Littell's Living Age/Volume 136/Issue 1752/Bright Spot on Saturn
Prof. Asaph Hall has succeeded in obtaining a number of observations of a bright spot which he had noticed on the night of December 7th, 1876, on the ball of Saturn, and thereby deducing a value of the period of the planet’s rotation, which is probably more accurate than any previous determination. The spot in question was two or three seconds in diameter, round and well defined, and of a brilliant white color. Besides Washington, it was, at Prof. Hall’s request, observed at several other American observatories, and the time of rotation concluded (assuming the spot to have no proper motion on the surface of the planet) is 10h. 14m. 23.8s. mean time. Sir William Herschel’s determination (given in the Philosophical Transactions for 1794) was 10h. 16m. 0.4s., and was derived from the different appearances of a quintuple belt in the winter of 1793–4. Prof. Hall points out a curious mistake, which had been copied into nearly all books on astronomy, assigning 10h. 29m. 16.8s. as Herschel’s value of Saturn’s rotation—this being in fact the time of rotation of Saturn’s ring, not that of the planet itself.