The discovery through a telescope that Venus had phases firmly established the theories of Copernicus.
When this Polish astronomer, in 1543, announced the astounding theory that the sun was the center of our solar system, he said that if the planets between the earth and the sun could be clearly seen, they would show phases like the moon. When in 1610, Galileo turned his telescope on Venus and beheld the beautiful little crescent, the followers of the ancient system of Ptolemy—who claimed that the earth was the central body around which the sun and planets revolved—were, for the first time, silenced, for they saw that the orbit of the earth must enclose the orbit of Venus, and that they must both revolve around the sun. Even a small telescope will show the phases of Venus, the crescent being far more beautiful than one would imagine.
"Calm star! who was it named thee Lucifer,
From him who drew the third of Heaven down with him?
Oh! it was but the tradition of thy beauty!
For if the sun hath one part, and the moon one,
Thou hast the third part of the host of Heaven—
Which is its power—which is its beauty!"
—Bailey.
When first seen through a telescope, in the evening twilight, Venus is gibbous, or nearly round. It has then just emerged on the farther side of the sun from us. After first becoming visible as a disk, it gradually assumes the figure of a half moon (this is at the point of its greatest elongation when it is at its highest in the sky); then as it again approaches the sun, it gradually becomes more and more like a slender crescent of light. When Venus is showing nearly a full face, it is the whole diameter of its orbit farther away than when it shows a slender crescent. In this position it is 160 millions of miles distant from the earth; when nearest the earth it is only 22 millions of miles distant. This is the reason why it appears much smaller when its full hemisphere is turned toward us than when it is seen as a small crescent. The proportion of this disk to the crescent is as 10 to 65. After becoming a "silver bow," Venus disappears from the evening sky, later to appear on the other side of the sun in the same guise, and gradually, as the days go by, changing to a half moon and on again through all its phases.
The orbit of Venus is tipped slightly from the plane of the earth's orbit; thus only on very rare occasions does it pass directly in front of the sun. Such a passage is called a transit. These transits occur in pairs—two transits being 8 years apart and the successive pairs being separated by 122 years. The last transit occurred on December 9th, 1874, and December 6th, 1882. They will not occur again until June 8th, 2004, and June 6th, 2012. There is only a slight possibility that some young reader may see this.
When passing before the face of the sun, Venus appears as a round, black spot surrounded by an illuminated ring of atmosphere. The spectroscope has shown that this atmosphere is about the same as the earth's, possibly a little more dense, and, strange to say, there is now strong evidence that water vapor and oxygen are conspicuous by their absence.
The dense atmosphere on Venus causes it to reflect light most beautifully but it also obscures its surface so that it is very difficult to see any markings. From time to time, when Venus passes between the sun and ourselves, it approaches to within 25 million miles of the earth, and even less—which is about 10 million miles closer than Mars comes at its nearest approach—yet we can clearly see the surface of Mars for its atmosphere is very tenuous, while the surface of Venus is almost completely hidden under thick white clouds. At such a time, Venus is also a slender crescent while Mars shows a full round face. The spectroscope combined with observations made upon such surface markings as may be seen in full daylight has, however, enabled us to determine that Venus rotates so slowly upon her axis that one face is always turned toward the sun. Thus the day on Venus must last forever and its night-side has only one long night. It has been conjectured that this condition may cause storms of frightful intensity upon its surface, for high air currents on the light torrid side probably flow rapidly to the dark, cold side, with the cold air near the surface of the dark, cold side flowing with equal rapidity to the torrid side. This state of affairs may tend to equalize the intensity of the two extremes of climate. Being almost 30 million miles closer to the sun than the earth, Venus may have a higher temperature than the earth, although the heat may be somewhat modified by the clouds which are always in the sky.
Sometimes a faint, grayish light has been observed on the dark side of the planet when it is nearest the earth. This was at one time thought to be a reflection from a cold and frozen hemisphere, but a more likely suggestion hints that it may be an electrical manifestation in the planet's atmosphere, similar to our Aurora.