3 day 13 hrs 13 min 42.1 sec. 7 days 3 hrs 42 min 33.4 sec. So near Jupiter is the fifth satellite that his disk, as seen from the surface of the satellite, would stretch more than half way from horizon to zenith. Referring to conditions which produce eclipses of sun and moon, illustrated on page 293, and remembering that the orbits of Jupiter's satellites nearly coincide with the plane of its path, it is clear that eclipses of the sun and of Jupiter's moons must occur every time a satellite goes round the planet. So there are nearly 9000 eclipses of the sun and moon annually, from some point or other of Jupiter's disk.
Eclipses vary from just a few minutes to nearly five hours in length.
Eclipses, occultations, and transits are predicted many years in advance in the Ephemeris , and are very interesting to observe, even with small telescopes. An opera glass will show at a glance the moons which are not in transit, occultation, or eclipse. Sometimes all four disappear for a time, though not again in the 19th century. Eclipses of all four moons exhibited the same discrepancy. Half this number, or 499, is the constant factor in 'the equation of light'.