74. The Angles under which the Orbits of Jupiter’s Moons are seen from
the Earth, at it’s mean distance from Jupiter, are as follow: The first,
3ʹ 55ʺ; the second, 6ʹ 14ʺ; the third, 9ʹ 58ʺ; and the fourth, 17ʹ 30ʺ.
And their distances from Jupiter, measured by his semidiameters, are
thus: The first, 5-2/3; the second, 9; the third. 14-23/60; and the
fourth, 25-18/60[15]. This Planet, seen from it’s nearest Moon, appears
1000 times as large as our Moon does to us; waxing and waneing in all
her monthly shapes, every 42-1/2 hours.
[Sidenote: Two grand discoveries made by the Eclipse of Jupiter’s
Moons.]
75. Jupiter’s three nearest Moons fall into his shadow, and are eclipsed
in every Revolution: but the Orbit of the fourth Moon is so much
inclined, that it passeth by Jupiter, without falling into his shadow,
two years in every six. By these Eclipses, Astronomers have not only
discovered that the Sun’s light comes to us in eight minutes; but have
also determined the longitudes of places on this Earth with greater
certainty and facility than by any other method yet known; as shall be
explained in the eleventh Chapter.
[Sidenote: The great difference between the Equatoreal and Polar diameters
of Jupiter.