Page:Popular Science Monthly Volume 92.djvu/899

 Popular Science Monthly

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��date in question, and such information always plays an important part in the selection of sites for eclipse observations. Generally speaking the chances of favor- able weather are best along a line from eastern Oregon through Idaho and Colo- rado, and this will be a favorite part of the track for the further reason that the sun will be higher in the sky at the time of the eclipse than farther east.

The world over there are about 70 total eclipses of the sun in the course of a century, but at any one place on the globe there is, on an average, only one in about 360 years. In the whole present territory of the United States (outlying possessions not included) there were only eight total solar eclipses during the nine- teenth century, and there will be the same number during the present century: viz, in 1918, 1923, 1925, 1945, 1954, 1979, 1984 and 1994.

A Case Where the Moon Obscures the Sun

The fundamental facts relating to a solar eclipse are quite simple. The moon, in her monthly revolution around the earth, occasionally passes between us and the sun. The moon has no light of her ow^n, and when she shines it is by reflected sunlight. In a solar eclipse her unilluminated side is turned toward us, so that we see her as a black disk, intervening in front of the sun. The reporters who write up scientific events for the newspapers often refer to this disk as "the shadow," through confusion

���The principal heavenly bodies, which will be visible near the sun [during the eclipse. Although many astronomers have given up the hope that any planet or planets moving within the orbit of Mercury, the planet nearest the sun, will ever be found, the search will be continued during the coming eclipse

��with eclipses of the moon, in which the darkening is due to the shadow of the earth. What we see is not a shadow, but the moon itself. The sun's diameter is about 400 times as great as the moon's, and the sun's average distance from the earth is about 390 times that of the moon. The attached diagram, which is correctly drawn to scale, shows the long, tapering shadow cast by the moon as she revolves through space, and shows why there is only a small area of the earth's surface from w'hich, at any one time, the sun is completely hidden by the moon. Owing to variations in the distance of the moon and the length of her shadow, there are some eclipses in which the latter does not reach all the way to the earth. Under these circumstances an observer directly in the line passing through the sun and moon sees, at the time of eclipse, a circle of sunlight extending all around the lunar disk, and the eclipse is said to be "an- nular."

The Moon's Shadow a Fast Traveler

The shadow cone on June 8 will first touch the earth at sunrise in the Pacific Ocean, not' far south of Japan. Thence it will sweep eastward, entering the United States in southwestern Washington at 2:55 P.M., Pacific Standard Time. It will then be traveling at a speed of 33 miles a minute. Striking southeast, it will cross, the Mississippi River at 5:37 P. M., Central Time, reach the coast of Florida at 6:42 p. m., Eastern Time, and leave the earth after reach- ing the vicinity of the Bahamas at sunset. The actual time re- quired for the journey across the United States (from 2:55 P. M., Pacific Time, to 6:42 P.M., Eastern Time) will be 47 minutes. "Day- light saving" necessi- tates the adding of an hour to these times. •

The coming eclipse will be observed by par- ties from all the leading observatories of Amer- ica. But for the un- happy state of public affairs abroad, we should

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