_Of the Eclipses of the_ Satellites _of_ Jupiter. The apparent diameters of the inferior Planets are so small, that when they pass betwixt us and the Sun, they only appear like small spots upon the Sun’s surface, without depriving us of any sensible quantity of his light. The shadow of the Earth likewise terminates before it reaches any of the superior Planets, so that they are never eclipsed by us; and the Earth when she is in conjunction with the Sun, only appears like a black spot upon his surface. But _Jupiter_ and his Moons mutually eclipse each other, as our Earth and Moon do; as also doth _Saturn_ and his Moons. The satellites of _Jupiter_ become twice hid from us, in one circulation round ♃; _viz._ once behind the body of _Jupiter_, _i. e._ when they are in the right line joining the centers of the Earth and ♃; and again they become invisible when they enter the shadow of _Jupiter_, which happens when they are at their Full, as seen from ♃, at which times they also suffer eclipses; which eclipses happen to them after the same manner as they do to our Moon, by the interposition of the Earth betwixt her and the Sun. [Sidenote: _Fig. 7._] Let S be the Sun, ABT the Earth’s orbit; and C ♃ D, an arch of _Jupiter_’s orbit, in which let _Jupiter_ be in the point ♃; and let CFDH be the orbit of one of _Jupiter_’s satellites, which we will here suppose to be the farthest from him. These satellites, while they move thro’ the inferior parts of their orbs, _viz._ from D thro’ H, I, to C, seem from the Earth and the Sun to have a retrograde motion; but when they are in the superior part of their orbit, they are then seen to move from West to East, according to their true motion. Now while they describe the superior part of their orbits, they will be twice hid from the Earth, once in the shadow of ♃, and once behind his body. If _Jupiter_ be more Westerly than the Sun, that is, when the Earth is in A, they will be first hid in the shadow F, and afterwards behind the body of ♃ in G: But when the Earth is in B, then they are first hid behind ♃’s body in E, and afterwards fall into the shadow F. While the satellites describe the inferior parts of their orbit, they only once disappear, which may be either in I or H, according to the position of the Earth, in which places they cannot be distinguished from the body of _Jupiter_. [Illustration: Plate 5 _Fig. I_ _Fig. II_ _Fig. III_ _Fig. IV_ _Fig. V_ _Fig. VI_ _Fig. VII_] When the satellites seen from ♃ are in conjunction with the Sun, their shadows will then fall upon ♃, and some part of his body be involved in darkness, to which part the Sun will be totally eclipsed.