This is exactly what takes place when the Moon gets between the Earth and the Sun and all of these bodies are in a straight line, as shown in Fig. 123; the Moon will then cover up the Sun and the shadow of the Moon will fall upon the Earth in a circle about 100 miles in diameter, as shown in Fig. 124. It is thus that the Sun is eclipsed. [Illustration: FIG. 124.—TOTAL ECLIPSE OF THE SUN, SHOWING PATH OF THE SUN.] [Illustration: FIG. 125.—TOTAL ECLIPSE OF THE SUN, FROM PHOTO.] [Illustration: FIG. 126.—ANNULAR ECLIPSE OF THE SUN.] [Illustration: FIG. 127.—PARTIAL ECLIPSE OF THE SUN.] As the Moon is traveling round the Earth and the Earth is turning round on its own axis, the Moon’s shadow moves across a path or trail that is only about 100 miles wide, and it moves very fast, too, for it usually takes less than five minutes for the Moon to sweep over the face of the Sun. There are three kinds of eclipses of the Sun. The first is called a _total eclipse_, and this takes place when the Moon covers the entire face of the Sun, as in Fig. 125. The second is called an _annular eclipse_ and this takes place when the Moon does not completely cover the Sun but leaves a bright ring exposed, as shown in Fig. 126. The reason the Moon covers the Sun completely during a total eclipse and does not cover all of it during an annular eclipse is because the orbit of the Moon around the Earth is not a perfect circle, and so sometimes the Moon is nearer the Earth than at other times and this makes the Moon seem larger or smaller, as the case may be. The third kind is called a _partial eclipse_. If we are not in the direct path of the shadow of the Moon we may see the Moon pass over only a part of the Sun, as shown in Fig. 127. The only total eclipses of the Sun which can be seen in the United States in the next 30 years are the following: ---------------+---------------+----------------------------------- Date of | Time of | Eclipse | Total Phase | Course of Moon’s Shadow ---------------+---------------+----------------------------------- 1918, June 18,| 2 minutes | Oregon to Florida 1922, Sept. 2,| 6 minutes | Pacific Ocean, U. S. & West Indies 1923, Sept. 10,| 3 minutes | U. S. & Atlantic Ocean 1930, April 28,| 2 seconds | U. S. & Canada 1945, July 9,| 1 minute | U. S., Canada, Scandinavia | | and Russia ---------------+---------------+------------------------------ Note: The eclipse of 1930 will be an annular eclipse. =Finding a Comet.=—To the naked eye a great _comet_ looks like a bright star with a long, glowing tail. In the long ago a comet was called a hairy star, for the early Greeks pictured the tail of a comet as being made of long hair and so from their language we get the word comet, which means hair. [Illustration: FIG. 128.—COMET SHOWING NUCLEUS, COMA AND TAIL.]