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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,
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 hare 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 flattened 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. (1.) The luminous part does not end abruptly; on the contrary, its light diminishes gradually, which diminution may be entirely explained by the twilight on the planet. The existence of an atmosphere
which diffuses the rays of light into regions where the snn has already set, has hence been inferred. Thus, on Venus, the evenings, like ours, are lighted by twilight, and the mornings by dawn. (2.) The edge of the enlightened portion of the planet is un • even and irregular. This appearance is doubtless the effect of shadows cast by mountains. Spots have been noticed on its disk which are considered to be traceable to clouds. Indeed, Herschel thinks that we never see the real body of the planet, but only its atmosphere loaded with vapors, which may mitigate the glare of the intense sunshine.
Satellites.—Venus is not known to have any moon.
Sign, e, i circle with Eqtutor and Meridian,
The Earth is the next planet we meet in passing outward from the sun. To the beginner, it seems strange enough to class our world among the heavenly bodies. They are brilliant, while it is dark and opaque; they appear light and airy, while it is solid and firm; we see in it no motion, while they are constantly changing their position; they seem mere points in the sky, while it is vast and extended. Yet at the very beginning we are to consider the earth as a planet shining brightly in the heavens, and appearing to other worlds as a star does to us: we are to learn that it is in motion, flying through its orbit with inconceivable velocity; that it is not fixed, but hanging in space, held by an invisible power of gravitation which it cannot evade; that it is small and insignificant beside the mighty globes that so gently shine upon us in the far-off sky; that our earth is only one atom in a universe of worlds, all firm and solid, and equally well fitted to be the abode of life.
Dimensions.—The earth is not "round like a ball," but flattened at the poles. Its form is that of an oblate spheroid. Its polar diameter is about 7,899 miles, and its equatorial about 7,925£. The compression is, therefore, about 26J miles. (See table