Letter | Published:

Dark Transit of Jupiter's First Satellite


ON May 18, at 8h., on observing Jupiter with my 10-inch reflector, p. 252, I saw three very dark spots—one near the planet's centre, and two others not far advanced upon the east limb. These I took to be the shadows of satellites, and on reference found that the shadows of Satellites I. and II. were really upon the planet; also Satellite I. itself. The latter was evidently the spot near the centre of the disk, and it appeared almost equally as black as the shadow. The satellite was situated close to the equatorial white spot, and in point of fact was projected upon the north-east borders of that object. The latter was estimated on the central meridian at 8h. 5m., so that its longitude, computed on the diurnal rate of 878°.46 (= rotation of 9h. 50m. 7.42s.), was 94°.3.

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