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on its axis. It is not ascertained whether it has an atmosphere.
Mercury. Mercury is the smallest of the planets. It is 3224 miles in diameter, and 36,583,825 miles from the sun. Its bulk is to that of the earth, nearly as 1 to 15; and its weight, as 0.165 to 1. A body weighing 1 pound on the earth, would weigh 1.03 pounds on Mercury. It is not known whether it revolves on its axis ; yet, as all the other planets do, it is naturally concluded that this does also. It revolves round the sun in 87 days, 23 hours; or little less than 3 months. It emits a very bright, white light. Mercury can be seen only a few days at a time. It is visible in the evening about the eastern elongation. It then disappears about 6 or 7 weeks, after which time it may be seen in the morning, rising before the sun. In about 10 weeks, it reappears in the west, setting after the sun. It has no'moon, nor any spots on its surface. Its hourly motion in its orbit is 111,000 miles. The heat near the poles of Mercury is not probably greater than that of the torrid zone. Near its equator, .water would continually boil, and most inflammable substances would be parched up, destroyed, or converted into vapor.
Venus. This is the most beautiful of the celestial luminaries, and the only star that is ever visible in the day time. This happens once in about 8 years ; when the planet is at its greatest north latitude, and near its farthest distance from the sun. Venus is 7687 miles in diameter, and its mean distance from the sun is 68, 368,008 miles. Its bulk, compared with that of the earth, is nearly as 8 to 9; and its weight, as 0.89 to 1. A body weighing 1 pound on the earth would weigh 0.98 pounds in Venus Its diurnal rotation on its axis is performed in 23 hours, 22 minutes, and it moves in its orbit 81,000 miles an hour. When Venus appears to the west of the sun, it rises before him in the morning, and is called the morning star; and when it appears to the east of the sun, it shines in the evening, after the sun sets, and is called the evening star; being in each situation, alternately, about 290 days. The axis of Venus is inclined 75 degrees towards the plane of its orbit.
Mercury and Venus are inferior planets. They are called inferior planets in relation to the earth; because they are below the Earth ; that is nearer to the sun or centre of the system. Mars, Jupiter, Saturn, and Herschel are called superior planets, because they are above the earth ; that is, farther from the centre of the system.
Mercury never appears more than 28 20 from the sun, nor Venus more than 47 48. Of course, they and the sun are never in opposition, i. e. on opposite sides of the earth. They have both, however, an inferior conjunction, when they pass between the earth and the sun; and a superior conjunction, when they pass behind the sun. In their inferior conjunctions, they sometimes pass directly over the sun's disc. This passage is called a transit. ' In their transits, they appear like small, round, black spots, moving rapidly over the face of the sun. This appearance proves them to be opaque bodies. The transits of Venus are not so frequent as those of Mercury, The last transit of Venus was in 1769; the next will be in 1874,
The last of Mercury was in 1815, and the next will be in 1822. The greatest heat on the planet Venus probably exceeds the heat of the torrid zone about as much, as that exceeds the average heat of 60 degrees north latitude.
The Earth. The Earth is a spherical body. This is obvious from the following considerations: First, From analogy; as all the other planets and heavenly bodies are spherical. Secondly, To people on shore, the mast of a ship appears before the hull; but, were the earth a plane, the hull would appear long before the mast, by reason of the much greater angle which it subtends. Thirdly, The earth has been sailed round by Magellan, Drake, Dampier, Anson, Cook, and many others. Fourthly, The boundary of the earth's shadow upon the moon, in a lunar eclipse, is always circular; and nothing but a spherical body can, in all situations, produce a circular shadow. The unevennesses of the earth's surface have no effect upon its shadow on the moon; for the height of the highest mountain on the globe, is only equal to 1500th part of the earth's diameter.
The earth is not a perfect sphere, but an oblate spheroid ; that is, its equatorial diameter is longer than its axis. The difference of these diameters is about 34 miles. The mean diameter of the earth, or the diameter in latitude 45 degrees, is 7928 miles.
of course, the equatorial diameter is 7945 miles, and the length of the earth's axis is 7911. The equatorial circumference of the earth is about 24,970 miles; its mean circumference, in latitude 45 degrees, 24,917; and its meridional circumference, 24,863. The number square miles on the earth's surface is 197,153,101 ; and 260,909,292,265 is the number of cubic miles contained in the earth. It performs a rotation on its axis once in 24 hours.
The earth is surrounded with a thin, invisible, elastic fluid, called air, the whole body of which forms what is called the atmosphere. The density of the air is not always the same, it being subject to be expanded by heat and contracted by cold. In its mean state it is found to be about 850 times lighter than water.
Notwithstanding the seeming inequality in the distribution of light and darkness, it is certain, that, throughout the whole world, there is nearly an equal proportion of light diffused on every part, if we disregard what is absorbed by clouds, yapors, and the atmosphere itself. The equatorial regions have indeed the most intense light during the day, but the nights are long and dark; while on the other hand, in the northerly and southeriy parts, though the sun shines less powerfully, yet the length of time that it appears above the horizon, with the longer duration of twilight, makes up for the seeming deficiency.
Mars. The diameter of Mars is 4189 miles, and its mean distance from the sun is 144,000,023 miles. Its annual revolution occupies 1 year, 321 days, 23 hours, 31 minutes, and its rotation on its axis 24 hours, 39 minutes, 22 seconds. It moves in its orbit at the rate of 36,000 miles an hour. Its bulk, compared with that of the earth, is as 7 to 21; and its density, as 7 to 10. One pound tou the earth would weigh 0.31 in this planet. Mars is of a fiery
red colour. By the telescope, dark spots are discoverable on its surface; but round its poles, particularly the southern, an intense and permanent brightness. Mars is an oblate spheroid. Its axis is to its equatorial diameter, as 98 to 103. It has an atmosphere of considerable extent.
Jupiter. Jupiter, the largest of the planets, is 89,170 miles in diameter, and 491,702,301 miles from the sun. Its bulk, compared with that of the earth is nearly as 1400 to 1; its density as 5 to 22; and its weight as 312 to 1. "One pound on the earth would weigh 2.33lbs. in Jupiter. Its shape is that of an oblate spheriod. Its polar diameter is to that of its equatorial, as 12 to 13; and the difference of their lengths, is upwards of 6000 miles. Its ecliptic and equator are nearly coincident; that is, its axis is nearly perpendicular to the plane of its orbit. Hence this planet has no sensible change of seasons. If its axis were inclined any considerable number of degrees towards the plane of its orbit, just so many degrees round each pole would, in their turn, be almost six years together in total darkness. It revolves on its axis in 9 hours, 55 min. utes; and round the sun in 11 years, 314 days, 18 hours, 45 minutes. Its hourly motion in its orbit is 30,000 miles. From a comparison of the most ancient, with the modern observations, there is some reason to conclude, that the period of its revolution is decreasing. Jupiter is surrounded by faint substances, called belts. These were discovered in 1665. They are parallel to each other, and to the equator of the planet. The quantity of light and heat enjoyed by Jupiter, is to that enjoyed by the Earth, as 37 to 1000.
Saturn. The diameter of Saturn is 79,042 miles, and its distance from the sun is 901,668,908 miles. Its bulk is proportioned to that of the earth nearly, as 1000 to 1. Its density, as 26 to 288, and its weight, as 98 to 1. A body weighing llb. on the earth, would weigh 1.02 on this planet. It is an oblate spheriod, its axis being to its equatorial diameter, as 10 to 11. It revolves on its axis in 10 hours, 16 minutes, 2 seconds, and round the sun in 29 years, 164 days, 7 hours, 21 minutes. Its hourly motion in its orbit is about 22,000 miles. The intensity of the sun's light and heat, is about 95 times greater at the Earth, than at Saturn. This planet has belts discoverable on its disc; but they are not so large or numerous as the belts of Jupiter. The most remarkable appearance, however, is a large ring, entirely separated from the planet itself, and yet completely surrounding it. The plane of the ring coincides with the plane of Saturn's equator, so that the axis of the planet makes a right angle with it. When the outer edge of the ring is turned towards the earth, it is invisible, except with telescopes of very great powers; either on account of its thinness, or of its almost total incapacity to reflect light. The ring is double, or is composed of two rings, having the same plane and the same centre. The outside diameter of the larger ring is 204,883 miles, and its inner diameter 190,248 miles; so that the breadth is 7318 miles, The outside diameter of the smaller ring is 184,393 miles, its inner diameter 146,345, and its breadth 19,024. The space between the rings is 2,977 miles. There is no visible connection
between the two rings. They both, however, revolve on a common axis, in 10 hours, 32 minutes, 15 seconds ; a period longer than that of Saturn's rotation by 16 minutes, 13 seconds. The ring is doubtless no less solid than the planet; and it is observed to cast a strong shadow upon it. Its light is also generally brighter than that of the planet. The thickness of the ring is probably less than 1000 miles, and its outer edge is not flat, but spherical. As the planet revolves round the sun, the plane of the ring is always parallel with itself, so that in each Saturnian year, it is twice turned edgewise towards the sun.
Herschei. This planet is called in England Georgium sidus, on the continent of Europe, Uranius, and generally, in this country, Herschel. There is no reason to believe that it had ever been observed by any inhabitant of the Earth before the 13th of March, 1781, when it was discovered by Dr. Herschel. Its diameter is 35,112 miles, and its distance from the sun, is 1,803,534,392. Its hourly motion in its orbit is 16,000 miles. Its bulk, compared with that of the Earth, is nearly as 90 to 1, and its weight as 16.84 to 1. A body on the Earth weighing llb. would weigh 0.93lb. in this planet. The period of its revolution round the sun is 83 years, 150 days, 18 hours. It has not yet been determined whether it revolves on an axis. Yet there can be no doubt of this fact, as its shape is that of an oblate spheroid. The quantity of light and heat, communicated to the Earth by the sun, is at least 360 times as great, as that enjoyed by Herschel; and the diameter of the sun, as seen from it, is not more than twice the apparent diameter of the planet Venus, as seen from the Earth. The plane of its orbit is nearly coincident with the plane of the ecliptic. Owing to its immense distance few discoveries have been made respecting it.
Satellites. A satellite, or moon, is a body revolving round a planet, and, in company with the planet, round the sun. Of these there are 18 in our system, distributed in the following manner: 1 to the Earth ; 4 to Jupiter; 7 to Saturn; and 6 to Herschel.
The Moon. The moon's diameter is 2180 miles. This is to the diameter of the Earth nearly as 20 to 73. Its surface is to that of the Earth as 1 to 13}; its bulk as 1 to 49; its density as 5 to 4 nearly; and its weight as 1 to 39. Its mean distance from the Earth is 239,029 miles, which is to the sun's mean distance nearly as 1 to 390. The angle which its orbit makes with the ecliptic varies from 5 degrees to 5 18. The moon revolves round the Earth in 27 days, 7 hours, 43 minutes. The interval of time between one new moon and the next, is 29 days, 12 hours, 44 minutes. If the Earth stood still, or had no revolution round the sun, every month would be of the former length; but as the Earth, during a lunary revolution, materially alters its place, it takes the moon 2 days 5 hours to regain what it has lost by the earth's motion. The moon's orbit, to a spectator on the sun, always appears concave. In different parts of its orbit the apparent size of the moon is found to vary. This is owing to the elliptical shape of the orbit. It is found hy
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 12į days, every lunar day being a little longer than 29of 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 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 eclipse at conjunction or opposition, unless at that time the moon be at, or near, the node.