Page:Astronomy for Everybody.djvu/235

For the same reason the satellites generally pass across the disk of the planet in that part of their course which lies on this side of it. The general rule is that, when a satellite has impinged on the planet, it looks brighter than the latter, owing to the darkness of the planet's limb. But, as it approaches the central regions, it may look darker than the background of the planet. Of course this does not arise from any change in the brightness of the satellite, but only from the fact, already mentioned, that the planet is brighter in its central regions than at its limb.

Yet more interesting and beautiful is the shadow of a satellite which, under such circumstances, may often be seen upon the planet, looking like a black body crossing alongside the satellite itself. Such a shadow is shown in the picture of Jupiter on page 204.

The phenomena of Jupiter's satellites, including their transits and those of their shadows, are all predicted in the astronomical ephemerides, so that an observer can always know when to look for an eclipse or transit.

The eclipses of the inner of the four older satellites occur at intervals of less than two days. By noting their times, an observer in unknown regions of the earth can determine his longitude more easily than by any other method. He has first to determine the error of his watch on local time by certain simple astronomical observations, quite familiar to astronomers and navigators. He thus finds the local time at which an eclipse of the satellite takes place. He compares this with the time predicted in the ephemeris. The difference gives his longitude