Abstract
ROTATION PERIOD OF THE SUN FROM FACULÆ.—The fifth part of vol. iv. of the Publications of the Astrophysical Observatory at Potsdam has recently appeared, and contains a determination by Dr. J. Wilsing of the rotation period of the sun from observations of faculæ. The previous determinations of the solar rotation have been based upon observations of the spots, or upon the relative displacement of lines in the spectra of the east and west limbs, for, as faculæ can usually only be seen well when near the limb, and therefore can seldom be watched for more than three consecutive days, and as they often undergo rapid changes, they did not seem well suited for such a discussion. Their irregular and often straggling shapes, too, render measures of their positions much less precise than those of spots. Notwithstanding these difficulties Dr. Wilsing's inquiry seems to have met with a measure of success. Of the faculæ shown on the solar photographs taken at Potsdam from 1884 March 14 to August 31, 144 groups were seen at three or more different epochs, at intervals of one or more semi-rotations. Arranging these according to their distribution in solar latitude, in zones of 3° wide, Dr. Wilsing finds practically the same rotation period for each zone from + 24° to - 33°, the difference from the mean of the daily angular motion only exceeding 2′ in a single instance, and in many cases amounting only to 20″ or 30′. As these differences are so small and follow no law, it would appear that, whilst, as Carrington and Spoerer have shown, the different spot zones have different rates of rotation, the layer of the faculæ rotates as a whole. Since the faculæ are certainly at a higher level than the spots, this conclusion is one which will fail to be accepted until we have much further and more convincing evidence than we have at present. In the present discussion it sometimes happens that a group of faculse is considered as identical with an earlier group seen two or three semi-rotations earlier, when the same part of the sun has been seen in the interval, but without showing the group, although the district has been favourably presented for displaying faculæ. In such a case, and particularly if several semi-rotations have elapsed, the two groups will be identified or not according to the rotation period assumed; so that if a single rotation period for the whole sun be assumed in the preliminary reductions of position for the sake of identification of the groups, there will be an inevitable tendency towards a single rotation period in the final result.
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Our Astronomical Column . Nature 38, 206–207 (1888). https://doi.org/10.1038/038206a0
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DOI: https://doi.org/10.1038/038206a0