Page:The Music of the Spheres.djvu/233

 Areas near the equator were found to complete a revolution in 25 days, but they were seen to forge ahead of those regions in higher latitudes; 45 degrees north and south latitude, the rotation period is 27½ days; 60 degrees north and south latitude, the period is 31.2 days, and in 80 degrees north and south latitude, the period is 35.3 days. This alone can be regarded as proof that the sun's apparent surface is not solid but is either in a liquid or gaseous state.

A total eclipse of the sun is caused by the moon passing between the earth and the sun, and since the moon is apparently the same size as the sun in the heavens, it hides the bright face of this luminary from our view; but the long shadow which trails behind the moon must touch the earth. The shadow moves along a narrow strip as the earth revolves and only those who are in this shadow can see the sun totally eclipsed.

If the shadow of the moon does not quite touch the earth, the moon's disk does not quite cover the sun. Then the dark body of the moon is seen surrounded by a ring of light. This is called an annular eclipse.

In a partial eclipse, the sun's disk suddenly becomes indented on one side, the indentation slowly increasing for some time, and then diminishing until it disappears altogether.

Total solar eclipses are extremely infrequent in any one place because of the smallness of the cone of the moon's shadow. This shadow draws a black streak across the earth, which, under favorable circumstances, may have a breadth of a little more than 160 miles and a length of 10,000 to 12,000 miles. The width of the shadow depends upon the nearness of the moon for if its shadow just touches the earth, the streak will have no sensible width. A partial solar eclipse is visible 2,100 miles or more on either side