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OF MARS. <3
Marx is the least bright and elegant of all the planets; its orbit lies between that of the Earth and Jupiter; but very distant from both. He appears of a dusky reddish hue; from the dulness of his appearance, many have conjectured that he is encompassed with a thick cloudy atmosphere; his light is not near so bright as that of Venus, though he is some,-times nearly equal to her in size,
Mars, which appears so inconsiderable in the heavens, is 5,30f) miles in diameter. Its distance from the sun is 146,000,000 miles. It goes round the sun in one year, 321 days, 23 hours; moving at the rate of 55,287 miles per hour. It revolves round jts a- is in about 24 hours, 40 minutes. To an inhabitant in Mars, the sun would appear one-third less in diameter than it does to us. Its apparent diameter, as viewed at a mean distance from the earth, in 3O seconds.
Mars, when in opposition to the sun$ is five times
nearer to us than when in conjunction. This has a
very visible effect on the appearance of the planet,
• causing him to appear much larger at some periods
Jhan at others.
The analogy between Mars and the earth is by far the greatest in the whole solar system; their jdiurnal motion is nearly the same; the obliquities of their respective ecliptics not very different. Of all the superior planets, that of Mars is by far the nearest Hke the earth; nor will the Martial year appear so dissimilar to our's, when we compare it with the long duration of the years of Jupiter, Saturn, and the Georgium Sidus^ It probably has a considerable atmosphere; for besides the permanent spots on its surface, Dr. Herschel has often perceived occasional changes of partial bright belts, and also once a darkish o:ie in a pretty high latitude; alterations which we can attribute to no other cause, than the variable disposition of clouds and vapours floating in the atmosphere of the planet.
A spectator in Mars- wiil rarely, if ever, see .Mercury, except when he sees it passing over the sun's disc. Venus will appear to him at about the same distance from the sun, as Mercury appears to Usl The earth will appear about the size of Venus, and never above 48 degrees from the sun; and will be^ by turns, a morning and evening star to the inhabitants of Mars. It appears, from the most accurate observations, that Mars is a spheroid, or flatted sphere, the equatorial diameter to the polar being in the proportion of about 131 to 12/; and there is reason to suppose that all the planets are of this figure.
or Jupiter. If.
Jupiter is situated still further in the system, revolving round the sun between Mars and Saturn. It is the largest of all the planets, and easily distinguished from them by his peculiar magnitude and light. To the naked eye it appears almost as large as Venus, but not. altogether so bright.
Jupiter revolves round its axis in nine hours, 5(> minutes; its revolution in its orbit to the same point «f the ecliptic, is 11 years, 314 days, 10 hours. The disproportion of Jupiter fo the earth, in size, is very great; viewing him in the heavens, we consider him as small in magnitude; whereas he is in reality 90,228 miles in diameter; his distance from the sun is 494,750,000 miles; he moves at the rate of rather more than 29,083 miles per hour. Its apparent diameter, as seen at a mean distance from the earth, is 39",
To an eve placed in Jupiter, the sun would not be a fifth part of the size he appears to us, and his disc be 25 times less. Though Jupiter be the largest of all the planets, yet its revolution round its axis is the swiftest. The polar axis is shorter than the equatorial one, and his axis perpendicular to the plane of his orbit.
Jupiter, when in opposition to the sun, is much nearer the earth, than when he is in conjunction with him; at those times he appears also larger* and tnore luminous than at other times.
In Jupiter, the days and nights are of an equal length, each being about five hours long. We have already observed, that the axis of his diurnal rotation is nearly at right angles to the plane of his. annual one; and consequently there can be scarce any difference in seasons: and here, as far asAve may reason from analogy, we may discover the footsteps of wisdom; for, if the axis of this planet were inclined by any considerable number of degrees, just so many degrees round each pole would, m their turn, be almost six years in darkness; and as Jupiter is of such an amazing size, in this case, immense regions of land would be uninhabitable.
Jupiter is attended by four satellites, or moons; these are invisible to the naked eye; but through a telescope they make a beautiful appearance. As our moon turns round the earth, enlightening the nights, by reflecting the light she receives from the sun, so these also enlighten the nights of Jupiter; and move round him in different periods of times, proportioned to their several distances: and as the moon keeps company with the earth in its annual revolution round the sun, so these accompany Jupiter in its course round that luminary.
In speaking of the satellites, \ve distinguish them according to their places; into the first, second, and so on; by the first, we mean that which is nearest to the planet.
The outermost of Jupiter's satellites will appear almost as big as the moon does to us; five times the diameter, and twenty-five times the disc of the sun. The four satellites must afFoid a pleasing spectacle to the inhabitants of Jupiter; for sometimes they will rise all together; sometimes be all together on the meridian, ranged one under another, besides frequent eclipses. Notwithstanding the distance of Jupiter and his sa4eliites from us, the eclipses thereof are of considerable use for ascertaining with accuracy the longitude of places. From the four satellites, the inhabitants of Jupiter will have four different kinds of months, and the number of them in their year not less than 4,500.
An.astronomer in Jupiter will never see Mercury, Venus, the Earth, or Mars', because, from the immense distance at which he is placed, they must appear to accompany the sun, and rise and set with him; but then he will have for the objects of observation, his own four moons, Saturn, his ring and satellites, and probably the Georgium Sidus.
OF SATURN, J?
Before the discovery of the Georgium Sidus, Saturn was reckoned the most remote planet in our system: he shines but with a pale feeble light, less bright than Jupiter, though less ruddy than Mars. The uninformed eye imagines not, when it is directed to this little speck of light, that it is viewing a large and glorious globe, one of the most stupendous of the planets, whose diameter is nearly 78,000 miles. We need not, however, be surprised at the vast bulk of Saturn, and its disproportion to its appearance in the heavens; for we are to consider that all objects decrease in their apparent magnitude, in proportion to their distance; but the distance of Saturn is immense; that of the earth from the sun is 96,000,000 miles; of Saturn, 916,500,000 miles.
The length of a planet's year, or the time of its revolution round its orbit, is proportioned to its distance from the sun. Saturn goes round the sun in 29 years, 167 days, six hours: moving at the rate of