PLANETS

From Agepedia

PLANETS (from nXavau, to wander); moving stars, which shine by reflecting the !ight of the sun, around which they re volve. Homer and Hesiod were already acquainted with Venus, but considered th" morning and evening stars as two different bodies. Democritus supposed that there were several planets. Pythagoras discovered the identity of the morning and evening stars; and, in the fourth year before Christ, Eudoxus brought the knowledge of the motions of the five planets then known, from the Egyptians to the Greeks. In addition to these five planets, Mercury, Venus, Mars, Jupiter and Saturn, five others have been discovered in modem times: Herschel (Georgium Sidus, or Uranus), Ceres, Pallas, Juno, and Vesta; so that, including the earth and moon, there are now known eleven primary and eighteen secondary planets (satellites, or moons). Like the earth, many of them, if hot all, have the motion of rotation on their axis, whence arise day and night, and a common motion around the sun, around which they revolve from west to east, through south, in elliptical orbits, generally making a small angle with the ecliptic, in different times, depending on their distances from the sun. The planet nearest the sun is Mercury, though thirtyseven millions of miles distant from it. It completes its revolution around the sun in eightyeight days, moving with a velocity of 315 miles a second. It is the smallest of the six old planets, its bulk being only one eighteenth of that of the earth. Its time of rotation on its axis is twentyfour hours five and a half minutes, and its eccentricity is much greater than that of either of the other five old planets, or of Uranus. Next to Mercury is placed Venus, at a distance of sixtyeight millions of miles from the sun, around which it revolves in 225 days, having a mean veloci ty of 21 miles a second. It turns on its axis in twrentythree hours twentyone minutes, as is known from observation of the spots on its surface. Mountains have also been observed in it, the height of some of which is computed to exceed eighteen miles. Seen from the earth, Venus and Mercury exhibit phases similar to those of the moon, sometimes appearing nearly full, sometimes half illuminated, or in the form of a crescent, and sometimes becoming invisible by turning to us the dark side. In size, Venus is nearly equal to the earth, and in her perigee approaches it within 27,600,000 miles, though in her apogee, she may recede 165,b'00,000 miles from it. We have no certain knowledge of a moon belonging to Venus ; tht, supposed discovery of one seems to have been founded on a mistake. Mercury and Ve nus appear, at times, like black spots passing across the face of the sun, whenever, in their motion in their orbits, like the moon in solar eclipses, they enter the plane of the ecliptic within a few hours of their inferior conjunction. This phenomenon is called a transit of Mercury or Venus. A transit of the latter planet is of rare occurrence, two only taking place in about 120 years. Those of Mercury are much more frequent. The next transit of Venus will take place in 1874; the next of Mercury, May 5 of the present year, 1832, and May 7,1835, both of which will be visible in the U. States. These two planets, which are nearer to the sun than the earth, are called the inferior planets, and those more distant are called the superior. Next in order to the earth (q. v.) and its moon (q. v.) is Mars, 143 million miles distant from the sun. In its orbit, which it accomplishes in one year and 322 days, it moves with a velocity of fifteen miles a second. It is flattened at the poles about one sixteenth of its diameter, and turns once in twentyfour hours thirtynine minutes on its axis, which is inclined to the plane of its orbit at an angle of sixtyone degrees. The surface of Mars is about one fourth that of the earth, and, his density being less, the quantity of matter is only one seventh. Spots and belts are often observed on Mars; from which it is conjectured that it has a dense atmosphere. Between Mars and Jupiter there is a great distance, which led to the supposition that there was some body between them; and this conjecture was verified, in the beginning of this century, by the discovery of four new planets. January 1, 1801, Piazzi (q.v.),at Palermo, discovered Ceres, which, at a distance of 263 million miles from the sun, completes its revolution in four years seven months, moving with a mean velocity of 11£ miles a second. On account of its small size, it is not visible to the naked eye, and, viewed through a telescope, has the appearance of a star of the seventh magnitude. This discovery was followed, Marfch 28, 1802, by that of Pallas by Olbers, at Bremen. It is about the same distance from the sun, and accomplishes its revolution in about the same time as Ceres. It is supposed to be rather larger than either Vesta, Juno or Ceres. This planet is distinguished from every other by the great inclination of its orbit to the ecliptic. Juno, which revolves around the sun in four years and four months, commonly appears like astar of the eighth magnitude, and was discovered September 1, 1804, by Harding, at Lilienthal. Finally, March 29, 1807, Olbers discovered Vesta, which appears of the fifth to the seventh magnitude, is 225 million of miles from the sun, and com pletes its revolution around the sun in three years and eight months. Jupiter, the largest of the known planets, at a distance of 490 million miles from the sun, accomplishes its revolution, at the rate of seven miles a second, in eleven years and 314 days, and is attended by four moons, which were discovered by Galilei, at Florence, January 7, 1610, and the largest of which has a diameter nearly equal to the semidiameter of the earth. The diameter of Jupiter itself is 11J times greater than the diameter of the earth ; its surface is 118 times, and its bulk 1281 times greater than that of the earth. In nine hours fiftysix minutes it revolves on its axis, which is inclined at an angle of eightyseven ^degrees to its orbit, and at the poles it is flattened one fourteenth of its diameter. On the surface of this planet belts parallel to the equator are usually observed. At nearly twice the distance of Jupiter, or 900 million miles from the sun, Saturn passes through its orbit, 5760 million miles in length, in twentynine years and 169 days, accompanied by seven moons (of which five were discovered in the seventeenth century by Huygens and Cassini, two in 1789 by Herschel), and by a very remarkable double ring, which is 21,000 miles from the surface of the planet, and 27,000 miles in breadth ; and the interval between them is about 3000 miles. According to Herschel, this ring completes its rotation in ten hours thirty minutes, while that of the planet itself is ten hours eighteen minutes. Finally, the knowledge of our solar system was enlarged, March 13, 1781, by Herschel's discovery of theGeorgium Sidus (Herschel, Uranus), which is 1800 million miles distant from the sun, and, accompanied by six satellites, accomplishes its revolution in eightyfour years nine days, at the rate of about four miles a second. Its surface is nineteen times larger than the earth's, but so much less solid, that its quantity of matter is only 17h times greater. To render the vast distances from the planets to the sun more comprehensible, an illustration, addressed to the senses, is often drawn from the velocity of a cannon ball, moving at the rate of eight miles a minute. With this velocity a cannon ball would go from the sun to Mercury in nine and a half years, to Venus in eighteen, to the earth in twentyfive, to Mars in thirtyeight, to Vesta in sixty, to Juno in sixtysix, to Ceres and Pallas in sixtynine, to Jupiter in 130, to Saturn in 238, and to Uranus or Herschel in 479, while it would go from the earth to the moon in twentythree days.