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are nearly 48° east and west of the sun,* or about 18° more than those of Mercury. She is therefore seen much -earlier in the morning and much later at night. She is "morning star" from inferior to superior conjunction, and "evening star" from superior to inferior conjunction. She is the most brilliant about five weeks before and after inferior conjunction, at which time the planet is bright enough to cast a shadow at night. If, in addition, at this time of greatest brilliancy, Venus is at or near her highest north latitude, she may be seen with the naked eye in full daylight.t This occurs once in eight years, in which interval the earth and planet return to the same situation in their orbits; eight complete revolutions of the earth about the sun occupying nearly the same time as thirteen of Venus. This happened last in February, 1862. A less degree of brilliancy is attained once in twenty-nine months, under somewhat the same circumstances..
Motion ra Space.—Unlike Mercury, Venus has an orbit the most circular of any of the principal
* This distance varies but little, owing to the slight eccentricity of Yenus's orbit.
f Arago relates that Bonaparte, upon repairing to the Luxembourg, when the Directory was about to give him a fete, was much surprised at seeing the multitude paying more attention to the heavens above the palace than to him or his brilliant staff. Upon inquiry, he learned that these curious persons were observing with astonishment a star which they supposed to be that of the Conqueror of Italy. The emperor himself was not indifferent when his piercing eye caught the clear lustre of Venus smiling upon him at midday.
planets. Her mean distance from the sun is about 66,000,000 miles, which varies at aphelion and perihelion within the limits of a half million miles against 15,000,000 miles in the case of the former planet. She makes a complete revolution around the sun in about 225 days, at the mean rate of 22 miles per second; hence her year is equal to about seven and one half of our months. This is a sidereal revolution, as it would appear to an observer at the sun, but a synodic revolution is 584 days. Mercury, we remember, catches up with the earth in 28 days after it reaches the point where it left the earth at the last inferior conjunction. But it takes Venus nearly two and a half revolutions to overtake the earth and come into the same conjunction again. This grows out of the fact that Venus has a longer orbit to travel through, and moves only about one-fifth faster than the earth, while Mercury travels nearly twice as fast. The planet revolves upon its axis in about 24 hours; so the day does not differ in length essentially from ours.
Distance From The Earth.—The distance of Venus from the earth, like that of Mercury, when in inferior conjunction, is the difference between the distances* of these two planets from the sun, and when in superior conjunction the sum of these distances.
* Let the pupil calculate the distances of the earth and Venus from each other, when in perihelion and aphelion, as in the case of Mercury, (See tables in Appendix.)
The figure represents its apparent dimensions at the extreme, mean, and least distances from us. The variation is nearly as the numbers 10, 18, and 65. It would be natural to think that the planet is the brightest when the nearest, and thus the largest,
EXTREME, MEAN. AND LEAST APPARENT SIZE OP VENUS.
but we should remember that then the bright side is toward the sun, and the unillumined side toward us. Indeed, at the period of greatest brilliancy of which we have spoken, only about one-fourth of the light is visible. - At this time, however, many observers have noticed the entire contour of the planet to be of a dull gray hue, as seen in the cut.
Dimensions.—Venus is about 7,500 miles in diameter. The volume of the planet is about four-fifths that of the earth, while the density is about the same. A stone let fall upon its surface would fall 14 feet in the first second: a pound weight removed to its equator would weigh about five-sixths of a pound. From this we see that the force of gravity does not decrease exactly in proportion to the size of the planet, any more than it increases with the mass of the sun. The reason of this is, that the body is brought nearer the mass of the small planet, and so feels its attraction more fully than when far out upon the extreme circumference of a large body,— the attraction increasing as the square of the distance from the particles decreases
Seasons.—As the axis of Venus is very much inclined from a perpendicular, its seasons are similar to those of Mercury. The torrid and temperate
of the nights is very marked. The heat and light are double that of the earth, while the circular form of its orbit gives nearly an equal length to its four seasons.
If the inclination of its axis is 75°, as some astronomers hold, its tropics must be 75° from the equator, and its polar circles 75° from the poles. The torrid zone is, therefore, 150° in width. The torrid and frigid zones interlap through a space of 60°, midway between the equator and poles.
Telescopic Features.—Venus, being an interior planet, presents, like Mercury, all the phases of the moon. This fact was discovered by Galileo, and was among the first achievements of his telescopic observations. It had been argued against the Copernican system that, if true, Venus should wax and wane like the moon. Indeed, Copernicus himself boldly declared that if means of seeing the planets more distinctly were ever invented, Venus would be found to present such phases. Galileo, with his telescope, proved this fact, and, by overthrowing that objection, again vindicated the Copernican theory. This planet is not sensibly fattened at the poles. It is thought to have a dense, cloudy atmosphere. This was established by the fact that at the transit of Venus over the sun in 1761 and 1769, a faint ring of light was observed to surround the black disk of the planet. The evidence of an atmosphere, as well as of mountains, rests very much upon the peculiar appearance attending its crescent shape.