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when near the horizon; and every luminous object, seen through a mist, is of a ruddy hue.
This phenomenon may be thus explained:-The momentum of the red, or least refrangible rays, being greater than that of the violet, or most refrangible rays, the former will make their way through the resisting medium, while the latter are either reflected or absorbed. The colour of the beam, therefore, when it reaches the eye, must partake of the colour of the least refrangible rays, and this colour must increase with the distance. The dim light, therefore, by which Mars is illuminated, having to pass twice through its atmosphere before it reaches the Earth, must be deprived of a great proportion of its violet rays, and consequently then be red. Dr. Brewster supposes that the difference of colour among the other planets, and even the fixed stars, is owing to the different heights and densities of their atmos. pheres.
THE ASTEROIDS, OR TELESCOPIC PLANETS.
ASCENDING higher in the solar system, we find, between the orbits of Mars and Jupiter, a cluster of four small planets, which present a variety of anomalies that distinguish them from all the older planets of the system. Their names
are Vesta, Juno, Ceres, and Pallas. They were all discovered about the beginning of the present century.
The dates of their discovery, and the names of their discoverers, are as follows:
Ceres, January 1, 1801, by M. Piazzi, of Palermo.
The scientific Bode* entertained the opinion, that the planetary distances, above Mercury, formed a geometrical series, each exterior orbit being double the distance of its next interior one, from the Sun; a fact which obtains with remarkable exactness between Jupiter, Saturn, and Herschel. But this law seemed to be interrupted between Mars and Jupiter. Hence he inferred, that there was a planet wanting in that interval; which is now happily supplied by the discovery of the four star-form planets, occupying the very space where the unexplained vacancy presented a strong objection to his theory.
4+3X23 28 4+3X21 52 4+3X25 100 4+3X23 = 196
Comparing these values with the actual mean distances of the planets from the Sun, we cannot but remark the near agreement, and can scarcely hesitate to pronounce that the respective distances of the planets from the Sun, were assigned according to a law, although we are entirely ignorant of the exact law, and of the reason for that law.-Brinkley's Elements, p. 89.
What new planets have been discovered within the present century? Where are they situated? What are the dates of their discovery, and the names of their discoverers? Why did Bode infer that there was a planet wanting between Mars and Jupiter?
These bodies are much smaller in size than the older planets-they all revolve at nearly the same distances from the Sun, and perform their revolutions in nearly the same periods, their orbits are much more eccentric, and have a much greater inclination to the ecliptic,—and what is altogether singular, except in the case of comets-all cross each other; so that there is even a possibility that two of these bodies, may, some time, in the course of their revolutions, come into collision.
(The orbit of Vesta is so eccentric, that she is sometimes farther from the Sun than either Ceres, Pallas, or Juno, although her mean distance is many millions of miles less than theirs. The orbit of Vesta crosses the orbits of all the other three, in two opposite points.
The student should here refer to the Figures, Plate I. of the Atlas, and veri fy such of these particulars as are there represented. It would be well for the teacher to require him to observe particularly the positions of their orbits, and to state their different degrees of inclination to the plane of the ecliptic.
From these and other circumstances, many eminent astronomers are of opinion, that these four planets are the fragments of a large celestial body which once revolved between Mars and Jupiter, and which burst asunder by some tremendous convulsion, or some external violence.' The discovery of Ceres by Piazzi, on the first day of the present century, drew the attention of all the astronomers of the age to that region of the sky, and every inch of it was minutely explored. The consequence was, that, in the year following, Dr. Olbers, of Bremen, announced to the world the discovery of Pallas, situated not many degrees from Ceres, and very much resembling it in size.)
From this discovery, Dr. Olbers first conceived the idea that these bodies might be the fragments of a former world; and if so, that other portions of it might be found either in the same neighbourhood, or else, having diverged from the same point, "they ought to have two common points of reunion, or two nodes in opposite regions of the heavens through which all the planetary fragments must sooner or later pass."
One of these nodes he found to be, in the constellation Virgo, and the opposite one, in the Whale; and it is a remarkable coincidence that it was in the neighbourhood of
In what particulars do these new planets differ from the older planets? How is it possible that two of them should ever come into collision? How is it that Vesta is sometimes farther from the Sun than either Ceres, Pallas, or Juno, when her mean distance is many millions of miles less than theirs? What is the position of her orbit with regard to their orbits? What theory in regard to the origin of these planets have some astronomers derived from these and some other circumstances? Who first conceived this idea? How came he to have this idea? Where did he imagine other fragments might be found? In what constellations did he find these nodes to be 7
the latter constellation that Mr. Harding discovered the planet Juno. In order therefore to detect the remaining fragments, if any existed, Dr. Olbers examined, three times every year, all the small stars in Virgo, and the Whale; and it was actually in the constellation Virgo, that he discovered the planet Vesta. Some astronomers think it not unlikely that other fragments of a similar description may hereafter be discovered. Dr. Brewster attributes the fall of meteoric stones to the smaller fragments of these bodies happening to come within the sphere of the Earth's attraction.
Meteoric stones, or what are generally termed aerolites, are stones which sometimes fall from the upper regions of the atmosphere, upon the Earth The substance of which they are composed, is, (for the most part, metallic but the ore of which it consists is not to be found in the same constituent proportions in any known substance upon the Earth. Their fall is generally preceded by a luminous appearance, a hissing noise, and a loud explosion; and, when found immediately aft their descent, they are always hot, and usually covered with a black crust, indicating a state of exterior fusion.
Their size varies from that of small fragments of inconsiderable weight, to that of the most ponderous masses. They have been found to weigh from 300 pounds to several tons; and they have descended to the Earth with a force sufficient to bury them many feet under the surface.
Some have supposed that they are projected from volcanoes in the Moon; others, that they proceed from volcanoes on the Earth; while others imagine that they are generated in the regions of the atmosphere; but the truth, probably, is not yet ascertained. In some instances, these stones have penetrated through the roofs of houses, and proved destructive to the inhabitants.
If we carefully compute the force of gravity in the Moon, we shall find, that if a body were projected from her surface with a momentum that would cause it to move at the rate of 8,200 feet in the first second of time, and in the direction of a line joining the centres of the Earth and Moon, it would not fall again to the surface of the Moon; but would become a satellite to the Earth. Such an impulse might, indeed, cause it, even after many revolutions, to fall to the Earth. The fall, therefore, of these stones, from the air, may be accounted for in this manner.
Mr. Harte calculates, that even a velocity of 6000 feet in a second, would be sufficient to carry a body projected from the surface of the Moon beyond the power of her attraction. If so, a projectile force three times greater than that of a cannon, would carry a body from the Moon beyond the point of equal attraction, and cause it to reach the Earth. A force equal_to this is often exerted by our volcanoes, and by subterranean steam. Hence, there is no impossibility in the supposition of their coming from the Moon; but yet I think the theory of aerial consolidation the more plausible.
Vesta appears, however, 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.
Where were Juno and Vesta actually found? How did Dr. Olbers discover Vesta? To what does Dr. Brewster attribute the fall of meteoric stones? What is meant by the expression, meteoric stones? Of what substance are they composed? In what respect do they differ from any metallic substances known on the Earth? What indications generally precede their fall? In what state are they found to be after their descent? What is their magnitude? What theories have been adopted to account for their origin? Explain how it is not impossible that they may come from the Moon. Describe the appearance of Vesta.
JUNO, the next planet in order after Vesta, revolves around the Sun in 4 years, 4 months, at the mean distance of 254 millions of miles, moving in her orbit at the rate of 41 thousand 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 millions 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.
CERES, the planet next in order after Juno, revolves about the Sun in 4 years, 73 months, at the mean distance of 2631 millions of miles, moving in her orbit at the rate of 41 thousand 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 colour, 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
What is the planet next in order after Vesta? In what time does she complete her revolution around the Sun? What is her mean distance from him? What the rate of her motion per hour? What is the length of her diameter? How much less, then, is her magnitude, than that of the Earth? How much light and heat does she receive from the Sun, compared with those received by the Earth? How much greater is her greatest distance from the Sun, than her least distance? How much less time does she occupy in moving through that half of her orbit which is nearest to the Sun, than she does in moving through that half which is farthest from him? What is her diameter according to Schroeter? According to the same astronomer, what is the density of her atmosphere, compared with that of the other planets? To what does he attribute the va riation in her brilliancy? What is the next planet in order after Juno? In what time does she complete her revolution about the Sun? What is her mean distance from him? What is the rate of her motion per hour? What is her diameter? How great is her magnitude, compared with that of the Earth? What is the intensity of the light and heat which she receives from the Sun, compared with that of those received by the Earth? Describe her appearance.
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. 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 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.
PALLAS, the next planet in order after Ceres, performs her revolution around the Sun in 4 years, 7 months, at the mean distance of 264 millions of miles, moving in her orbit at the rate of 41 thousand 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 labour 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.
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 splendour and magnitude; appearing to the naked eye almost as resplendent as Venus, although it is more than seven times her distance from the Sun.
How high, according to Schroeter, is the atmosphere formed by this nebulous light? Why did astronomers congratulate themselves at the discovery of this planet? What again introduced confusion and dificulty into their system? How were they at length enabled to solve the difficulty? What planet is the next in order after Ceres? In what time does she complete her revolution around the Sun? What is her mean distance from him? What is the rate of her motion in her orbit per hour? What is her diameter? How great is it compared with the diameter of the Moon? What is the differ ence between the respective distances of Ceres and Pallas from the Sun? What is the difference between the times of their sidereal revolutions? Why is the calculation of the latitude and longitude of the asteroids a labour of extreme dificulty? Have any of the asteroids rotations on their axes? Which is the largest planet of the solar system? How may Jupiter be readily distinguished from the fixed stars? How much farther is he from the Sun than Venus?