VISITS TO THE MOON AND PLANETS
The other three are well away from these; Mr. Perrine's sixth and seventh are at nearly the same distance, and may be regarded as a twin pair. Then comes this tangle of wires far outside everything else, looking like a whole lot of orbits; but it is really only one orbit—that of Mr. Melotte's eighth satellite. The fact is that this satellite is so far from Jupiter that the Sun gets quite an undue pull upon it; it is like the man in the Bible who was trying to serve two masters, the result being unsatisfactory, as you remember. But it is even more important to notice that, like Phcebe, it is going round its planet in the wrong direction. We now know of three such satellites; that of Neptune has been known to go round "retrograde" for a number of years, but as it was the single exception to the usual rule no one paid much attention to it. When, however, Professor Pickering found Phœbe, and especially when this going round in the wrong direction had been such a trouble to him, he looked for some explanation; and the one he suggested he illustrated by an experiment with a gyroscope, which we will repeat here. I have a gyroscope mounted in a wooden frame which I can grip firmly with both hands, or one of you can perhaps hold it after it has been set spinning. Now you see when I turn slowly in this direction (which is the same as that in which the gyroscope is spinning) nothing much happens; but if I turn round in the opposite direction, the gyroscope turns upside down. Try it yourself: you
- Since these lectures were given, a ninth satellite to Jupiter, also going round the wrong way, has been found in America. This makes four retrograde satellites.