or flowing robe. Venus is the only planet mentioned in the sacred writings, or by the most ancient poets, such as Homer and Hesiod.

2. Venus is a spherical body, whose diameter is about 7687 miles, and she revolves on her axis, from west to east, in 23 hours and 21 minutes, which is the length of her day. This planet is, therefore, about the size of the Earth; and its relative mean distance from the Sun is nearly 7, that of the Earth being considered as 10.

The magnitude or size of Venus, is .8828, its mass .9243, and its density 1.04701; the size, mass, and density of the Earth being respectively considered as unity or 1. And a body weighing one pound on the Earth, will weigh one pound nearly on the surface of Venus.

3. The inclination of the orbit of Venus, to the plane of the ecliptic, is 3° 23′ 30′′, and its eccentricity is the least of any of the planets, being about 492,000 miles.

The secular decrease of the inclination of the orbit to the plane of the ecliptic is 4.6". The greatest geocentric latitude north is 3° 13', and the greatest geocentric latitude south is 70 55'. The greatest heliocentric latitude, which is equal to the inclination of the orbit to the plane of the ecliptic, will take place in 1825, on or about the 1st of of April, 25th of July, and 13th of November.

4. The planet Venus offers the same phenomena as Mercury, with this difference, that its phases are much more sensible, its oscillations more extensive, and their period more considerable. The greatest elongations of Venus vary from 45° to 47° 42'; its mean elongation is, therefore, 46° 21', and the mean length of its entire oscillations, or synodic revolution, is 584 days.

5. The retrogradations of Venus commence or finish, when the planet, approaching the Sun in the evening or receding from that body in the

morning, is distant from the Sun about 28° 48′. The mean arc of retrogradation is about 16° 42′, and its mean duration 42 days.

This planet is never seen in the eastern part of the heavens when the Sun is in the western, but always seems to attend that body in the evening, or to give notice of its approach in the morning, and never receding from the Sun more than about 47 degrees. This proves that the orbit of Venus includes that of Mercury, but is included by that of the Earth.

6. From inferior to superior conjunction Venus is to the westward of the Sun, and therefore rises before the Sun, and by the splendour of her appearance, being much noticed, is called the morning star. From superior to inferior conjunction, she appears to the eastward of the Sun, and therefore does not set till after the Sun, and is then called the evening star.

When in the former of these situations, Venus was called by the Greeks Phosphorus, and in the latter Hesperus. The evening and morning stars were at first supposed to be different, and it is said that Pythagoras was the first person who discovered they were the same.

When Venus is an evening star, and at her greatest distance from the Sun, or what is termed her greatest eastern elongation, she appears, when viewed with a telescope, to have a semicircular disc, like the Moon in the last quarter, with its convexity turned to the west. From that time during her app approach to the Sun, her splendour increases for a while, though the quantity of the illuminated disc diminishes like the Moon; but her apparent diameter, when measured by the distance of the horns, is found to be increased. At the time of her greatest elongation, Venus appears to be stationary, with respect to the Sun, for some time. After this her motion eastward becomes slower than the Sun's, and then she approaches nearer to the Sun, as just remarked. At a certain point she becomes stationary with respect to the fixed stars, and then her motion becomes retrograde, or appears to be directed westward with respect to the fixed stars. At last she approaches the Sun, so as to be Lost in his light; but, after some time, she is to be seen to the 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 dise 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 Copernican system of the World, that can be ob

tained.

7. The apparent diameter of Venus, like Mercury, is 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 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.

8. 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 Mersury; for 8 periodic revolutions of Mercury are equal nearly to 13 of Venus; and there are no other intervening whole

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 8 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, n, 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 1882.

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 transit 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.