Page:Popular Science Monthly Volume 60.djvu/249

Rh day before the event whether the sky will be clear or clouded. Under these circumstances it is not surprising that many people do not travel to the scenes of total eclipses. Expeditions to eclipses were practically unknown until half a century ago. Before that time man received with varied emotions those which Providence sent him, but did not travel far to seek them. Now, expeditions half way round the earth are common. This is not due entirely to the greater scientific zeal of the present day; probably few living astronomers would care to journey to the antipodes for an eclipse, under the conditions of travel which prevailed one or two centuries ago.

About seventy total eclipses of the sun occur each century. The average duration is, perhaps, three minutes, which amounts to about three and a half hours per century. If some Wandering Jew, at the beginning of the Christian era, had started to observe total eclipses of the sun, and had visited every one possible since that time, he would have had less than three whole days for observation. The time, indeed, would have been much less, since many of these eclipses occurred on the ocean, or at inaccessible regions of the earth, and clouds undoubtedly obscured the sky during half the time of totality. During the last half century, since spectroscopic observations have been carried on, the time during which an individual could have obtained favorable observations has been little, if any, more than a single hour. Under these circumstances the wonder is that it has been possible to accomplish so much. Many men, however, have worked at different stations along the narrow but extended path of totality, and every device which ingenuity could suggest has been utilized in order to obtain as much as possible in the brief seconds of totality. Nothing has contributed so much to increase the amount and accuracy of the results as photography. There is hardly a line of investigation which cannot be done more quickly and better by photographic than by visual methods. Nevertheless it would be a mistake to abandon visual observations altogether.

It may hardly need to be stated that for the most part scientific observations of total eclipses have for their object the promotion of our knowledge about the sun. No one, who understands at all how intimate is our dependence upon that great body, will question the wisdom of such efforts. In order to understand why certain problems can be better studied when the sun's face is covered by the moon, it may be well to outline our knowledge on the subject.

The sun, the center of our system, is an exceedingly hot, intensely bright, highly condensed, gaseous body. Its distance is a little less than 93,000,000 miles. Its volume is more than a million times that of the earth. Its specific gravity is somewhat greater than that of water. A gaseous body, denser than water, is something very different