may be situated, one nearly behind the other, so as to appear binary, though immensely distant.

The double star Epsilon, Boötes, is beautiful, composed of two stars, one light red, the other a fine. blue. Plate viii. Fig. 3, represents this star, as seen by telescopes of different magnifying powers.

The double star Zeta, in the constellation Hercules, is composed of two stars; the greater a beautiful bluish white, the less a fine ash color.

The star Delta, of the Swan, is binary, composed of two stars very unequal in their apparent magnitude; the larger white, the less reddish.

The pole-star is binary, composed of two stars of very unequal magnitude; the larger white, the less red. In Plate viii. Fig. 4, is represented the treble star in the left fore foot of the constellation Monoceros, one of the most beautiful objects of the kind in the heavens.

The Beta, in the constellation Lyra, or the Harp, is quadruple, white, but three of them a little inclined to red.

* We are not told his manner of counting.

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