VENUS

, in Astronomy, one of the inferior planets, but the brightest and to appearance the largest of all the planets; and is designed by the mark , supposed to be a rude representation of a female figure, with her trailing robe.

Venus is easily distinguished from all the other planets, by her whiteness and brightness, in which she exceeds all the rest, even Jupiter himself, and which is so considerable, that in a dusky place she causes an object to project a sensible shadow, and she is often visible in the day-time. Her place in the system is the second from the sun, viz, between Mercury and the earth, and in magnitude is about equal to the earth, or rather a little larger according to Dr. Herschel's observations.

As Venus moves round the sun, in a circle beneath that of the earth, she is never seen in opposition to him, nor indeed very far from him; but seems to move backward and forward, passing him from side to side, to the distance of about 47 or 48 degrees, both ways, which is her greatest elongation.

When she appears west of the sun, which is from her inferior conjunction to her superior, she rises before him, or is a morning star, and is called Phosphorus, or Lucifer, or the Morning Star; and when she is eastwards from the sun, which is from her superior conjunction to her inferior, she sets after him, or is an evening star, and is called Hesperus, or Vesper, or the Evening star: being each of those in its turn for 290 days.

The real diameter of Venus is nearly equal to that of the earth, being about 7900 miles; her apparent mean diameter seen from the earth 59″, seen from the sun, or her horizontal parallax, 30″; but as seen from the earth 18″.79 according to Dr. Herschel: her distance from the sun 70 million of miles; her eccentricity 7/1000 ths of the same, or 490,000 miles; the inclination of her orbit to the plane of the ecliptic 3° 23′; the points of their intersection or nodes are 14° of II and ; the place of her aphelion 4° 20′; her axis inclined to her orbit 75° 0′; her periodical course round the sun 224 days 17 hours; the diurnal rotation round her axis very uncertain, being according to Cassini only 23 hours, but according to the observations of Bianchini it is in 24 days 8 hours; though Dr. Herschel thinks it cannot be so much. See also Planets.

Venus, when viewed through a telescope, is rarely seen to shine with a full face, but has phases and changes just like those of the moon, being increasing, decreasing, horned, gibbous, &c: her illuminated part being constantly turned toward the sun, or directed toward the cast when she is a morning star, and toward the west when an evening star.

These different phases of Venus were first discovered by Galileo; who thus fulfilled the prediction of Copernicus: for when this excellent astronomer revived the ancient Pythagorean system, asserting that the earth and planets move round the sun, it was objected that in such a case the phases of Venus should resemble those of the moon; to which Copernicus replied, that some time or other that resemblance would be found out. Galileo sent an account of the first discovery of these phases in a letter, written from Florence in 1611, to William de Medici, the duke of Tuscany's ambassador at Prague; desiring him to communicate it to Kepler. The letter is extant in the preface to Kepler's Dioptrics, and a translation of it in Smith's Optics, p. 416. Having recited the observations he had made, he adds, “We have hence the most certain, sensible decision and demonstration of two grand questions, which to this day have been doubtful and disputed among the greatest masters of reason in the world. One is, that the planets in their own nature are opake bodies, attributing to Mercury what we have seen in Venus: and the other is, that Venus necessarily moves round the sun; as also Mercury and the other planets; a thing well believed indeed by Pythagoras, Copernicus, Kepler, and myself, but never yet proved, as now it is, by ocular inspection upon Venus.”

Cassini and Campani, in the years 1665 and 1666, discoyered spots in the face of Venus: from the appearances of which the former ascertained her motion round her axis; concluding that this revolution was performed in less than a day; or at least that the bright spot which he observed, finished its period either by revolution or libration in about 23 hours. And de la Hire, in 1700, through a telescope of 16 feet, discovered spots in Venus; which he found to be larger than those in the moon.

The next observations of the same kind that occur, are those of signior Binanchini at Rome, in 1726, 1727, 1728, who, with Campani's glasses, discovered several dark spots in the disc of Venus, of which he gave an account and a representation in his book entitled Hesperi et Phosphori Nova Phenomena, published at Rome in 1728. From several successive observations Bianchini concludes, that a rotation of Venus about her axis was not completed in 23 hours, as Cassini imagined, but in 24 1/3 days; that the north pole of this rotation faced the 20th degree of Aquarius, and was elevated 15° above the plane of the ecliptic, and that the axis kept parallel to itself, during the planet's revolution about the sun. Cassini the son, though he admits the accuracy of Bianchini's observations, disputes the conclusion drawn from them, and finally observes, that if we suppose the period of the rotation of Venus to be 23 h. 20 min. it agrees equally well with the observations both of his father and Bianchini; but if she revolve in 24 d. 8 h. then his father's observations must be rejected as of no consequence.

In the Philos. Trans. 1792, are published the results of a course of observations on the planet Venus, begun in the year 1780, by Mr. Schroeter, of Lilientha<*>, Bremen. From these observations, the author infers, | that Venus has an atmosphere in some respects similar to that of our earth, but far exceeding that of the moon in density, or power to weaken the rays of the sun: that the diurnal period of this planet is probably much longer than that of other planets: that the moon also has an atmosphere, though less dense and high than that of Venus: and that the mountains of this planet are 5 or 6 times as high as those on the earth.

Dr. Herschel too, between the years 1777 and 1793, has made a long series of observations on this planet, accounts of which are given in the Philos. Trans. for 1793. The results of these observations are: that the planet revolves about its axis, but the time of it 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″.79; from whence it may be inferred, that this planet is somewhat larger than the earth, instead of being less, as former astronomers have imagined.

Sometimes Venus is seen in the disc of the sun, in form of a dark round spot. These appearances, called Transits, happen but seldom, viz, when the earth is about her nodes at the time of her inferior conjunction. One of these transits was seen in England in 1639 by Mr. Horrox and Mr. Crabtree; and two in the present century, viz, the one June 6, 1761, and the other in June 1769. There will not happen another of them till the year 1874. See Parallax.

Except such transits as these, Venus exhibits the same appearances to us regularly every 8 years; her conjunctions, elongations, and times of rising and setting, being very nearly the same, on the same days, as before.

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 3/4 of Venus's diameter. It had the same phases as Venus, but without any well defined form; and its diameter scarce exceeded 1/4 of the diameter of Venus. Dr. Gregory (Astron. lib. 6, prop. 3) thinks it more than probable that this was a satellite; and supposes that the reason why it is not more frequently seen, is the unfitness of its surface to reflect the rays of the sun's light; as is the case of the spots in the moon; for if the whole disc of the moon were composed of such, he thinks she could not be seen so far as to Venus.

Mr. Short, in 1740, with a reflecting telescope of 16 1/2 inches focus, perceived a small star near Venus: with another telescope of the same focus, magnifying 50 or 60 times, and fitted with a micrometer, he found its distance from Venus about 10′; and with a magnifying power of 240, he observed the star assume the same phases with Venus; its diameter seemed to be about 1/3, or somewhat less, of the diameter of Venus; its light not so bright and vivid, but exceeding sharp and well defined. He viewed it for the space of an hour; but never had the good fortune to see it after the first morning. Philos. Trans. number 459, p. 646, or Abr. vol. 8, p. 208.

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 1/4 of that of the planet. Others have also thought they saw a like appearance. And indeed it must be acknowledged, that Venus may have a satellite, though it is difficult for us to see it. Its enlightened side can never be fully turned towards us, but when Venus is beyond the sun; in which case Venus herself appears little bigger than an ordinary star, and therefore her satellite may be too small to be perceived at such a distance. When she is between us and the sun, her moon has its dark side turned towards us; and when Venus is at her greatest elongation, there is but half the enlightened side of the moon turned toward us, and even then it may be too far distant to be seen by us. But it was presumed, that the two transits of 1761, and 1769, would afford opportunity for determining this point; and yet we do not find, although many observers directed their attention to this object, that any satellite was then seen in the sun's disc; unless we except two persons, viz, an anonymous writer in the London Chronicle of May 18, who says that he saw the satellite of Venus on the sun the day of the transit, at St. Neot's in Huntingdonshire; that it moved in a track parallel to that of Venus, but nearer the ecliptic; that Venus quitted the sun's disc at 31 minutes after 8, and the satellite at 6 minutes after 9; and M. Montaign at Limoges, whose account of his observations is in the Memoirs of the Academy of Paris, from whence the following certificate is extracted:—Certificate. “We having examined, by order of the Academy, the remarks of M. Baudouin on a new observation of the satellite of Venus, made at Limoges the 11th of May by M. Montaign. This fourth observation, of great importance for the theory of the satellite, has shewn that its revolution must be longer than appeared by the first three observations. M. Baudouin believes it may be fixed at 12 days; as to its distance, it appears to him to be 50 semidiameters of Venus; whence he infers that the mass of Venus is equal to that of the earth. This mass of Venus is a very essential element to astronomy, as it enters into many computations, and produces different phenomena: &c.