Sun-spots are, as a rule, thousands of miles across and usually appear in groups which cover an immense area on the surface of the sun, but, since the sun lies 93,000,000 miles away, a spot large enough to contain the earth would appear to us about the size of a pinhead on a pincushion. Right here one should be warned again not to attempt to look at the sun without adequate protection (such as smoked glasses) for the eyes. A photograph taken through a large telescope is the most satisfactory. Although these dark blotches--which are only comparatively dark--were called "spots" and were later drawn as saucer-like depressions, modern research and increased power in the telescopes has revealed them as gigantic solar tornadoes. These tornadoes usually last from a few days to several months, and are especially numerous in cycles of 11 years. During sun-spot activity eruptive prominences rise to heights of many thousands of miles and streams of electrons are shot forth with such force that they may possibly even penetrate deeply into our own atmosphere causing disturbances such as magnetic storms and auroras. [Figure 135: SUN-SPOTS. The earth is so small compared with the huge bulk of the sun that it would be lost in one of these "spots." Photograph by the Mount Wilson Observatory.] If a total solar eclipse occurs at a time when these great magnetic storms are raging in particular fury on the sun's surface, the corona always appears brighter and more intricate than when the sun is peaceful. Sun-spots were very numerous in 1915 and 1916. The next spot maximum will occur about 1926, then in 1937 and so on. For three years or four years they appear with great frequency, then declining to a minimum for three or four years, rise to a maximum again; thus the period from maximum to maximum is about 11 years. Through watching these black spots disappear around one edge of the sun and then reappear after an interval of time on the other side, Galileo discovered that the sun turned completely around on its axis in 25 days. [Figure 136: SUN-SPOTS, DIRECT PHOTOGRAPH. Compare with photograph on page 214, also made on same day by the Mount Wilson Observatory.] Further study disclosed the still stranger fact that the sun does not turn around in a solid body. 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. AN ECLIPSE OF THE SUN