Visual Education (Jan 1923-Dec 1924)

March, 1923 83 Visualized Astronomy Edwin B. Frost Director, Yerkes Observatory, University of Chicago PART II THE advance of astronomy due to the use of photography has been so great that it is necessary for us to select only a few of its most significant applications in the space that is left us in this article. We shall now consider the case of an eclipse of the sun, one of the striking spectacles of nature, full of interest alike to the trained astronomer and to the untutored native of the wilds of Australia. At every fifth or sixth new moon our satellite passes across the face of the sun : there are always two eclipses of the sun in a year, which may be partial or total; but at any one place on the earth a total solar eclipse is a very rare event. The last one certainly visible at Chicago occurred on August 13, 1151. Progress of the Eclipse The eclipse begins with the dark body of the moon indenting very slightly the disk of the sun. The intrusion becomes greater and greater, and finally only a narrow sickle of sunlight is left. Then comes the sudden transformation : totality has begun. The sky has become dark enough to see the brighter stars. Around the black moon there appears a sort of glory, of a pale lavender color. Perhaps scarlet eruptions like flames are seen around the edge of the sun. (It is scarcely seventy years since these were thought to be volcanic eruptions at the edge of the moon !) After two or three minutes, generally, although sometimes after six minutes, the advancing moon discloses the edge which was first covered, and a burst of sunlight shocks the observer with its brilliance. Then this sickle of light grows, until, after another hour, the whole sun is uncovered. The Accurate and Unemotional Camera It is in recording such a phenomenon as this that photography is of inestimable service, for in the few exciting moments it is very difficult for the most skilled artist to get, an accurate drawing of the corona. To attempt to depict its true colors is still more difficult, although this was successfully accomplished at the eclipse of 1918 RISE OF A SOLAR PROMINENCE As photographed with the Spectroheliograph, by Edison Pettit These pictures show the rise of a gigantic mass of gas from the sun's edge on May 29, 1919. In the lowest picture this prominence covers about 320,000 miles and has a height of 125,000 miles. In the middle picture (an hour and a quarter later) it has risen to a height of 135,000 miles. The picture at the top shows it two and one-half hours later, when it had become detached and reached an altitude of 200,000 miles. Intermittently during the day it rose, reaching a maximum height of 470,000 miles. The principal gases included in this mass were hydrogen, helium and the vapor of calcium. On the scale of this picture the earth would have a diameter of a thirtieth of an inch. by Professor Howard Russell Butler of Princeton. Indeed, it is only by photography that we have any certain record of such a phenomenon, and therefore during the few seconds of totality the cameras are clicking and a carefully-planned program is being executed by the observers at an eclipse station. Short exposures of a second or so are needed for the brighter parts of the corona close to the sun, while exposures of thirty seconds, sixty seconds, or even of the whole length of totality, are required for securing the faintest details : no single photograph can correctly depict the faintest streamers and extensions and at the same time give the details of the brighter portions. Some of the most interesting spectroscopic records of an eclipse must be obtained during the second when totality begins and the second when it ends, and nothing but photography can accomplish this. Here the use of the movie camera is of great service, giving a score of pictures of what is going on during the great transformation of the spectrum in those two seconds. Filming an Eclipse Indeed, the whole visible record of an eclipse offers a fine opportunity for a film; but be it noted that the ordinary lens of a movie camera is wholly unsuited to this purpose. Such a camera would make the sun the size of a pin-head, and the image would be burned out during all of the exposure except at totality. The scale must be larger for the picture to be of any use, and a first-class photographic lens with a focal length of some twentyfour inches must be substituted for the regular lens of the movie camera.