150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



THE CORONA 



The total light of the corona is roughly one-millionth that of the 

 noonday sun or one-half that of the full moon. The coming of a 

 total eclipse, particularly the last few seconds before totality, the 

 engulfing shadow of the moon coming from the west, the sudden 

 coming of darkness and then the gorgeous corona make a never-to- 

 be-forgotten spectacle, still further enhanced by the beauty of the 

 rosy red prominences. Regarding the 1925 eclipse, tlie New York 

 newspaper folk state that no phenomenon of modern life had so 

 many words written per unit of duration of the event as that particu- 

 lar eclipse. 



The corona may be photographed by cameras large or small, fast 

 or slow. In spite of the excellent qualities of modern photographic 

 technique, it is still impossible to get an adequate photograpli that 

 will represent all the details of the corona that are readily visible to 

 the naked eye. The inner corona is very brilliant, while the outlying 

 portions of the coronal streamers fade off gradually to nothingness. 

 From the lofty perch of Pike's Peak in 1878 the corona was detected 

 to 12 diameters from the sun's edge, a distance of over 10 million 

 miles. An exposure long enough to portray the faint outlying streamers 

 will have the inner corona quite burnt out through overexposure. 



With cameras of great length it is necessary to counteract by 

 mechanical means the diurnal motion. This may be done by mounting 

 the telescope horizontally and then using a coelostat mirror or by 

 pointing the object glass directly to the sun by means of erecting a 

 tower. In 1900 the Smithsonian Institution used a camera of 135 

 feet focus, and its great length required it to be arranged horizontally. 

 Unfortunately, the heat of the sun may warp the plane mirror of the 

 coelostat and spoil the focus. The result has been that in recent years, 

 when exact definition is desireil, it is obtained by a camera pom ted 

 directly at the sun. For moderate focal lengths up to about 15 feet 

 this may be obtained by mounting on a polar axis, but for greater 

 lengths the tower telescope is necessary with the diurnal motion 

 counteracted by a motion of the photographic plate. The Swarthmore 

 astronomers have obtained exquisite definition \vith focal lengths of 

 63 and 65 feet. Equally good definition may be secured with cameras 

 of 15 feet focus mounted on a polar axis. The question is whether it 

 is more desirable to photograph on the smaller scale and then enlarge 

 four times if the greater scale is needed or to obtain scale without 

 enlargement by the greater focal length. As for the final definition 

 of the large-scale photographs, there is little to choose between the 

 two methods. The individual astronomer must decide the problem 

 for himself, depending on a number of factors, chief among which is 

 the equipment he owns and the experience he has had. Then he must 

 decide whether it is more convenient and cheaper to take from home 



