Showing the distance of Venus from the Sun, her eccentri city, longitude of the ascending node, &c. Mean distance in miles Eccentricity in miles Longitude of ascending node at the be ginning of 1801 2s. 14° 52′ 40′′ Longitude of the perihelion at the same time : Greatest equation of the centre January, 1825 Geocentric longitude at the same time 10 18 51 0 1 36 0 0 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 disc, 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 : bodies seem to move daily from east to west, was at no great distance from the Earth; and that all the celestial bodies were created solely for its use and ornament. Heraclitus imagined the Earth to have the shape of a canoe; Anaximander supposed it to be cylindrical; and Aristotle, the great oracle of antiquity, gave it the form of a timbrel. Such of the ancients, however, as understood any thing of astronomy, and especially the doctrine of eclipses, must have been acquainted with the round figure of the Earth; as the ancient Babylonian astronomers, who had calculated eclipses long before the time of Alexander, and Thales, the Grecian, who predicted an eclipse of the Sun. A very little reflection, and a very little travelling either by sea or land, must soon convince any one that the Earth is of a spherical form. For let a person occupy any station in a level country, and mark carefully the objects within the range of his horizon, let him then advance in any direction, and as he moves the objects behind him gradually disappear, and new objects in front come in view. Before he has travelled twenty miles in the same direction, he will find that every object that was at first visible to him is lost to his view, and that he is now in the centre of a new horizon. As a similar change takes place at every part of the globe where the same experiment has been tried, it follows that the Earth is a spherical body. The same inference may be deduced from observing the appearance of a ship at a distance at sea, or from observing the gradual rising of the coast as the ship approaches the shore. In the former case, the top of the mast is first seen, and as the vessel approaches the land, the whole of her gradually becomes visible. In the latter, the hills, or the higher parts of the buildings, are first discovered, but by degrees every part of the building and even the beach itself is seen. These are appearances which can only be reconciled with the spherical figure of the Earth. The same conclusion may be drawn from observing the altitude of the pole star, after travelling north or south a considerable number of miles. In travelling northward its altitude will be increased; but in travelling south it will be diminished. The globular figure of the Earth is also inferred from the operation of levelling, in which it is found necessary to make, an allowance for the difference between the true and apparent level; and the allowance which is made, and found to answer, is on the principle that the Earth is spherical. Another proof of the Earth being of a spherical form, is obtained from its shadow in an eclipse of the Moon; for when the shadow of the Earth falls on the Moon she is eclipsed, and the shadow always appears circular upon the face of the Moon, when she is not totally eclipsed, although the Earth is constantly turning on its axis. Hence it follows, that the body which projects the shadow, must be spherical. But the most convincing proof of the spherical figure of the Earth, is, that many navigators have sailed round it; not on an exact circle, it is true, because the winding of the shores would not admit of it, but by going in and out as the shores happened to lie, and still keeping the same course, they have at last arrived at the port from which they departed. The first who succeeded in this daring enterprise was Ferdinand Magellan, a Portuguese, in the year 1519, and who completed his voyage in 1124 days; in the year 1557, Francis Drake performed the same in 1056 days; in the year 1586, Sir Thomas Cavendish made the same voyage in 777 days; in the year 1598, Oliver Noort, a Hollander, in 1077 days; Van Schouten, in the year 1615, in 749 days; Jac. Heremites and Joh. Huygens, in the year 1623, in S02 days: and many others have performed the same navigation, particularly Anson, Bougainville, and Cook. Some of these navigators sailed eastward, some westward, till they again arrived in Europe, whence they set out; and in the course of their voyage observed, that all the phenomena, both of the heavens and Earth, confirmed the doctrine of the spherical figure of the Earth. The unevenness or irregularity of the Earth's surface, such as mountains and valleys, afford no objection to its being considered as a globular body; for the loftiest mountains bear no greater proportion to the vast magnitude of the Earth, than grains of sand to the size of an artificial globe of thirteen inches in diameter. This is the reason that no deviation from the spherical figure of its shadow is perceptible in an eclipse of the Moon. 2. From the most accurate measurement, lately made by mathematicians, it is found that the terrestrial meridian is nearly an ellipse; that the figure of the Earth is not exactly a sphere, but nearly an oblate spheroid, its equatorial diameter being about 25 miles longer than its axis or polar diameter, and its mean diameter 7914 miles. 3. By the application of a new theory of most probable results to the determination of the magnitude and figure of the Earth, Dr. Adrain has found the ratio of the axis to the equatorial diameter to be at 320 to 321, the true mean diameter of the Earth, considered as a globe, to be 7918.7 miles, and consequently its circumference 24877.4 miles, and a degree of a great circle equal to 69.1039 miles. According to La Place the polar diameter is to the equitorial as 331 to 332; he makes the equatorial diameter 7924 miles: hence the polar diameter is 7900 miles, and the mean diameter 7912. In the preceding part of this work, the mean diameter of the Earth has been taken equal to 7920 miles, its circumference 24,880 miles, and the length of a degree 69 miles; the same numbers shall therefore be used in the subsequent part: they are nearly those given by Dr. Adrain, and which are considered to be the most exact measures of the magnitude of the Earth. Although every one of the observations which have just been made, (in the preceding article) respecting the figure of the Earth, affords sufficient evidence that the surface of the Earth is curved, yet none of them, except, perhaps, the form of the shadow on the disc of the Moon in a lunar éclipse, entitles us to infer that the figure of the Earth is that of a globe, or perfect sphere. It was natural, however, for those who first discovered that the Earth had a round shape, to suppose that it was truly spherical. This, however, is now known not to be the case; its true figure being that of an oblate spheroid, or sphere flattened a little at the poles, and raised about the equator: so that the axis or polar diameter is less than the equatorial. What first led to this discovery was the observations of some French and English philosophers in the East Indies and other parts, who found that pendulums required longer time to perform their vibrations the nearer they were to the equator; for Richer in a voyage to Cayenne, near the equator, found that it was absolutely necessary to shorten the pendulum of his clock about one eleventh part of a Paris inch, in order to make it vibrate in the same time as it did in the latitude of Paris. From this it appeared that the force of gravity was less at places near the equator than at Paris; and consequently that those parts |