Page:In the high heavens.djvu/112

 The distance of this new moon from the centre of the planet appears to be about 112,500 miles, and the period of each revolution is about 11 hours 57 minutes. It will thus be noticed that the satellite revolves round Jupiter in a period which exceeds that required by Jupiter to accomplish a rotation on his own axis, namely, 9 hours 55 minutes. The new satellite is so close to the surface of Jupiter that the difficulty of this detection is greatly enhanced by the fact that it is so frequently hidden by the great globe. Only for a comparatively small part of each revolution does the little body appear well away from the margin of the planet. When most remote it will be at a distance of 36 seconds from the edge, that is, about two-thirds of the diameter of Jupiter. Then six hours later it will be at a similar distance on the opposite side of its orbit. It is often difficult to observe one of the large satellites when in the act of transit across the planet's disc, so that we can hardly be surprised that the transits of an object which is such an extremely small fraction of their size should not be perceived.

Of course, there is a notable difference between the case of a transit of a satellite over its primary and that of a planet, like Venus or Mercury, in front of the sun's disc. In the latter case the planet appears as a black spot against the brilliant background. In fact, it may be remembered that an unsuccessful search for an intra-Mercurial planet has actually been conducted in the manner thus suggested by seeing if it could not be observed during the progress of the transit. But the case is very different when a satellite of Jupiter transits over the face of the planet. The lustre of the satellite,