Our Astronomical Column

Abstract

JUPITER'S SATELLITES.—In a previous note we referred to the period of rotation of the third satellite of Jupiter as determined by recent observations made by Mr. Douglass at the Lowell Observatory. In the current number of the Astronomischen Nachrichten (No. 3432) he communicates a more detailed description of the observations, together with reproductions of the surface markings, which led him to the determination of the length of the period of rotation. Attempts were at first made to discover signs of surface movement within three to five hours of continuous observation. None, however, could be detected, so that the 24-hour period had to be discarded as untenable. A large series of sketches showed that the satellite's period was nearly the same as its period of revolution, namely, 7d. 5˙ih. ± 1˙2h. The surface markings seem to take the form of lines, their maximum width being estimated at less than 0″˙1, or 200 miles. The fourth satellite was also observed minutely, and on its surface were seen markings which are described as similar to those found on the third. Its period of rotation was also noted as being probably nearly equal to its period of revolution round its primary. Prof. W. H. Pickering's discovery of the variability in the elongation of the first satellite seems to be confirmed by Mr. Douglass, who found this satellite at the time of observation “most remarkably elongated.”