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. [Figure 137] 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 of the region where the eclipse is total. [Figure 138: RELATIVE POSITION OF THE SUN, MOON AND EARTH AT THE TOTAL SOLAR ECLIPSE OF SEPTEMBER 10, 1923. (The two words at the point of the shadow's contact with the Earth are "Catalina" and "San Diego.") Drawing by Dr. Mars F. Baumgardt, Curator of Clark Observatory, Los Angeles, Calif.] Since the shadow of the moon passes across the earth at the furious rate of 1300 miles an hour, a total eclipse of the sun lasts for only a few minutes,--and for this length of time, short as it is, some thanks is due to the fact that the earth, in revolving upon its axis, carries the observer and the ground upon which he stands along the same direction in which the shadow is moving. Total eclipses are of extremely rare occurrence and only happen about once in every three hundred years for any selected spot on the earth's surface. An interesting description of a total eclipse may be found in Flammarion's "Astronomy for Amateurs": "The luminous disk of the sun is gradually corroded. Another disk, as black as ink, creeps in front of it and little by little invades it entirely. The atmosphere takes on a wan, sepulchral hue; astonished nature is hushed in profound silence; an immense veil of sadness spreads over the world. Night comes on suddenly, and the stars shine out in the Heavens. It seems as though by some mysterious cataclysm the Sun had disappeared forever." Professor Moulton in his "Astronomy" gives another splendid description: