arising as it does from sunbeams only, is equal to the lustre of the face of the planet, except in so far as inequalities in the intrinsic reflecting powers of the two bodies may suffice to cause a difference. The shadow of the new satellite on the globe of the planet, though no doubt it would be an extremely small point, would still nevertheless be intensely black in comparison with the surrounding surface, and therefore it might be expected that it ought to be comparatively easy to see when sufficient optical power was available. It must, however, be observed that the diameter of this shadow is considerably less than the tenth of a second, and therefore far too minute to be recognisable as a dark spot. As there is but little variation in Jupiter's distance from the sun, it will be almost equally well displayed at every opposition, if not to observatories in the British Islands, then to observatories elsewhere. Thus, for instance, if the opposition happened to be in June, as will sometimes occur, then, though the planet will be very low down for observers in our latitudes, yet it will be very favourably placed for astronomers in the southern hemisphere. Thus we may hope that we shall speedily accumulate a considerable quantity of observations relative to the new object.
To realise all that is implied by this discovery of an additional moon to the four previously known members of Jupiter's system, it will be necessary to refer to another point. Every one who knows anything of astronomy is aware that the distances of the several satellites from the centre of the planet, and the periodic times in which they revolve, are connected by a definite relation. This is, of course, an immediate inference from Kepler's famous law.
