solute length both to the earth and moon, though very different as to the number of days; we having 365 natural days, and the Lunarians only 12, every day and night in the moon being as long as 29 on the earth. Mars is not so bright as Venus, nor even as Jupiter, though nearer to the sun. Its colour is a little reddish. Some spots have been observed upon its surface, from which its rotation round its axis, and the inclina tion of its axis to the plane of its orbit, have been determined. This planet some times appears gibbous, but never horned, like the moon, which shews that his orbit includes that of the earth, and that he shines by a borrowed light. Ceres Ferdinandea is a very small planet, situated next without Mars: it was discovered on the first day of the present century by Mr. Piazzi, an Italian astronomer. Pallas is another very small planet, discovered by Dr. Olbers of Bremen, on the 28th of March, 1802. Two others have also been discovered, one by M. Harding, and the other by a pupil of Dr. Olbers. To these have been given the names of Juno and Vesta. These planets Dr. Herschel proposes to call asteroids, because they are so much smaller than any of the other planets. Jupiter is the brightest planet next to Venus. When viewed by a telescope, seve. ral belts are observed across its disc, paral. lel to its equator: these belts are variable, and are supposed to be ranges of clouds in the atmosphere of the planet. Jupiter is surrounded by four moons of different sizes, which move about it in different times. These moons are sometimes eclipsed by the shadow of Jupiter falling upon them; and the eclipses have been found of great use in de. termining the longitudes of different places on the earth: Ex. Suppose two observers of an eclipse, one at London, the other at the Cape of Good Hope, the eclipse will appear at the same instant of time to both; but being situated under different meridians, they count different hours, according to which the difference of their longitude is found. Thus, if an emersion of a satellite is observed at London 9h 33′ 12′′, and at another place 10h 46′ 45′′, the difference of time is 1h 13 33', of course that other place is 18° 23' 15" east of London. The eclipses of Jupiter's satellites have been applied also to measure the velocity of light: by comparing the times of the apparent entrance and emersion of the satellites with tables calculated for the mean distances of the earth from the satellite, the visible emersion at the least distance is found to happen about eight minutes sooner; and at the greatest distance about eight minutes later than the tables: consequently a ray of light is about 16 minutes in passing through the earth's orbit, or eight minutes in coming from the sun to the earth. If therefore the distance be 95,000,000 of miles, the velocity of light per second is 95,000,000 198 thousand miles in 8 × 60 a second nearly. equal to Saturn can hardly be seen by the naked eye. When examined by a telescope, it exhibits a very remarkable appearance. It is surrounded by a thin, flat, broad luminous ring, which surrounds the body of the planet, but does not touch it. This ring casts a strong shadow upon the planet, and is divided into two, by a distinct line in the middle of its breadth. The rings are circular, but appear elliptical from being viewed obliquely. According to Dr. Herschel, the dimensions of the rings, and the space between, are as follows: Besides this ring, Saturn has seven moons of different sizes, and its body is surrounded also by belts, like those of Jupiter. The Herschel planet, with its six satel lites, have been entirely discovered by Dr. Herschel. It cannot be seen without a te lescope, but it does not require a powerfal one. The satellites cannot be seen without the most powerful telescopes. We shall subjoin in the opposite page a table which will contain a number of particulars relating to the planets, that will be found of great utility to the reader. DR. MASKELYNE'S VIEW OF THE PLANETARY SYSTEM FOR 1801, DEC. 1. sun. More accuApparent Mean diMean distances rate propormean diame-ameters Mean diame- from the sun in tional numters, as seen as scenters in English round numbers bers of the from the from the miles. of miles. preceding earth. mean distances. * Of the small planetary bodies, Juno and Vesta, enough is not yet known to reduce the facts to a tabular form. OF COMETS. Besides these planets already mentioned, there are some other bodies which revolve round the sun, called comets. They move in very eccentric ellipses, and their periods of revolution are so long, and so uncertainly known, that few are ever observed twice. They are only seen by us when they are in that part of their orbit which is nearest to the sun, and then they move so fast, that they soon become again invisible to us. The number of comets is unknown; numbers of small ones have been discovered by telescopes. Their distances are inconceivably great, and most of them move entirely beyond the planetary orbits; though some have descended below Mars. Their appearances are very different. Some appear only a faint vapour; others have a nucleus or solid part in the middle. When they approach the sun,they put forth the appearance of a beard or tail of luminous matter, which is sometimes of astonishing length. These tails are always directed from the There are three comets, viz. of 1680, 1744, and 1759, that deserve to have a farther account given of them. The comet of 1680 was remarkable for its near approach to the sun; so near, that in its perihelion it was not above a sixth part of the diameter of the luminary from the surface thereof. The tail, like that of other comets, increased in length and brightness as it came nearer to the sun; and grew shorter and fainter as it went farther from him and from the earth, sun. till that and the comet were too far off to be any longer visible. The comet of 1744 was first seen at Lausanne in Switzerland, December 13, 1743, N. S. From that time it increased in brightness and magnitude as it was coming nearer to the sun. Its diameter, when at the distance of the sun from us, measured about one minute, which brings it out equal to three times the diameter of the earth. It came so near Mercu the earth. He says, that in the evening of January 23, this comet appeared exceedingly distinct and bright, and the diameter of its nucleus nearly equal to that of Jupiter. Its tail extended above 16 degrees from its body; and was in length, supposing the sun's parallax 10", no less than 33 millions of miles. Dr. Bevis, in the month of May, 1744, made four observations of Mercury, and found the places of that planet, calculated from correct tables, differed so little from the places observed, as to shew that the comet had no influence upon Mercury's motion. The nucleus, which had before been always round, on the 10th of February appeared oblong, in the direction of the tail, and seemed divid. ed into two parts, by a black stroke in the middle. One of the parts had a sort of beard brighter than the tail; this beard was surrounded by two unequal dark strokes, that separated the beard from the hair of the comet. The odd phenomena disappeared the next day, and nothing was seen but irregular obscure spaces like smoke in the middle of the tail; and the head resumed its natural form. February 15, the tail was divided into two branches; the eastern part about seven or eight degrees long, the western 24. On the 23d, the tail began to be bent; it showed no tail till it was as near to the sun as the orbit of Mars; the tail grew longer as it approached nearer the sun; and at its greatest length was computed to equal a third part of the dis ap tance of the earth from the sun. The comet of 1759 did not make any considerable pearance by reason of the unfavourable situation of the earth all the time its tail might otherwise have been conspicuous; the comet being then too near the sun to be seen by us; but deserves our particular consideration, as it was the first that ever had its return foretold. With respect to the real nature and use of the comets in the OF THE FIXED STARS. ry, that if its attraction had been propor- system, we are entirely unacquainted. they are observed not to change their see them all at one view. The whole number of stars visible to the naked eye is about 3186. But seldom above one-third of that number can be seen at one time. From the earliest ages they have been divided into groups, or constellations, which have been called by the names of various animals and objects, from a supposed resemblance to them; such as the Great Bear, the Little Bear, the Swan, &c. The fixed stars are placed at a distance from us so great, that it cannot be ascertained by any means yet known hence, they must shine by their own light, and not by the light which they receive from our sun, as the planets do. Though it has been formerly mentioned that the relative situations of the fixed stars do not vary, yet in the course of several ages, some variations have been observed among them. Some of the larger stars have not the same precise situations that ancient observations attribute to them, and new stars have appeared, while some others which have been described, are now no longer to be found. Some stars are likewise found to have a periodical increase and decrease. Many of the fixed stars, upon examination with the telescope, are found to consist of two. Besides the phenomena already mentioned, there are many nebulæ, or parts of the heavens, which are brighter than the rest. The most remarkable of these is a broad irregular zone or belt, called the Milky-way. There are others much smaller, and some so small, that they can be seen only by telescopes. If the telescope be directed to these nebula, they are resolvable into clusters of stars, which appear as white clouds in instruments of less force. Dr. Herschell has rendered it highly probable, both from observation and well-grounded conjecture, that the starry heavens is replete with these nebulæ or systems of stars, and that the Milky-way is that particular nebula in which our sun is placed. Reasoning analogically from the circumstances with which we are acquainted, we may deduce, that the universe consists of nebulæ or distinct systems of stars: that each nebula is composed of a prodigious number of suns or bodies that shine by their own native splendour; and that each individual sun is destined to give light to numbers of worlds that revolve about it. What an august, what an amazing conception does this give of the works of the Creator! Instead of one world and one sun, we find thousands and thousands of suns, ranged around us at immense distances, all attended by innumerable worlds, all in rapid motion, yet calm, regular, and harmonious, invariably keeping the paths prescribed them; and these worlds peopled with myriads of intelligent beings, formed for endless progression in perfection and felicity. We shall now, in the form of a table, give the names of the constellations, and the number of stars observed in each by different astronomers. Several stars observed by the ancients are now no more to be seen, but are destroyed; and new ones have appeared which were unknown to the ancients. Some of them have also disappeared for some time, and again become visible. We are also assured from the observations of astronomers, that some stars have been observed which never were seen before, and for a certain time they have distinguished themselves by their superlative lustre; but after wards decreasing, they vanished by degrees, and were no more to be observed. One of these stars being first seen and observed by Hipparchus, the chief of the ancient astronomers, set him upon composing a catalogue of the fixed stars, that by it posterity might learn whether any of the stars perish, and others are produced afresh. After several ages, another new star appeared to Tycho Brahie and the astronomers who were contemporary with him; which put him on the same design with Hipparchus, namely, the making a catalogue of the fixed stars. |