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475. Vesta appears like a star of the 5th or 6th magnitude, shining with a pure steady radiance, and is the only one of the asteroids which can be discerned by the naked eye.
Juno revolves around the Sun in 4 years, 4 months, at the mean distance of 254,000,000 of miles, moving in her orbit at the rate of 41,000 miles an hour. Her diameter is estimated at 1393 miles. This would make her magnitude 183 times less than the Earth's. The light and heat which she receives from the Sun is seven times less than that received by the Earth.
The eccentricity of her orbit is so great, that her greatest distance from the Sun is nearly double her least distance; so that, when she is in her perihelion, she is nearer the Sun by 130,000,000 of miles, than when she is in her aphelion. This great eccentricity has a corresponding effect upon her rate of motion; for being so much nearer, and therefore so much more powerfully attracted by the Sun at one time than at another, she moves through that half of her orbit which is nearest the Sun, in one-half of the time that she occupies in completing the other half.
According to Schroeter, the diameter of Juno is 1425 miles; and she is surrounded by an atmosphere more dense than that of any of the other planets. Schroeter also remarks that the variation in her brilliancy is chiefly owing to certain changes in the density of her atmosphere; at the same time, he thinks it not improbable that these changes may arise from a diurnal revolution on her axis.
476. Ceres revolves about the Sun in 4 years, 7 months, at the mean distance of 263,500,000 of miles, moving in her orbit at the rate of 41,000 miles an hour. Her diameter is estimated at 1582 miles, which makes her magnitude 125 times less than the Earth's. The intensity of the light and heat which she receives from the Sun is about 7 times less than that of those received by the Earth.
Ceres shines with a ruddy color, and appears to be only about the size of a star of the 8th magnitude. Consequently she is never seen by the naked eye. She is surrounded by a species of cloudy or nebulous light, which gives her somewhat the appearance of a comet, forming, according to Schroeter, an atmosphere 675 miles in height.
Ceres, as has been said, was the first discovered of the asteroids. At her discovery, astronomers congratulated themselves upon the harmony of the system being restored. They had long wanted a planet to fill up the great void between Mars and Jupiter, in order to make the system complete in their own eyes; but the successive discoveries of
475. What is the appearance of Vesta? Juno's period? Distance from the Sun? Rate of motion? Diameter? Relative magnitude? Light and heat? Eccentricity of orbit? Effect upon her orbitual motion? 476. Ceres' period and mean distance? Rate of motion? Diameter? Relative magnitude? Light and heat? Color and apparent size? How seen? What said of her atmosphere? Her discovery?
Pallas and Juno again introduced confusion, and presented a difficulty which they were unable to solve, till Dr. Olbers suggested the idea that these small anomalous bodies were merely the fragments of a larger planet, which had been exploded by some mighty convulsion. Among the most able and decided advocates of this hypothesis, is Dr. Brewster, of Edinburgh.
477. PALLAS performs her revolution around the Sun in 4 years, 7 months, at the mean distance of 264,000,000 of miles, moving in her orbit at the rate of 41,000 miles an hour. Her diameter is estimated at 2025 miles, which is but little less than that of our Moon. It is a singular and very remarkable phenomenon in the solar system, that two planets (Ceres and Pallas), nearly of the same size, should be situated at equal distances from the Sun, revolve about him in the same period, and in orbits that intersect each other. The difference in the respective distances of Ceres and Pallas is less than a million of miles. The difference in their sidereal revolutions, according to some astronomers, is but a single day.
The calculation of the latitude and longitude of the asteroids is a labor of extreme difficulty, requiring more than 400 equations to reduce their anomalous perturbations to the true place. This arises from the want of auxiliary tables, and from the fact that the elements of the star-form planets are very imperfectly determined. Whether any of the asteroids has a rotation on its axis, remains to be ascertained.
The following Table contains most that is known of the asteroids to the present date (1855):
477. Periodic revolution of Pallas? Mean distance from the Sun? Rate of motion? Diameter? What remarkable phenomenon mentioned?
PRIMARY PLANETS-JUPITER AND SATURN.
478. JUPITER is the largest of all the planets belonging to the solar system. It may be readily distinguished from the fixed stars, by its peculiar splendor and magnitude; appearing to the naked eye almost as resplendent as Venus, although it is more than seven times her distance from the Sun.
When his right ascension is less than that of the Sun, he is our morning star, and appears in the eastern hemisphere before the Sun rises; when greater, he is our evening star, and lingers in the western hemisphere after the Sun sets.
Nothing can be easier than to trace Jupiter among the constellations of the zodiac; for in whatever constellation he is seen to-day, one year hence he will be seen equally advanced in the next constellation; two years hence, in the next; three years hence, in the next, and so on; being just a year, at a mean rate, in passing over one constellation.
The exact mean motion of Jupiter in its orbit, is about one-twelfth of a degree in a day; which amounts to only 30° 20′ 32" in a year.
For 12 years to come, he will, at a mean rate, pass through the constellations of the zodiac, as follows:
479. Jupiter is the next planet in the solar system above the asteroids, and performs his annual revolution around the Sun in nearly 12 of our years, at the mean distance of 495,000,000 of miles; moving in his orbit at the rate of 30,000 miles an hour.
The exact period of Jupiter's sidereal revolution is 11 years, 10 months, 17 days, 14 hours, 21 minutes, 25% seconds. His exact mean distance from the Sun is 495,533,837 miles; consequently, the exact rate of his motion in his orbit, is 29,943 miles per hour.
480. He revolves on an axis, which is nearly perpendicular to the plane of his orbit, in 9 hours, 55 minutes, and 50 seconds; so that his year contains 10,471 days and nights; each about 5 hours long.
His form is that of an oblate spheroid, whose polar diameter
478. Comparative size of Jupiter? How distinguished from the fixed stars? When morning star, &c.? Is he easily traced? 479. His position in the system? His periodic time? Distance from the Sun? Rate of motion? 480. Time of diurnal revolution? Position of axis? Length of his days? Number in his year? His form? Cause of his oblateness? Difference of equatorial and polar diameters? The Earth?
is to its equatorial, as 13 to 14. He is therefore considerably more flattened at the poles than any of the other planets, except Saturn. This is caused by his rapid rotation on his axis; for it is an universal law that the equatorial parts of every body, revolving on an axis, will be swollen out, in proportion to the density of the body, and the rapidity of its motion.
The difference between the polar and equatorial diameters of Jupiter, exceeds 6000 miles. The difference between the polar and equatorial diameters of the Earth, is only 26 miles. Jupiter, even on the most careless view through a good telescope, appears to be oval; the longer diameter being parallel to the direction of his belts, which are also parallel to the ecliptic.
481. By this rapid whirl on its axis, his equatorial inhabitants are carried around at the rate of 26,554 miles an hour; which is 1600 miles farther than the equatorial inhabitants of the Earth are carried, by its diurnal motion, in twenty-four hours.
The true mean diameter of Jupiter is 86,255 miles; which is nearly 11 times greater than the Earth's. His volume is, therefore, about thirteen hundred times larger than that of the Earth. (For magnitude as compared with that of the Earth, see Map I.) On account of his great distance from the Sun, the degree of light and heat which he receives from it is 27 times less than that received by the Earth.
When Jupiter is in conjunction, he rises, sets, and comes to the meridian with the Sun; but is never observed to make a transit, or pass over the Sun's disc; when in opposition, he rises when the Sun sets, sets when the Sun rises, and comes to the meridian at midnight, which never happens in the case of an interior planet. This proves that Jupiter revolves in an orbit which is exterior to that of the Earth.
482. As the variety in the seasons of a planet, and in the length of its days and nights, depends upon the inclination of its axis to the plane of its orbit, and as the axis of Jupiter has little or no inclination, there can be no difference in his seasons, on the same parallels of latitude, nor any variation in the length of his days and nights. It is not to be understood, however, that one uniform season prevails from his equator to his poles; but that the same parallels of latitude on each side of his equator, uniformly enjoy the same season, whatever season it may be. About his equatorial regions there is perpetual summer; and at his poles everlasting winter; but yet equal day and equal night at each. This arrangement seems to have been kindly ordered by the beneficent Creator; for had his axis been inclined to his orbit, like that of the Earth, his polar winters would have been alternately a dreadful night of six years' darkness.
481. Motion at Jupiter's equator? His mean diameter? His volume? Light and heat? Does he ever transit the Sun? What proof that his orbit is exterior to that of the Earth? 482. What of the seasons of Jupiter? What apparent manifestation of Divine Wisdom?
483. Jupiter, when viewed through a telescópe, appears to be surrounded by a number of luminous zones, usually termed belts, that frequently extend quite around him. These belts are parallel not only to each other, but, in general, to his equator, which is also nearly parallel to the
TELESCOPIC VIEW OF JUPITER.
ecliptic. They are
subject, however, to considerable variation, both in breath and number. Sometimes eight have been seen at once; sometimes only one, but more usually three. Dr. Herschel once perceived his whole disc covered with small belts, though they are more usually confined to within 30° of his equator, that is, to a zone 60° in width.
Sometimes these belts continue for months at a time with little or no variation, and sometimes a new belt has been seen to form in a few hours. Sometimes they are interrupted in their length; and at other times, they appear to spread in width, and run into each other, until their breadth exceeds 5000 miles.
484. Bright and dark spots are also frequently to be seen in the belts, which usually disappear with the belts themselves, though not always, for Cassini observed that one occupied the same position more than 40 years. Of the cause of these variable appearances, but little is known. They are generally supposed to be nothing more than atmospherical phenomena, resulting from, or combined with, the rapid motion of the planet upon its axis.
Different opinions have been entertained by astronomers respecting the cause of these belts and spots. By some they have been regarded as clouds, or as openings in the atmosphere of the planet, while others imagine that they are of a more permanent nature, and are the marks of great physical revolutions, which are perpetually agitating and changing the surface of the planet. The first of these opinions sufficiently explains the variations in the form and magnitude of the spots, and the parallelism of the belts.
488. How does Jupiter appear through a telescope? Where are his belts usually seen? Their number? Are they permanent? 484. What else seen upon Jupiter's surface? Are they permanent? Is the cause of these phenomena well understood? What different opinions?