wise Creator has attempered every dwelling place in fis empire to the physical constitution of the beings which he has placed in it. From a variety of facts which have been observed in relation to the production of caloric, it does not appear probable, that the degree of heat on the sur face of the different planets depends on their respective distances from the Sun. It is more pro:able, that it depends chiefly on the distribution of the substance of caloric on the surfaces, and throughout the atmospheres of these bodies, in different quantities, according to the different situations which they occupy in the suit system; and that these different quantities of caloric are put into action by the influence of the solar rays, so as to produce that degree of sensible heat requisite to the wants, and to the greatest benefit of each of the planets. On this hypothesis, which is corroborated by a great variety of facts and experiments, there may be no more sensible heat experienced on the planet Mercury, than on the surface of Herschel, which is fifty times farther removed from the Sun. Owing to the dazzling brightness of Mercury, the swiftness of its motion, and its nearness to the Sun, astronomers have made but comparatively few discoveries respecting it. When viewed through a telescope of considerable magnifying power, it exhibits at different periods, all the various phases of the Moon; except that it never appears quite full, because its enlightened hemisphere is never turned directly towards the Earth, only when it is behind the Sun, or so near to it, as to be hidden by the splendour of its beams. Its enlightened hemisphere being thus always turned towards the Sun, and the opposite one being always dark, prove that it is an opaque body, similar to the Earth, shining only in the light which it receives from the Sun. The rotation of Mercury on its axis, was determined from he daily position of its horns, by M. Schroeter, who not only discovered spots upon its surface, but several mountains in its southern hemisphere, one of which was 103 miles high:-nearly three times as high as Chimborazo, in South America. It is worthy of observation, that the highest mountains which have been dis. covered in Mercury, Venus, the Moon, and perhaps we may add the Earth, are all situated in their southern hemispheres. During a few days in March and April, August and September, Mercury may be seen for several minutes, in the morning or evening twilight, when its greatest elongations happen in those months; in all other parts of its orbit, it is too near the Sun to be seen by the naked eye. The greatest On what does the degree of heat at the different planets probably depend? Why have astronomers been able to make but comparatively few discoveries respecting Mercury? What is its appearance when viewed through a telescope of considerable magnify ing power? What circumstances prove that it is an opaque body, shining only with the light of the sun? How was the rotation of Mercury on its axis determined, and by whom? What did he discover on its surface? What was the altitude of the highest mountain which he saw? In which hemisphere are the highest mountains which have been discovered in Mercury, Venus, and the Moon, situated? Does the same fact exist in regard to the Earth? During what months may Mercury be seen for a few days, an in what parts of the day? Why is it visible at these times, and not at others? distance that it ever departs from the Sun, on either side, varies from 16° 12′, to 28° 48', alternately. The distance of a planet from the Sun, as seen from the Earth, (measured in degrees.) is called its elongation. The greatest absolute distance of a planet from the Sun is denominated its aphelion, and the least its perihelion. On the diagram, exhibiting the Relative Position of the Planets' Orbits, [Plate I.] these points are represented by little dots in the orbits at the extremities of the right lines which meet them; the Perihelion points being above the Ecliptic, the Aphelion points below it. The revolution of Mercury about the Sun, like that of all the planets, is performed from west to east, in an orbit which is nearly circular. Its apparent motion as seen from the earth, is, alternately, from west to east, and from east to west, nearly in straight lines; sometimes, directly across the face of the Sun, but at all other times, either a little above, or a little below it. Being commonly immersed in the Sun's rays in the evening, and thus continuing invisible till it emerges from them in the morning, it appeared to the ancients like two distinct stars. A long series of observations was requisite, before they recognised the identity of the star which was seen to recede from the Sun in the morning with that which approached it in the evening. But as the one was never seen until the other disappeared, both were at last found to be the same planet, which thus oscillated on each side of the Sun. Mercury's oscillation from west to east, or from east to west, is really accomplished in just half the time of its revolution, which is about 44 days; but as the Earth, in the meantime, follows the Sun in the same direction, the apparent elongations will be prolonged to between 55 and 65 days. The passage of Mercury over the Sun's disc, is denominated a Transit. This would happen in every revolution, if the orbit lay in the same plane with the orbit of the Earth. But it does not; it cuts the Earth's orbit in two opposite points, as the ecliptic does the equator, but at an angle three times less. See diagram, Relative Position of the Planets' Orbits, and their Inclination to the Plane of the Ecliptic. [Plate I.] The dark lines denote sections in the planes of the planets' orbits. The dotted lines continued from the dark lines denote the inclination of the orbits to the plane of the Ecliptic, which inclination is marked in figures on them. Let the student fancy as many circular pieces of paper, intersecting each other at the several angles of inclination What are the greatest distances which it departs from the Sun, on either side? What is the Elongation of a planet? What is its Aphelion? What is its Perihelion? In what direction does Mercury revolve about the Sun? What is the figure of its orbit? Describe its apparent motion, as seen from the Earth. How did it appear to the ancients? What was the cause of this appearance? How were these apparently two distinct stars at last found to be but one? What is the actual period of each elongation of Mercury What the apparent period? What is the cause of this difference? What does the expres sion, transit of Mercury, signify? Why does it not make a transit at every revolution? marked on this diagram, and he will be enabled to understand more easily what is meant by the inclination of the planets' orbits. It will be perceived on the diagram, that the inclination of Mercury's orbit to the plane of the ecliptic is 7° 9''. These points of intersection are called the Nodes of the orbit. Mercury's ascending node is in the 16th degree of Taurus; its descending node in the 16th degree of Scorpio. As the Earth passes these nodes in November and May, the transits of Mercury must happen, for many ages to come, in one of these months. The following is a list of all the Transits of Mercury from the time the first was observed by Gassendi, November 6, 1631, to the end of the present century. 1631 Nov. 6. 1644 Nov. 6. 1776 Nov. 2. 1707 May 5. 1710 Nov. 6. 8. 1835 Nov. 7. 1845 May 1848 Nov. 9. 1861 Nov. 11. 1868 Nov. 4. 1878 May 6. 1881 Nov. 7. 1891 May 9. 1894 Nov. 10. By comparing the mean motion of any of the planets with the mean motion of the Earth, we may, in like manner, determine the periods in which these bodies will return to the same points of their orbit, and the same positions with respect to the Sun. The knowledge of these periods will enable us to determine the hour when the planets rise, set, and pass the meridian, and in general, all the phenomena dependent upon the relative position of the Earth, the planet, and the Sun; for at the end of one of these periods they commence again, and all recur in the same order. We have only to find a number of sidereal years, in which the planet completes exactly, or very nearly, a certain number of revolutions; that is, to find such a number of planetary revolutions, as, when taken together, shall be exactly equal to one, or any number of revolutions of the Earth. In the case of Mercury, this ratio will be, as 87.969 is to 365.256. Whence we find, that, 7 periodical revolutions of the Earth, are equal to 29 of Mercury: 13 periodical revolutions of the Earth, are equal to 54 of Mercury: 33 periodical revolutions of the Earth, are equal to 137 of Mercury: 46 periodical revolutions of the Earth, are equal to 191 of Mercury. Therefore, transits of Mercury, at the same node, may happen at intervals of 7, 13, 33, 46, &c. years. Transits of Venus, as well as eclipses of the Sun and Moon, are calculated upon the same principle. The sidereal revolution of a planet respects its absolute motion; and is measured by the time the planet takes to revolve from any fixed star to the same star again. The synodical revolution of a planet respects its relative motion; and is measured by the time that a planet occupies in coming back to the same position with respect to the Earth and the Sun. The sidereal revolution of Mercury, is 874. 23h. 15m. 44s. Its synodical revolution is found by dividing the whole circumference of 360° by its relative motion in respect to the Earth. Thus, the mean daily motion of Mercury is What are the points where the orbits of the planets intersect the orbit of the Earth called? Where is Mercury's ascending node? Where is its descending node? In what months must the transit of Mercury occur for many ages to come? Why must they occur in these months? How can we determine the periods in which the planets will return to the same points of their orbits, and the same positions in respect to the Sun? Why is it useful to know these periods? State the method of making the computation. What will the ratio be in the case of Mercury? State the ratio between the periodi cal revolutions of the Earth and Mercury. At what intervals then may transits of Mercury at the same node happen? Upon what principle are transits of Venus and eclipses of the Sun and Moon, calculated? What is the sidereal revolution of a planet? What is the synodical revolution? What is the time of the sidereal revo lution of Mercury? State the method of computing the time of the synodical revo Jution. Compute the synodical revolution of Mercury. 14732.555; that of the Earth is 3548" 318; and their difference is 11194 .237, being Mercury's relative motion, or what it gains on the Earth every day. Now by simple proportion, 11184.237 is to 1 day, as 360° is to 115d. 21h. 3', 25′′, the period of a synodical revolution of Mercury. The absolute motion of Mercury in its orbit, is.109,757 miles an hour; that of the Earth, is 68,288 miles: the difference, 41,469 miles, is the mean relative motion of Mercury, with respect to the Earth. VENUS. THERE are but few persons who have not observed a beautiful star in the west, a little after sunset, called the evening star. This star is Venus. It is the second planet from the Sun. It is the brightest star in the firmament, and on this account easily distinguished from the other planets. If we observe this planet for several days, we shall find that it does not remain constantly at the same distance from the Sun, but that it appears to approach, or recede from him, at the rate of about three fifths of a degree every day; and that it is sometimes on the east side of him, and sometimes on the west, thus continually oscillating backwards and forwards between certain limits. As Venus never departs quite 48° from the Sun, it is never seen at midnight, nor in opposition to that luminary; being visible only about three hours after sunset, and as long before sunrise, according as its right ascension is greater or less than that of the Sun. At first, we behold it only a few minutes after sunset; the next evening we hardly discover any sensible change in its position; but after a few days, we perceive that it has fallen considerably behind the Sun, and that it continues to depart farther and farther from him, setting later and later every evening, until the distance between it and the Sun, is equal to a little more than half the space from the horizon to the zenith, or about 46°. It now begins to return towards the Sun, making the same daily progress that it did in separating from him, and to set earlier and earlier every succeeding evening, until it finally sets with the Sun, and is lost in the splendour of his light. A few days after the phenomena we have now described, What is the rate per hour of the absolute motion of Mercury in its orbit? Of the Earth? What is the mean relative motion of Mercury with respect to the Earth? What beautiful star sometimes appears in the west a little after sunset? What is the comparative dis tance of Venus from the Sun? What is its comparative brightness? In what direction is its apparent motion? Why is it never seen at midnight, nor in opposition to the Sun? At what times is it visible? How long after sunset is it when we first behold it in the west ? Describe its changes of position. we perceive, in the morning, near the eastern horizon, a bright star which was not visible before. This also is Venus, which is now called the morning star. It departs farther and farther from the Sun, rising a little earlier every day, until it is seen about 46° west of him, where it appears stationary for a few days; then it resumes its course towards the Sun, appearing later and later every morning, until it rises with the Sun, and we cease to behold it. In a few days, the evening star again appears in the west, very near the setting-sun, and the same phenomena are again exhibited. Such are the visible appearances of Venus. Venus revolves about the Sun from west to east in 224} days, at the distance of abont 68 millions of miles, moving in her orbit at the rate of 80 thousand miles an hour. She turns around on her axis once in 23 hours, 21 minutes, and 7 seconds. Thus her day is about 25 minutes shorter than ours, while her year is equal to 7 of our months, or 32 weeks. The mean distance of the Earth from the Sun is estimated at 95 millions of miles, and that of Venus being 68 millions, the diameter of the Sun, as seen from Venus, will be to his diameter as seen from the Earth, as 95 to 68, and the surface of his disc as the square of 95 to the square of 68, that is, as 9025 to 4626, or as 2 to 1 nearly. The intensity of light and heat being inversely as the squares of their distances from the Sun, Venus receives twice as much light and heat as the Earth. Her orbit is within the orbit of the Earth; for if it were not, she would be seen as often in opposition to the Sun, as in conjunction with him; but she was never seen rising in the east while the Sun was setting in the west. Nor was she ever seen in quadrature, or on the meridian, when the Sun was either rising or setting. Mercury being about 23° from the Sun, and Venus 46°, the orbit of Venus must be outside of the orbit of Mercury. The true diameter of Venus is 7621 miles; but her apparent diameter and brightness are constantly varying, according to her distance from the Earth. When Venus and the Earth are on the same side of the Sun, her distance In what direction, and in what time, does Venus revolve about the Sun? What is her distance from the Sun? What is the rate per hour of her motion in her orbit? In what time does she revolve on her axis? How are the lengths of her day and year, compared with those of the Earth? How much larger does the Sun appear at Venus than he does at the Earth? How much more light and heat does she receive from him, than the Earth? How much farther is Venus from the Sun than Mercury? On which side of the orbit of Mercury must her orbit be? What is her true diameter? In what proportion do her ap parent diameter and brightness constantly vary? What is her distance from the Earth when they are both on the same side of the Sun ? |