Drawings of Jupiter's third satellite

The accompanying drawings of detail on the third satellite of Jupiter, were made through the 24 inch Clark glass of the Lowell Observatory during parts of February and March, 1 8 9 7. Beginning in the latter part of January occasional drawings were made, but the atmosphere was not perfectly steady and there always seemed to be an element of uncertainty in the markings suspected. On February zoth, after making a few drawings of that uncertain kind, an interval of extremely good seeing revealed the »Great Northern Belt « in longitude z6o 0 to zo 0 , with perfect distinctness and definition, indicating the character of the markings to be expected. On the following night the vertical line in longitude o 0 was seen, and on the third night this vertical line was seen to have moved towards the left (terrestrial west) giving at once a rough estimate of the period of rotation. During the first week insufficient care was taken with the orientation; so it is probable that certain early drawings are slightly in error in that respect, and allowance has been made for that in constructing the map. From February zoth to March 1 8 t the drawings were compared and carefully studied from night to night, but, beginning on March 2d and continuing through this series, no comparisons were made with previous observations, in order to make each night's work absolutely independent. The smaller series of eight drawings shows several a ttempts to discover signs of surface movement within the three to five hours of continuous observation. None could be detected and the rotation, therefore, is not approximately twenty-four hours; it must therefore be very closely one week, as shown by the larger series of sketches, or nearly the same as this satellite's period of revolution about Jupiter. Recent observations (Flagstaff, May 14th and subsequent dates) not only confirm the map but indicate a period of rotation of 7d 5~1 ± 1~2, or .one hour longer than its period about its primary. This difference of one hour, it will be observed, is less than the probable error. An ephemeris will be published in a subsequent article, but for the present, it is sufficient to state that all drawings of detail made from January to July, 1897, can be identified by the inferior conjunction with Jupiter, during which the detail in the third column of the larger collection …