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but has not quite accomplished her revolution around the Sun: the consequence is, that the Moon's nodes fall back in the ecliptic at the rate of about 1910 annually; so that the eclipses happen sooner every year by about 19 days.
As the Moon passes from one of her nodes to the other in 173 days, there is just this period between two successive eclipses of the Sun, or of the Moon. In whatever time of the year, then, we have eclipses at either node, we may be sure that in 173 days afterwards, we shall have eclipses | at the other node
As the Moon's nodes fall back, or retrograde in the ecliptic, at the rate of 19 every year, they will complete a backward revolution entirely around the ecliptic to the same point again, in 18 years, 225 days in which time there would always be a regular period of eclipses, if any complete number of lunations were finished without a remainder. But this never happens; for if both the Sun and Moon should start from a line of conjunction with either of the nodes in any point of the ecliptic, the Sun would perform 18 annual revolutions and 222° of another, while the Moon would perform 230 lunations, and 85° of another, before the node would come around to the same point of the ecliptic again: so that the Sun would then be 138° from the mode, and the Moon 850 froin the Sun.
But after 223 lunations, or 18 years, 11 days,* 7 hours, 42 minutes, and 31 seconds, the Sun, Moon, and Earth, will return so nearly in the same position with respect to each other, that there will be a regular return of the same eclipses for many ages. This grand period was discovered by the Chaldeans, and by them called Saros. If, therefore, to the mean time of any eclipse, either of the Sun or Moon, we add the Chaldean period of 18 years and 11 days, we shall have the return of the same eclipse. This mode of predicting eclipses will hold good for a thousand years. In this period there are usually 70 eclipses; 41 of the Sun, and 29 of the Moon.
The number of eclipses in any one year, cannot be less than two, nor more than seven. In the former case, they will both be of the Sun; and in the latter, there will be five of the Sun, and two of the Moon those of the Moon will be total. There are sometimes six; but the usual number is four: two of the Sun, and two of the Moon.
The cause of this variety is thus accounted for. Although the Sun usually passes by both nodes only once in a year, he may pass the same node again a little before the end of the year. In consequence of the retrograde motion
*If there are four leap years in this interval, add 11 days; but if there are five, add only ten days.
How far do the Moon's nodes fall back in the ecliptic annually, and how much sooner do the eclipses happen every year? In what time does the Moon pass from one of her nodes to the other? What is the length of the time which elapses between two successive eclipses of the Sun or the Moon? After there have been eclipses at one node, in what time may we be sure that there will be eclipses at the other? In what time do the Moon's nodes comple'e a backward revolution around the ecliptic? Why is there not always a regular period of eclipses in this time? If the Sun and Moon should both start from a line of conjunction with either node, how many revolutions would the Sun perform, and how many lunations the Moon, before the node would come around to the same point again? After how many lunations will the Sun, Moon, and Earth, return so nearly to the same position with respect to each other, that there will be a regular return of the same eclipses for many ages? What nation discovered this grand period, and what did they call it? What is the mode of predicting eclipses, with which this fact furnishes us? How many eclipses are there usually in this period? What is the least, and what the greatest number of eclipses, in any one year? In the former case, what eclipses will they be? What, in the latter? What is the usual number of eclipses in the year, and what eclipses are they? Please explain the cause of this variety.
of the Moon's nodes, he will come to either of them 173 days after passing the other. He may, therefore, return to the same node in about 346 days, having thus passed one node twice and the other once, making each time, at each, an eclipse of both the Sun and the Moon, or, sir in all. And, since 12 lunations, or 354 days from the first eclipse in the beginning of the year, leave room for another new Moon before the close of the year, and since this new Moon may fall within the ecliptic limit, it is possible for the Sun to be eclipsed again. Thus there may be seven eclipses in the same year. Again: when the Moon changes in either of her nodes, she cannot come within the lunar ecliptic limit at the next full, (though if she be full in one of her nodes, she may come into the solar ecliptic limit at her next change,) and six months afterwards, she will change near the other node; thus making only two eclipses.
The following is a list of all the solar eclipses that will be visible in Europe and America during the remainder of the present century. To those which will be visible in New-England, the number of digits is annexed.
The eclipses of 1838, 1854, 1869, 1875, and 1900, will be very large. In those of 1815, 1858, 1861. 1873, 1875, and 1880, the Sun will rise eclipsed.
In that of 1844, the Sun will set eclipsed. Those of 1838, 1854, and 1875, will be annular. The scholar can continue this table, or extend it backwards, by adding or subtracting the Chaldean period of 18 years, 11 days, 7 hours, 64 minutes, and 31 seconds.
MARS is the first of the exterior planets, its orbit lying immediately without, or beyond, that of the Earth, while those of Mercury and Venus are within.
Mars appears to the naked eye, of a fine ruddy complexion; resembling, in colour, and apparent magnitude, the star Antares, or Aldebaran, near which it frequently passes. It exhibits its greatest brilliancy about the time
What is the position of Mars in the solar system? Describe its appearance to the naked eye. When does it exhibit its greatest brilliancy 7
that it rises when the Sun sets, and sets when the Sun rises; because it is then nearest the Earth. It is least brilliant when it rises and sets with the Sun; for then it is five times farther removed from us than in the former case. Its distance from the Earth at its nearest approach is about 50 millions of miles. Its greatest distance from us is about 240 millions of miles. In the former case, it appears nearly 25 times larger than in the latter. When it rises before the Sun, it is our morning star; when it sets after the Sun, it is our evening star.
The distance of all the planets from the Earth, whether they be interior or exterior planets, varies within the limits of the diameters of their orbits;} for when a planet is in that point of its orbit which is nearest the Earth, it is evidently nearer by the whole diameter of its orbit, than when it is in the opposite point, on the other side of its orbit. The apparent diameter of the planet will also vary for the same reason, and to the same degree.
Mars is sometimes seen in opposition to the Sun, and sometimes in superior conjunction with him; sometimes gibbous, but never horned. In conjunction, it is never seen to pass over the Sun's disc, like Mercury and Venus. This proves not only that its orbit is exterior to the Earth's orbit, but that it is an opaque body, shining only by the reflection of the Sun.
The motion of Mars through the constellations of the zodiac is but little more than half as great as that of the Earth; it being generally about 57 days in passing over one sign, which is at the rate of a little more than half a degree each day. Thus, if we know what constellation Mars enters to day, we may conclude that two months hence it will be in the next constellation; four months hence, in the next; six months, in the next, and so on.
Mars performs his revolution around the Sun in 1 year and 101 months, at the distance of 145 millions of miles; moving in its orbit at the mean rate of 55 thousand miles an hour. Its diurnal rotation on its axis is performed in 24 hours, 39 minutes, and 21 seconds; which makes its day about 44 minutes longer than ours,
Why is it most brilliant at this time? What are its least and greatest distances from us? How much larger does it appear in the former case than in the latter? Within what limits does the distance of all the planets from the Earth vary? With what does the apparent diameter of a planet vary? What moon-like phases has Mars? What does the fact, that it never assumes the crescent form at its conjunction, prove, in regard to its situation? How do we know it to be opaque? What is the rate of its motion through the constellations of the zodiac, compared with that of the Earth? How long is it in passing over one sign? At what rate per day is this? How, then, if we know in what constellation it is at any one time, may we determine in what constellation it will be at any subsequent time? In what time does it perform its revolution around the Sun? What is its distance from the Sun ? What is the mean rate of its motion in its or bit per hour? In what time does it perform its revolution on its axis? What, then, is the length of its day, compared with that of the Earth?
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
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 28. 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 inore 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 planets, 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 synodical 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, compared 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 roe reasonably conclude that Mars has no satellite? Describe the 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 pla nets 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?