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of the most powerful telescopes of the world has it ever certainly been seen by the human eye. Its orbit is decidedly eccentric. Owing to the ellipticity of the planet, it possesses the remarkable peculiarity that its major axis, and, therefore, the perihelion point of its orbit, performs a complete revolution in about a year.

It has sometimes been questioned whether these satellites are round bodies, like the planets and most other satellites. Some observers, especially Barnard and W. H. Pickering, noticed curious changes in the form of the first satellite as it was crossing the surface of the planet. At one time it looked like a double body. But Barnard, by careful and repeated study, showed that this appearance was partly due to the varying shade of the background on which the satellite was seen projected upon the planet, and partly to the differences in the shade of various parts of the satellite itself.

During their course around the planet these bodies present many interesting phenomena, which can be observed with a moderate sized telescope. These are their eclipses and transits. Of course Jupiter, like any other opaque body, casts a shadow. As the satellites make their round they nearly always pass through the shadow during that part of their course which is beyond the planet. Exceptions sometimes occur in the case of the fourth and most distant satellite, which may pass above or below the shadow, as our moon passes above or below that of the earth. When a satellite enters the shadow, it is seen to fade away gradually, and finally to disappear from sight altogether.