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    Naturevolume 101page292 (1918) | Download Citation



    KODAIKANAL OBSERVATORY REPORT.—The report of the Director of the Kodaikanal Observatory for the year 1917 has been received. The weather during the year was generally unfavourable, according to Indian standards, but substantial progress in many departments of solar research is recorded. Direct photographs of the sun were obtained on 294 days, monochromatic images of the disc in K light on 328 days, prominence plates on 262 days, and Hα disc plates on 255 days. Judging by the mean latitude of spots, it would appear that the maximum of the sunspot cycle was not reached, though the northern hemisphere may possibly have, attained its greatest activity. The prominences, with a mean daily number of 19·8, were in excess of those recorded in 1916; the northern hemisphere continued to be more active than the southern, as regards both prominences and spots, and also as shown by prominences projected as absorption markings on the disc. Work with the large grating-spectrograph included the spectrum of Venus (see NATURE, vol. ci., p. 192), sun and arc comparison spectra, and the spectrographic determination of the solar rotation, in addition to experimental work on the “pole effect” in the iron arc. It has been found that most of the iron arc lines in the region between 4337 and 4494 show a tendency to shift towards the red with increasing exposure time, indicating that they are unsymmetrically widened towards the red to a very slight degree. The vertical motion-shift of 3 km./sec. reported by Perot for the B group of oxygen (telluric lines) was not confirmed by observations made at Kodaikanal. An attempt to photograph the conjunction of Regulus and the sun on August 22, in infra-red light, was unsuccessful, but the sky was not sufficiently clear to give a satisfactory test of the method. Time determinations and meteorological and seismological observations were also carried on.

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