It will be seen from Fig. 282 that the shadow of a moon does not fall upon the part of the planet's disk that is covered by the moon: hence we may observe the transit of both the moon and its shadow. The shadow appears as a small black spot, which will precede or follow the moon according to the position of the earth in its orbit. Fig. 283 shows two moons of Jupiter in transit. 260. _Occultations of Jupiter's Satellites._--The eclipse of a moon of Jupiter must be carefully distinguished from the _occultation_ of a moon by the planet. In the case of an eclipse, the moon ceases to be visible, because the mass of Jupiter is interposed between the sun and the moon, which ceases to be luminous, because the sun's light is cut off; but, in the case of an occupation, the moon gets into such a position that the body of Jupiter is interposed between it and the earth, thus rendering the moon invisible to us. The third satellite, _m''_ (Fig. 282), is invisible from the earth _E_, having become _occulted_ when it passed behind the planet's disk; but it will not be _eclipsed_ until it passes into the shadow of Jupiter. 261. _Jupiter without Satellites._--It occasionally happens that every one of Jupiter's satellites will disappear at the same time, either by being eclipsed or occulted, or by being in transit. In this event, Jupiter will appear without satellites. This occurred on the 21st of August, 1867. The position of Jupiter's satellites at this time is shown in Fig. 284. [Illustration: Fig. 284.] Saturn. THE PLANET AND HIS MOONS. 262. _The Orbit of Saturn._--The orbit of Saturn is rather more eccentric than that of Jupiter, its eccentricity being somewhat more than one-twentieth. Its inclination to the ecliptic is about two degrees and a half. The mean distance of Saturn from the sun is about eight hundred and eighty million miles. It is about a hundred million miles nearer the sun at perihelion than at aphelion. 263. _Distance of Saturn from the Earth._--The mean distance of Saturn from the earth at opposition is eight hundred and eighty million miles _minus_ ninety-two million miles, or seven hundred and eighty-eight million; and at conjunction, eight hundred and eighty million miles _plus_ ninety-two million, or nine hundred and seventy-two million. Owing to the eccentricity of the orbit of Saturn, his distance from the earth at opposition and at conjunction varies by about a hundred million miles at different times; but he is so immensely far away, that this is only a small fraction of his mean distance. 264. _Apparent Size and Brightness of Saturn._--The apparent diameter of Saturn varies from about twenty seconds to about fourteen seconds. His apparent size at his extreme and mean distances from the earth is shown in Fig. 285. [Illustration: Fig. 285.]