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observation, that the moon always turns the same side towards the Earth. Hence it must perform a rotation on an axis, and the time of this rotation must be equal to the time of the moon's synodic revolution, or 29 days, 12 hours, 44 minutes. Hence, also, though the lunar year is of equal length with ours, yet it contains only about 12 days, every lunar day being a little longer than 294 of our days. The side of the moon, which is towards the Earth, during its day, receives light both from the sun and from the Earth ; and, during its night, only the light of the Earth. The other side of the moon has, half of the time, the light of the sun; and the other half is in total darkness. The spots, visible on the moon, are occasioned by the mountains and vallies on its surface. These mountains were formerly supposed to be of a very great height. This, however is a mistake. The highest observed by Herschel, is 7,500 feet. Very few of the others are more than 2500 feet. It is not determined whether the moon has an atmosphere. No Glouds or vapours, however, can be discovered near its surface. When the moon is in conjunction with the sun, she is said to be new, and is then invisible : As she goes eastward she appears horned, till she gets 90 degrees from the sun, when she appears half enlightened, or dichotomized; from thence, till she comes into opposition, she appears more than half enlightened or gibbous ; and at opposition she appears full. From opposition to conjunction her apparent bright part decreases, as it before increased. Mr. Bouguer, from experiments on lunar light, concludes that 300,000 moons would not make a stronger light, than that of clear bright sunshine. The light of the moon condensed by the best mirrors produces no sensible effect upon the thermometer. The earth in the course of a month shows the same phases to the lunarians, as the moon does to us ; the earth is at the full, at the time of new moon, and new at the time of full moon. The surface of the earth being about 13 times greater than that of the moon, it affords 13 times more light to the moon, than the moon does to us.
It is remarkable, that, when the moon is full, near the middle of September, there is less difference between the times of two successive risings, than there is, when she is full at any other season of the year. By this means she affords an almost immediate supply of light, after sunset, for a whole week together, which is very beneficial at that season for gathering in the fruits of the earth. Hence this full moon is called the Harvest Moon.
Eclipses. An eclipse of the moon is caused by its entering into the earth's shadow, and consequently it must happen at the full moon, or when she is in opposition to the sun, as the shadow of the earth must lie opposite to the sun. An eclipse of the sun is caused by the interposition of the moon between the earth and sun, and therefore it must happen when the moon is in conjunction with the sun, or at the new moon.
If the plane of the moon's orbit coincided with the plane of the ecliptic, there would be an eclipse at every conjunction and opposition; but the plane of the moon's orbit being inclined to the plane of the ecliptic, there can be no eclipsc at conjunction or opposition, unless at that time the moon be at, or near, the node.
The ecliptic limits of the sun are to those of the moon, as 17 21 to 11 34, or nearly as 3 to 2, and hence there will be more golar than lunar eclipses, in about that ratio. But more lunar than golar eclipses are seen at any given place, because a lunar eclipse is visible to a whole hemisphere of the earth at once; whereas a solar eclipse is visible to a part only, and therefore there is a greater probability of seeing a lunar, than a solar eclipse. Since the moon is as long above the horizon as below, every spectator may expect to see half the number of lunar eclipses which happen.
If the earth had no atmosphere, when the moon was totally eclipsed, she would be invisible; but by the refraction of the atmosphere, some rays will be brought to fall on the moon's surface, on which account the moon is rendered visible, and of a dusky red color.
An eclipse of the moon arising from a real deprivation of light, must appear to begin at the same instant of time to every place on that hemisphere of the Earth, which is next the moon. Hence, it affords a ready method of finding the longitudes of places upon the Earth's surface.
The diameters of the sun and moon are supposed to be divided into 12 equal parts, called digits, and an eclipse is said to be so many digits, according to the number of those parts, which are inTolved at the greatest darkness.
The greatest number of eclipses, which can happen in a year, is seven, and when this happens, five will be of the sun, and two of the moon. The least number which can happen is two, and these must be both solar; for in every year there must be two solar eclipses. The mean number in a year is about four.
In a total eclipse of the sun, the planets, and some of the brightest of the fixed stars have been seen,
Jupiter's Moons. These are four in number, and were discov. ered by Galileo, Jan. 8, 1600. Their distances from the planet, periodical times, &c. may be learnt from the tables at the close of our account of the solar system. The first and third are larger than the earth : the second and fourth are considerably less than Venus, though larger than Mars. They all revolve on their axis, and also round the planet, from west to east.
The progressive motion and velocity of light was discovered by observations on the satellites of Jupiter. These satellites are eclipsed at regular intervals, and tables of the times when these eclipses are to happen, are constantly published. It is found that, when the earth is exactly between Jupiter and the sun, his satellites appear eclipsed 8 minutes sooner, than they would be according to the tables; but that, when the earth is at its greatest distance from Jupiter, these eclipses happen about 8 minutes later, than the tables predict. Hence it follows that light takes up 16.4 minutes in passing over the diameter of the earth's orbit, which is about 190 millions of miles. This is nearly at the rate of 200,000 miles a second. By means of these satellites also Jupiter's dis. tance from the earth may be discovered, and the longitudes of places on the earth's surface,
Satellites of Saturn. Of these Huygens discovered the fourth in 1665; Cassini the fifth in 1671, the third in 1672, the first and second in 1684; and Herschel the sixth in 1787, and the seventh in 1788. These last are nearer to Saturn, than the other five; but, to prevent confusion in the numbers with regard to former observations, they are called the sixth and seventh. The tables exhibit their periods and distances from their primary. The third satellite is the largest of all; the first and fourth are nearly of the same size.
Satellites of Herschel. These are six in number. The second and fourth were discovered by Herschel in 1787; and, what is entirely singular in our system, he observed, that their orbits made an angle nearly perpendicular with the ecliptic of the primary. The other four were also discovered by Herschel. The first and fifth in 1790, and the other two in 1794. Their light is extremelv faint; but the fourth is somewhat the brightest. The sixth, at its greatest distance, is farther removed from the earth than any body, if we except the comets, that is known to belong to our system. Of all the bodies hitherto described, the satellites of Herschel alone revolve from east to west, or in a retrograde direction.
Asteroids.* These bodies were entirely unknown, till the commencement of the present century. They appear of the size of stars of the 8th magnitude. It was owing to their diminutive size, that Herschel refused them a place among the planets, and gave them the name of Asteroids, though they are really primary planets, revolving round the sun.
Ceres was discovered by Joseph Piozzi, at the royal observatory at Palermo, January 1, 1801. It appears like a star of the 7th or 8th magnitude. Its diameter is estimated by Dr. Herschel at 160 miles, but this cannot be relied on as exact. All the asteriods are too small to be measured with precision. Their orbits are all between those of Mars and Jupiter. Ceres revolves in 4 years, 7 months, 10 days. Its mean distance from the sun is 263,663,000 miles.
Pallas was discovered by Dr. Olbers of Bremen, March 28, 1802. It appears sometimes like a star of the 7th magnitude, and some. times considerably less. Its diameter is 110 miles. Its periodical revolution is 4 years, 7 months, 11 days; and its distance from the sun 267,438,000 miles. The orbits of Ceres and Pallas are said to cross each other.
Juno was discovered by Mr. Harding, at Lilienthal, near Bremen, September 16t. 1804. It appears like a star of the 8th magnitude. Its periodical revolution is a little longer than those of Ceres and Pallas, Its diameter is 119 miles. Its distance from the sun is 286,541,000 miles.
Vesta was discovered by Dr. Olbers, March 29, 1807. It may be seen by the naked eye, like a star of the fifth or sixth magnitude, and very much like the planet Herschel. The angle which its diameter subtends, is about half a second. Its periodical revo
* From usne star, and sudos appearance,
lution is 3 years, 2 months, 5 days, and its mean distance, 206,596,000 miles. These elements all require to be corrected by future obr. servations.
TABLE OF ASTEROIDS.
Names. When discovered. Periodical Distance from Inclination Eccent
time. The sun. of the Orbit.tricity.
y. m. d. Vesta March 29, 1807 3 2 5206,596,000 7 8 10.095 Ceres January 1, 180114 og 10.263,663,0001 10 37 10.097 Pallas March 28, 1802 4 7 11 267,438,000 34 40 10.246
longer than Juno Septem. 1, 1804|the two last. 286,541,000 21 0.25
Thus, of the 30 bodies, beside the comets, belonging to our system, only eight were known to the ancients; viz, the Sun, Mercury, Venus, the Earth, the Moon, Mars, Jupiter, and Saturn. Of the remaining 22, 4 were discovered in the 16th century; viz. Jupiter's 4 moons, by Galileo: 5 in the 17th century; viz. Saturn's fourth moon by Huygens; and his first, second, third, and fifth, by Cassini : 9 in the 18th century; viz. Saturn's sixth and seventh moons, the planet Herschel, and his six moons, all by Dr. Herschel : and four already in the 19th ; viz. Ceres, by Piozzi; Pallas, by Olbers; Juno, by Harding : and Vesta, by Olbers.
Comets. Comets are bodies revolving in very eccentric ellipses about the sun in one of the foci. When a comet is west of the sun, and moving towards it, it is said to be tailed ; because a train of light follows it, in manner of a tail. When the sun and the comet are on opposite sides of the earth, the train is principally hid behind the body of the comet, and the little that appears has the form of a border of hair, or corna, whence it is called hairy ; and whence the name cornet is derived. The substance of the bodies of comets must be extremely solid, or they would be dissipated in their perihelion, or nearest approach to the sun. According to Sir Isaac Newton, the comet of 1680 endured a heat 28,000 times as great as that of the sun, in midsummer; or about 9,000 times as great as the heat of boiling water; or 2000 times as great as the heat of red hot iron. Little is ascertained respecting the real magnitudes of comets. Their apparent magnitudes are also very various. That which appeared in the time of Nero, was, as Seneca relates, apparently as large as the sun ; and that of 1652, according to Hevelius, did not seem to be less than the moon, though of a very pale, dim light.
The number of comets belonging to our system has never been ascertained. Conjecture has limited it to 450. The elements of 97 of them have been determined with some degree of accuracy. The angles, which their orbits made with the plane of the ecliptic, were found to vary from 1 to 88 degrees. The perihelion distance of the comet of 1351, was just equal to the earth's mean distance. The perihelion distance of 24 of the others, was greater than this, and of the remaining 72, less. The least distance of the comet of 1680, was only 122,000 miles from the surface of the sun ; while its greatest distance was 12,189,000,000 miles. The perihelion distance of the comet of 1759 is about 52,000,000 miles ; its aphe. lion distance 3,342,500,000. These are the only two comets whose periods are known, That of the latter is about 76 years. It appeared in 1759, 1682, 1607, 1531, and 1456 ; and will probably reappear in 1835. The period of the former is 575 years. It appeared in 1680, 1106, 531, and in 44, before Christ, and probably will not re-appear, till 2255. There is also strong reason to conelude, that the comet of 1264 was the same with that of 1556. If so, its period is 292 years; and it ought to appear again in 1848. Dr. Halley imagined, that the comet of 1661 was the same with that of 1532; and that its period was 129 years; but in 1790, it was found to have violated its engagements. Dr. Halley had the honor first to foretel the return of a comet. It was the comet of 1759. The velocity of a comet increases as it approaches the sun. That of 1680, in its perihelion, moved with the amazing velocity of 880,000 miles an hour. The comet of 1744, had a tail of the length of 23,000,000 of miles; and that of 1759, of more than 40,000,000. The orbits of comets make very different angles with the plane of the ecliptic: 50 out of the 97, whose elements have been calculated, had a direct motion, or from west to east; and 47 from east to west. The comet of 1680, on the 11th November, at 1 hour, 6 minutes, P. M. was only 4000 miles north of the orbit of the earth. If the earth at that time, had been in the part of its orbit nearest to the comet, their mutual gravitation must have caused a change in the plane of the earth's orbit, and in the length of our year.