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Meandistances Propor'n Inclinai on Periodical Revo-Sidereal Revoel from the sun. of Light of orbits to lution. lution.
& Heat. the Ecliptid
Satellites. Distance from Saturn. Periodic Revolution. Synodic Revolutn.
Satellites. Distance from Herschel. Periodic Revolution. SynodicRevolu'n.
OF THE FIXED STARS.
Those stars, which, when seen by the naked eye, or through telescopes, keep constantly in the same situation with respect to each other, are called fixed stars. They are easily distinguished from the planets by their twinkling. They appear of various magnitudes. This may arise from their different sizes, or distances, or both. Astronomers have distinguished them, from their apparent magnitudes, into six classes. The first contains those of the largest apparent size, the second those which appear next in bigness; and so on to the sixth, which includes all those that can just be seen without telescopes. Those, which can be seen only by the help of the telescope, are called telescopic stars.
The stars in the preceding table are so numerous, that it would be impossible to furnish names for them all and retain those names in the memory. To remedy this inconvenience the ancients distributed them into constellations, to which they gave the names of birds, beasts, fishes, &c. from an imaginary resemblance between the forms of the constellations, and of those animals. The stars of each constellation are numbered, according to their magnitude, by the letters of the Greek alphabet, a is the largest, B the second, ay the third, &c. This division of the heavens was very ancient ; for some of the constellations are mentioned by Homer and Hesiod, by Amos and Job.
The whole number of the constellations is 90. Of these 48 are ancient, and 42 modern ; 33 north of the Zodiac, 12 in the Zodiac, and 45 south of it. Those stars, which have never been arranged into constellations, are called unformed stars. Those, whose distance from the nearest pole is less than the latitude of the place, never set below the horizon, and are called circumpolar stars. The eircles, which they appear to describe in consequence of the earth's rotation, are called circles of perpetual apparition. Those stars, whose distance from the farthest pole, is less than the latitude of the place, never rise above the horizon. They also receive the same name ; and the circles, which they appear to describe, are called circles of perpetual occultation.
The real number of the fixed stars cannot be ascertained. Before the invention of the telescope, it was not supposed to surpass 3000. But since that event it has been found, that the greater the
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 29 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 clouds 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 apposition, 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 eclipse 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 1721 to 11 34, or nearly as 3 to 2, and hence there will be more solar than lunar eclipses, in about that ratio. But more lunar than solar eclipses are seen at any given place, because a lunar eclipse is risible 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 inFolved 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 & minutes later, than the tables predict. Hence it follows that light takes up 161 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 distance from the earth may be discovered, and the longitudes of places on the earth's surface.