Its mean sidereal revolution is performed in 686.9796458 solar days; or In 686 days, 23 hours, 30 minutes, 41.4 seconds. Its synodical revolution is performed in 779.936 solar days; or in 779 days, 22 hours, 27 minutes, and EO seconds. Its form is that of an oblate spheroid, whose polar diameter is to its equatorial, as 15 is to 16, nearly. Its mean diameter is 4222 miles. Its bulk, therefore, is 7 times less than that of the Earth; and being 50 millions of miles farther from the Sun, it receives from him only half as much light and heat. The inclination of its axis to the plane of its orbit, is about 283°. Consequently, its seasons must be very similar to those of the Earth. Indeed, the analogy between Mars and the Earth is greater than the analogy between the Earth and any other planet of the solar system. Their diurnal motion, and of course the length of their days and nights, are nearly the same; the obliquity of their ecliptics, on which the seasons depend, are not very different; and, of all the superior planets, the distance of Mars from the Sun is by far the nearest to that of the Earth; nor is the length of its year greatly different from ours, when compared with the years of Jupiter, Saturn, and Herschel. To a spectator on this planet, the Earth will appear alternately, as a morning and evening star; and will exhibit all the phases of the Moon, just as Mercury and Venus do to us; and sometimes, like them, will appear to pass over the Sun's disc like a dark round spot. Our Moon will never appear more than a quarter of a degree from the Earth, although her distance from it is 240,000 miles. If Mars be attended by a satellite, it is too small to be seen by the most powerful telescopes. When it is considered that Vesta, the smallest of the asteroids, which is once and a half times the distance of Mars from us, and only 269 miles in diameter, is perceivable in the open space, and that without the presence of a more conspicuous body to point it out, we may reasonably conclude that Mars is without a moon. The progress of Mars in the heavens, and indeed of all the superior pla nets, will, like Mercury and Venus, sometimes appear direct, sometimes retrograde, and sometimes he will seem stationary. When a superior planet first becomes visible in the morning, west of the Sun, a little after, its conjunction, its motion is direct, and also most rapid. When it is first seen east of the Sun, in the evening, soon after its opposition, its motion is retrograde. These retrograde movements and stations, as they appear to a In what time does it perform its mean sidereal revolution? In what time, its sy nodical revolution? What are its form and dimensions? What, then, is its bulk, compared with the Earth's, and how much less light and heat does it receive from the Sun? What is the inclination of its axis to the plane of its orbit? How are its seasons, compated with those of the Earth? In what particulars is there a greater analogy between Mars and the Earth, than between the Earth and any other planet in the solar system? What must be the appearance of the Earth to a spectator at Mars? What is the greatest distance from the Earth at which our Moon will appear to him to be? Why may we reasonably conclude that Mars has no satellite? Describe th" progress of Mars through the heavens. spectator from the Earth, are common to all the planets, and demonstrate the truth of the Copernican system. The telescopic phenomena of Mars afford peculiar interest to astronomers. They behold its disc diversified with numerous irregular and variable spots, and ornamented with zones and belts of varying brilliancy, that form, and disappear, by turns. Zones of intense brightness are to be seen in its polar regions, subject, however, to gradual changes. That of the southern pole is much the most brilliant. Dr. Herschel supposes that they are produced by the reflection of the Sun's light from the frozen regions, and that the melting of these masses of polar ice is the cause of the variation in their magnitude and appearance. He was the more confirmed in these opinions by observing, that after the exposure of the luminous zone about the north pole to a summer of eight months, it was considerably decreased, while that on the south pole, which had been in total darkness during eight months, had considerably increased. He observed, farther, that when this spot was most luminous, the disc of Mars did not appear exactly round, and that the bright part of its southern limb seemed to be swollen or arched out beyond the proper curve. The extraordinary height and density of the atmosphere. of Mars, are supposed to be the cause of the remarkable redness of its light. It has been found by experiment, that when a beam of white light passes through any colourless transparent medium, its colour inclines to red, in proportion to the density of the medium, and the space through which it has travelled. Thus the Sun, Moon, and stars, appear of a reddish colour What system, do these retrograde movements and stations, common to all the planets as seen from the Earth, serve to establish? What are the telescopic phenomena of Mars? How does Dr. Herschel account for them? How may the remarkable redness of the light of Mars be accounted for? when near the horizon; and every luminous object, secu 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 refran gible rays, the former will nake 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 atinospheres. 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+3X1 Venus The Earth Mars 10 Saturn 4+3X21 = 52 100 = 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.-Brink ley'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 sume 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 suck 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 remaihable coincidence that it was in the neighbourhood of In what particulars do these new planets differ from the older planets? How is it pos Bible 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? 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 explo sion; and, when found immediately after 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 re spect 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 |