west of the Sun, and appears in the morning before he rises As she proceeds to the westward, her illuminated disc is seen as a crescent continually increasing, at the same time that her diameter is diminishing. When she has got 450 to the west of the Sun, her disc is a semicircle; and as she again approaches the Sun, it increases till she is lost in the Sun's rays; her orb being almost a circle, but its diameter not more than one-sixth of what it was at the former conjunction. The superior conjunction takes place after the western elongation, and the inferior after the eastern elongation. At the former of these periods, Venus is the breadth of her orbit farther from the Earth than at the latter; for at the time of the superior conjunction, she is on the opposite side of the Sun to what the Earth is; but at the time of the inferior conjunction, Venus and the Earth are on the same side of the Sun. This planet appears to keep on the same side of the Sun for 290 days together, although this is a longer period than she takes to perform a complete revolution round that body: she is therefore an evening star during 290 days, and a morning star somewhat longer. This may appear strange to those who are but little acquainted with astronomy; but when it is considered that the Earth is all the while moving round the Sun the same way, though not so fast as Venus, the difficulty vanishes; because she must continue to appear on the same side with the Earth, till the excess of her daily motion above that of the Earth's motion amounts to 1790, or nearly to half a circle; which, at the rate of 27' per day, will be in about 290 days, as above stated. After the superior conjunction, the orb of Venus increases in magnitude as she approaches her greatest eastern elongation, but the enlightened part diminishes, just reversing the order of what has already been stated to take place from the inferior conjunction to her greatest western elongation. The different phases or appearances of Venus, described above, were first discovered by Galileo in 1611, which fulfilled the prediction of Copernicus, who foretold, before the discovery of the telescope, that the phases of the inferior planets would be one day discovered to be similar to those of the Moon. The accomplishment of this prediction affords some of the strongest and most convincing proofs of the truth of the the Copernican system of the World, that can be ob tained. 7. The apparent diameter of Venus, like Meris very variable: her greatest diameter being about 58", and her least diameter about 10". The apparent diameter of Venus, when she is at her mean distance from the Earth, is about 16". It was long doubted whether Venus be surrounded by an atmosphere or not; but this question has been completely settled by the very nice and accurate observations of the German astronomer Schroeter, who has ascertained the existence of a pale faint light extending along the line of the dark hemisphere of this planet, which he supposes to be a kind of twilight, occasioned by the Sun illuminating its atmosphere. From this circumstance, Schroeter has been enabled to ascertain the density of this atmosphere, and that it extends to a very great height, which must prevent the Sun from overpowering the inhabitants with his heat and splendour, which are supposed to be nearly twice as great as on the Earth's surface. Dr. Herschel, after a long series of observations on this planet, accounts of which are given in the Philosophical Transactions for 1793, says, that the planet revolves about its axis, but the time of its rotation is uncertain; that the position of its axis is also very uncertain; that the planet's atmosphere is very considerable; that the planet has probably hills and inequalities on its surface, but he has not been able to see much of them, owing perhaps to the great density of its atmosphere: as to the mountains of Venus, no eye, he says, which is not considerably better than his, or assisted by much better instruments, will ever get a sight of them: and that the apparent diameter of Venus, at the mean distance from the Earth, is 18.8"; whence it may be inferred, that this planet is somewhat larger than the Earth, instead of being less as former astronomers have asserted. S. When Venus, in her inferior conjunction, is in or near one of her nodes, she appears in the form of a circular black spot on the Sun's disc, and a transit of Venus takes place. The apparent diameter at the time of this planet's transit, according to Dr. Brinkley and other astronomers, is about 57". The transits of Venus are not so frequent as those of Mercury; for 8 periodic revolutions of Mercury are equal nearly to 13 of Venus; and there are no other intervening whol numbers till 335 periodic revolutions of the Earth, which are equal nearly to 382 per. of Venus. Hence a transit of Venus, at the same node, may happen after an interval of S years. If it does not take place after an interval of 8 years, it cannot happen till after 235 years. At present the ascending node of Venus, as seen from the Sun, is in 2 signs, 140, and the descending node in Ss, 140. The Earth, as seen from the Sun, is in the former longitude in the beginning of December, and in the latter in the beginning of June. Hence the transits of Venus will happen for many ages to come in December and June. Those of Mercury, as has already been observed, will take place in May and November. In the years 1761 and 1769, there were transits of Venus, being at those periods in her descending node: the next transit at that node will happen in 2004. But a transit was observed at the ascending node in the year 1639, by Horrox, who had previously computed it, from having corrected the tables of Venus by his own observations, all other astronomers having been ignorant of its occurring. This transit will again happen at the end of 235 years from that time, or in the year 1875, and the next in 1982. When a transit of Venus is observed, it not only proves that she is an opaque body, and that her orbit is included by the Earth's, but it is of admirable use in determining what is called the Sun's parallax, which is of so much use in astronomy, as we shall see in a subsequent chapter. In 1672 and 1686, Cassini, with a telescope of 34 feet, thought he saw. a satellite move round this planet, at the distance of about of Venus's diameter. It had the same phases as Venus, but without any well defined form; and its diameter scarcely exceeded of the diameter of Venus. M. Montaign, of Limoges in France, preparing for observing the trans of 1761, discovered in the preceding month of May, a small star, about the distance of 20' from Venus, the diameter of it being about that of the planet. Some other astronomers have asserted that they perceived the like appearance. And indeed it must be acknowledged that Venus may have a satellite, though it is difficult for a spectator on the surface of the earth to see it. See Dr. Hutton's Philosophical and Mathematical Dictionary. TABLE. Showing the distance of Venus from the Sun, her eccentri city, longitude of the ascending node, &c. Mean distance in miles ginning of 1801 Longitude of the perihelion at the same time Greatest equation of the centre 68,514,044 January, 1825 0 1 36 8 The line of the apsides had a sidereal motion in antecedentia, or contrary to the order of the signs, of 4' 27.8" in a century. But in longitude this motion will appear direct at the rate of 47.4" in a year, or about 1° 19' 2" in a hundred years. The nodes have a direct motion in longitude of 31.4", or about 52′ 20′′ in a century. The secular decrease of the inclination of the orbit to the ecliptic is 4.6". The inclination of the axis of this planet to the plane of its orbit, according to some astronomers, is not known; but, according to others (with a great degree of probability,) it is 75 degrees; therefore her tropics are only 150 from her poles; and her polar circles are as far distant from her equator. When Venus is observed with a good telescope, she exhibits bright and dark spots on her dise, and the mountains on her surface are supposed to be 101 miles in height; but, according to some astronomers, the mountains are six times higher than any on our globe. From the best observations the height of the atmosphere of Venus has been calculated to be about 50 miles. QUESTIONS.. What is said of Venus, what is the length of her year, and what is the mean hourly motion in her orbit? What is the diameter of Venus, what is the length of her day, &c.? What is the inclination of the orbit of Venus to the ecliptic? What is her eccentricity in miles ? In what time does she perform her synodic revolution ? What is the mean arc of retrogradation, and what is its duration ? When is Venus a morning star, and when an evening star? What is the mean apparent diameter of Venus? When does a transit take place ? CHAPTER VII. 1. The Earth is a spherical body, nearly resembling the figure of a globe; it performs its revolution round the Sun, in an orbit between that of Venus and Mars, in 365 days, 5 hours, 48 minutes, 51 seconds, which is the length of the tropical year; (Art. 14, p. 31,) and it revolves on its axis in 23 hours, 56 minutes, 4 seconds, of mean solar time, which is the length of a sidereal day, (Art. 6, p. 29.) The mean hourly motion of the Earth in its orbit is about 68,000 miles, which is 90 times faster than the velocity of sound. In the early ages of the world, many fanciful and absurd notions, respecting the figure of the Earth, prevailed; some of which were adopted because they appeared to agree with the slight and inaccurate observations of the vulgar, whilst others represented this matter in the way which best accorded with their preconceived opinions in philosophy and religion. The most general opinion was, that the Earth was a great circular plane, extending on all sides to an infinite distance; that the firmament above, in which the heavenly |