If the orbit of Venus lay exactly in the plane of the Earth's orbit, she would pass centrally across the Sun's disc, like a dark round spot, at every inferior conjunction; but as one half of her orbit lies about 34° above the ecliptic, and the other half as far below it, she will always pass the Sun a very little above or below it, except when her inferior conjunction happens in, or near, one of her nodes; in which case she will make a transit. [Relative position of the Planet's Orbits, Plate I—Plane of Venus—Inclination 3° 23'.] This phenomenon, therefore, is of very rare occurrence: it can happen only twice in a century; because it is only twice in that time that any number of complete revolutions of Venus, are just or nearly equal to a certain number of the Earth's revolutions. The principle which was illustrated in predicting the transits of Mercury applies equally well to those of Venus; that is, we must find such sets o numbers, (representing complete revolutions of the Earth and Venus,) an shall be to each other in the ratio of their periodical times, or as 365.256 is te 224.7. Thus; the motion of Venus, in the Julian years, is 2106591".52 that of the Earth for the same period being 129627".45, the ratio will be Why does not Venus pass centrally across the Sun's disc at every inferior conjunction what circumstances will she make a transit, across the sun? How often can this phe nomenon happen? Why can it not happen oftener? State the method of predicting ta transits of Venus 29627 $188591~ 53. As the two terms of this fraction cannot be reduced by a common divisor, we must multiply them by such numbers as will make one A multiple of the other; accordingly, 13 times the denominator will be nearly equal to 8 times the numerator; and 475 times the denominator will equal 291 tines the numerator. By combining these two periods and their multiples by addition and sub. traction, we shall obtain the period of all the transits that have ever happened. Thus; 291-8X7-235, another period; and 291-6X8-243, another period, and so on. Whence we find that, 8 periodical revolutions of the Earth, are equal to 13 of Venus. 235 periodical revolutions of the Earth, are equal to 382 of Venus. 243 periodical revolutions of the Earth, are equal to 395 of Venus. 251 periodical revolutions of the Earth, are equal to 408 of Venus. 291 periodical revolutions of the Earth, are equal to 475 of Venus. Hence a transit of Venus may happen at the same node, after an interval of 8 years; but if it do not happen then, it cannot take place again, at the same node, in less than 235 years. The orbit of Venus crosses the ecliptic near the middle of Gemini and Sagittarius; and these points mark the position of her nodes. At present, her ascending node is in the 14th degree of Gemini, and her descending node, in the same degree of Sagittarius. The Earth passes her ascending node in the beginning of December, and her descending node, in the beginning of June. Hence, the transits of Venus, forges to come, will happen in December and June. The rst, transit ever known to have been seen by any human being, took place at the ascending node, December 4th, 1639.* If to this date, we add 235 years, we shall have the time of the next ransit at the same node, which will accordingly happen in 1874. There will be another at the same node in 1882, *This phenomenon was first witnessed by Horrox, a young gentleman about 21 years f age, living in an obscure village 15 miles north of Liverpool. The tables of Kepler, constructed upon the observations of Tycho Brahe, indicated a transit of Venus in 1631, but none was observed. Horrox, without much assistance from books and instruments, set himself to inquire into the error of the tables, and found that such a phenomenon might De expected to happen in 1639. He repeated his calculations during this interval, with all the carefulness and enthusiasm of a scholar ambitious of being the first to predict and observe a celestial phenomenon, which, from the creation of the world, had never been witnessed. Confident of the result, he communicated his expected triumph to a confidential friend residing in Manchester, and desired him to watch for the event, and to take observations. So anxious was Horrox not to fail of witnessing it himself, that he commenced his observations the day before it was expected, and resumed them at the rising of the Sun on the morrow. But the very hour when his calculations led him to expect the visible appearance of Venus upon the Sun's disc, was also the appointed hour for the public worship of GoD on the Sabbath The delay of a few minutes might deprive bim forever of an opportunity of observing the transit. If its very commencement were not noticed, clouds might intervene, and conceal it until the Sun should set: and nearly a entury and a half would elapse before another opportunity would occur. He had been waiting for the event with the most ardent anticipation for eight years, and the result promised much benefit to the science Notwithstanding all this, Horrox twice suspended his observations, and twice repaired to the House of God, the Great Author of the bright worlds he delighted to contemplate. When his duty was thus performed, and he Fad returned to his chamber the second time, his love of science was gratified with full success; and he saw what no mortal eye had observed before! If any thing can add interest to this incident, it is the modesty with which the young astronomer apologizes to the world, for suspending his observations at all. "I observed it," says he, "from sunrise till nine o'clock, again a little before ten, and lastly at noon, and from one to two o'clock; the rest of the day being devoted to higher duties, which might not be neglected for these pastimes." After how long an interval may a transit of Venus happen again at the same node? If it do not happen then, how long a period must elapse before it will occur again at the same node? Where does the orbit of Venus cross the ecliptic, and where are her nodes? In what months, for ages to come, will the transits of Venus happen, and why? At which node, and when, did the first transit of Venus ever known to have been observed, take place? When will the next two transits occur? Pight years afterwards. It is not more certain that this phenomenon will recur, than that the event itself will engross the attention of all the astronomers then living upon the Earth. It will be anticipated, and provided for, and observed, in every inhabited quarter of the globe, with an intensity of solicitude which no natural phenomena, since the creation, has ever excited. The reason why a transit of Venus should excite so great an interest, is, because it may be expected to solve an important problem in astronomy, which has never yet been satisfactorily done :-a problem whose solution will make known to us the magnitudes and masses of all the planets, the true dimensions of their orbits, their rates of motion around the Sun, and their respective distances from the Sun, and from each other. It may be expected, in short, to furnish a universal standard of astronomical measure. Another consideration will render the observation of this transit peculiarly favourable; and that is, astronomers will be supplied with better instruments, and more accurate means of observation, than on any former occasion. So important, says Sir John Herschel, have these observations appeared to astronomers, that at the last transit of Venus, in 1769, expeditions were fitted out, on the most efficient scale, by the British, French, Russian, and other governments, to the remotest corners of the globe, for the express purpose of making them. The celebrated expedition of Captain Cook to Otaheite, was one of them. The general result of all the observations made on this most memorable occasion, gives 8".5776 for the Sun's horizontal parallax. The phenomena of the seasons, of each of the planets, like those of the Earth, depend upon the inclination of the axis of the planet, to the plane of its orbit. The inclination of the axis of Venus to the plane of her orbit, though not precisely known, is commonly estimated at 75°; which is more than three times as great as the inclination of the Earth's axis to the plane of the ecliptic. The north pole of Venus' axis inclines towards the 20th degree of Aquarius; the Earth's towards the beginning of Cancer; consequently, the northern parts of Venus have summer in the signs where those of the Earth have winter, and vice versa. The declination of the Sun on each side of her equator. must be equal to the inclination of her axis; and if this ex tends to 75°, her tropics are only 15° from her poles, and her polar circles 15° from her equator. It follows, also, that Why will the next transit excite a very great and universal interest? Upon what do the phenomena of the seasons of each of the planets depend? What is the estimated inclination of the axis of Venus to the plane of her orbit ? How does this inclination compare with that of the Earth's axis to the plane of the ecliptic? What seasons have the northern parts of Venus, when those of the Earth have winter? How do we know this? To what must the declination of the Sun on each side of her equator be equal? How far are Der tropics from her poles, and her polar circles from her equator } the Sun must change his declination more in one day at Venus, than in five days on the Earth; and consequently, that he never shines vertically on the same places for two days in succession. This may perhaps be providentially ordered, to prevent the too great effect of the Sun's heat, which, on the supposition that it is in inverse proportion to the square of the distance, is twice as great on this planet as it is on the Earth. At each pole, the Sun continues half a year* without setting in summer, and as long without rising in winter; consequently, the polar inhabitants of Venus, like those of the earth, have only one day and one night in the year; with this difference, that the polar days and nights of Venus are not quite two thirds as long as ours. Between her polar circles, which are but 15o from her equator, there are two winters, two summers, two springs, and two autumns, every year. But because the Sun stays for some time near the tropics, and passes so quickly over the equator, the winters in that zone will be almost twice as long as the summers. TELESCOPIC APPEARANCES OF VENUS. When viewed through a good telescope, Venus exhibits not only all the moon-like phases of Mercury, but also a variety of inequalities on her surface; dark spots, and brilliant shades, hills, and valleys, and elevated mountains. But on account of the great density of her atmosphere, these in * That is, half of Venus' year, or 16 weeks. How much more must the Sun change his declination in one day at Venus than on the Earth? Why, perhaps, is this so ordered? How many days and nights have her polar inhabitants during the year? How long are these days and nights, compared with those of our polar inhabitants? How many, and what seasons, has Venus between her pola circles? What is the length of the winters in this zone, compared with that of the summers? What appearances, besides her moon-Lke phases, does Venus exhibit when seen through a good telescope? equalities are perceived with more difficulty than those up· on the other planets. The mountains of Venus, like those of Mercury and the Moon, are highest in the southern hemisphere. According to M. Schroeter, a celebrated German astronomer, who spent more than ten years in observations upon this planet, some of her mountains rise to the enormous height of from 10 to 22 miles.* The observations of Dr. Herschel do not indicate so great an altitude ; and he thinks, that in general they are considerably overrated. He estimates the diame ter of Venus at 8,649 miles; making her bulk more than one sixth larger than that of the Earth. Several eminent astronomers affirm, that they have repeatedly seen Venus attended by a satellite, and they have given circumstantial details of its size and appearance, its periodical revolution, and its distance from her It is said to resemble our Moon in its phases, its distance, and its magnitude. Other astronomers deny the existence of such a body, because it was not seen with Venus on the Sun's disc, at the transits of 1761, and 1769. THE EARTH. THE Earth is the place from which all our observations of the heavenly bodies must necessarily be made. The ap parent motions of these bodies being very considerably affected by her figure, motions, and dimensions, these hold an important place in astronomical science. It will therefore be proper to consider, first, some of the methods by which they have been determined. If, standing on the sea-shore, in a clear day, we view a ship leaving the coast, in any direction, the hull or body of the vessel first disappears ; afterwards the rigging, and lastly, the top of the mast vanishes from our sight. Those on board the ship, observe that the coast first sinks below the horizon, then the buildings, and lastly the tallest spires of the city * 1st, 22.05 miles; 2d, 18.97 miles; 3d, 11.44 miles; 4th, 10.84 miles. Why is it more difficult to perceive the inequalities on her surface than those on the other planets? In which hemisphere are her mountains highest? What does M. Schroe ter make the altitude of some of the highest? Is this estimate confirmed by the observa tions of Dr. Herschel? How long is the diameter of Venus, according to Herschel's es timate? How much larger, then, must she be than the Earth? Some astronomers affirm that they have seen Venus attended by a satellite, why do others deny the existence of such a body? Why is it important, in an astronomical view, to be acquainted with the figure, dimensions, and motions of the Earth? Mention some of the proofs of the convexity of its surface? |