Page:In the high heavens.djvu/104

 minute, of all the objects of which our telescopes can take cognisance? Why is it that an apparition of a great comet with a blazing tail half across the sky would not have for astronomers half the interest possessed by this little stranger? Why is it that sun-spots, lunar craters, the ice-caps of Mars, and his newly discovered lakes, the revelations as to Venus, were for the moment forgotten or unheeded; why did everyone become so eager to learn all about this tiny little moon of Jupiter, and on the tiptoe of expectation for every further item of intelligence from the Lick Observatory?

Let me say at once that this extreme interest in the little object appears most natural. I am glad to share in it, for there seem to be certain very good reasons why such interest should be felt. In the first place, Jupiter has always been a favourite telescopic object. The globe itself is so vast that the features are on a sufficiently large scale to be discernible with comparatively small instrumental power. Thus it is that the cloud belts on the great planet are familiar objects to everyone who has ever used a telescope. Then, too, the ever- varying positions of the four older satellites make them a spectacle that is always attractive. At one time we are watching an eclipse, in which a satellite plunges into Jupiter's shadow and disappears for a while. At another time we note how the wanderer vanishes by occultation behind the body of the great globe around which it revolves. Then, too, there are the singularly delicate and beautiful phenomena of the transits of the satellites in front of the planet. In this case not only is the satellite itself often to be traced in the act of crossing, but, as a far more striking manifestation, its dark shadow is thrown on the brilliant globe. The