A little ramble through 19th century astronomy - shadows of jupiter's satellites in the 1870s

 Academy, Saturday 16^th January 1875

 Shadows of Jupiter's Satellites—During the last four years Mr. Burton has frequently observed that the shadows of Jupiter's satellites projected on the disc of the planet during transit were elliptical, and that this was, as a rule, the case only when Jupiter was near quadrature, and the shadow therefore seen obliquely.

Mr. Burton's explanation of the phenomenon is that the shadow falls on cumulus clouds, which give rise to the markings on Jupiter’s disc, these clouds being dark and therefore invisible wherever the shadow falls, but in full sunshine scattering the light in all directions. The shadow will thus present exactly the same appearance as a cylindrical hole which sends no light to the eye but allows light from the bright clouds forming its boundary to pass; and such a hole, when viewed obliquely, will appear the more elliptical the greater its depth.

From his estimations of the ellipticity on different occasions, Mr. Burton has deduced a depth of atmosphere of from 3,000 to 9,000 miles, a result which would accord well with the small density of Jupiter as a whole, only a quarter that of the earth. On the hypothesis that the bright clouds are scattered at different levels in an atmosphere of considerable thickness, the observed decrease of brightness near the limb is explained by supposing the sunlight to pass freely into space through interstices between the clouds near the limb, so that none of it is received back again by the eye. Mr. Burton's paper is given in the Monthly Notices for December.

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