Page:The Music of the Spheres.djvu/264

 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