73. The Orbits of these Moons are represented in the Scheme of the Solar
System by four small circles marked 1. 2. 3. 4. on Jupiter’s Orbit ♃;
but are drawn fifty times too large in proportion to it. The first Moon,
or that nearest to Jupiter, goes round him in 1 day 18 hours and 36
minutes of our time; and is 229 thousand miles distant from his center:
The second performs it’s revolution in three days 13 hours and 15
minutes, at 364 thousand miles distance: The third in 7 days three hours
and 59 minutes, at the distance of 580 thousand miles: And the fourth,
or outermost, in 16 days 18 hours and 30 minutes, at the distance of one
million of miles from his center. The Periods of these Moons are so
incommensurate to one another, that if ever they were all in a right
line between Jupiter and the Sun, it will require more than
3,000,000,000,000 years from that time to bring them all into the same
right line again, as any one will find who reduces all their periods
into seconds, then multiplies them into one another, and divides the
product by 432; which is the highest number that will divide the product
of all their periodical times, namely 42,085,303,376,931,994,955,904
seconds, without a remainder.
[Sidenote: Parallax of their Orbits, and distances from Jupiter.