SOLAR ECLIPSE EXPEDITIONS— MITCHELL 151 



the complete polar axis and mounting for the shorter focal length or 

 take the plate holders and the clock mechanism for moving the plate 

 and then build the tower from materials that may be found locally. 



It is interesting to note that at eclipse expeditions success has 

 always been greater in the direct photography of the corona than 

 in any other branch of the investigations. The self-evident reason 

 is that every well-equipped expedition, no matter what their other 

 programs, attempts to photograph the corona and usually with several 

 cameras. However, there is another reason not so apparent, which 

 is that perfection of focus and seeing, although desirable, are not 

 absolutely essential in obtaining successful photographs of coronal 

 details. A lack of perfect focus plays havoc with spectroscopic 

 photographs but detracts little from the corona for the reason that its 

 structure is nebulous or filmy in detail. However, no astronomer will 

 be satisfied with anything short of absolute perfection in the deter- 

 mination of the best focus. 



THE SHAPE OF THE CORONA 



The general form of the corona can be predicted in advance of the 

 eclipse. Near sunspot minimum are found the extended streamers 

 along the sun's equator and the short plumelike polar brushes, whereas 

 near times of spot maxima the corona is nearly circular in shape, thus 

 resembling a gigantic dahlia. Fortunately, photographs of all scales 

 except the small can be utilized to determine the coronal shape by a 

 very simple type of measurement devised by Ludendorff , of Potsdam. 

 In the author's Eclipses of the Sun, 1935 edition, page 500, a chart 

 gives all the eclipses since 1893 successfully observed (but not in- 

 cluding 1936). The surprising fact is that the most elongated type 

 of corona does not take place exactly at sunspot minimum, or at 

 phase zero of the curve. As the interval from maximum to maxi- 

 mum is 11 years, it is seen from the chart that 2 years or more before 

 spot minimum the corona is quite as elliptical as at zero phase. 

 The most pronounced ellipticity takes place 1 % years before minimum 

 of spots, and likewise the corona closest in shape to a circle takes 

 place V/i years before spot maximum. The recent eclipse of 1932 took 

 place 1.2 years before spot minima and yet its shape placed it at the 

 highest point of the curve. The succeeding eclipse of 1934 was 

 only 0.2 years after spot minimum but instead of having a pronounced 

 ellipticity the corona had lost its minimum type characteristics. 

 The 1936 corona 2.4 years after minimum spots, in shape resembling 

 an irregular five-pointed star, had lost all of the minimum features 

 except the brushes at one pole. Although about 1 year ahead of 

 the predicted spot maximum, the corona of 1937 will be more nearly 

 circular than that of 1936 and will be the typical sunspot maximum 

 eclipse. 



