Our Astronomical Column

    Abstract

    THE DEARBORN OBSERVATORY, CHICAGO.—The annual report from Prof. Hough to the Board of Directors of the Chicago Astronomical Society, dated May last, has been issued. The planet Jupiter has been made a special object of study with the great equatorial, the first observation having been secured on May 6, 1880, and the last on January 30, 1881. The observations made at the Dearborn Observatory do not support the idea that the surface of the planet is “subject to sudden and rapid changes, which may be accomplished in a few days or even a few hours.” On the contrary, the observations in question show that all minor changes in the markings or spots have been slow and gradual. “In fact the principal features have been permanent, no material change being detected by micrometer measurement.” With regard to the rotation of Jupiter, the discussion of the measures on the great red spot made from September 25, 1879, to January 27, 1881, or over a period of 490 days, gave for the mean value 9h. 55m. 35·2s., but when the individual observations are compared with it, a well-marked maximum displacement of the centre of the spot, to the amount of 1°·4, is exhibited, apparently indicating that it gradually oscillated to this extent in longitude, which on the surface of Jupiter corresponds to about 3200 miles. The observations however may be well represented by making the period of rotation a function of the time; thus the period 9h. 55m. 33·2s. + 0·18s. √t is found to satisfy all the measures with a mean maximum error of 0″·5: the zero-epoch being September 25, 1879, and t the number of days after that date. The mean-rotation period derived from observations of polar spots is 9h. 55m. 35·1s., that deduced from the small spots indicating an average displacement during two months of 2″, or about 4600 miles. The rotation resulting from the observations of equatorial spots is 9h. 50m. 9·8s. with uniform motion. Prof. Hough states that the actual size of the great red spot, as seen with the Chicago telescope (18½ inches aperture) is—length, 29,600 miles; breadth, 8300 miles; and he remarks that smaller telescopes make the approximate length considerably less than the real value.

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    Our Astronomical Column . Nature 24, 477–478 (1881). https://doi.org/10.1038/024477a0

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