"JWS" Events, and Transits

One of the reasons that JWS events are relatively uncommon is that the movements of the three inner Galileans (Io, Europa & Ganymede) are synchronised due to gravitational attraction. Not only are their orbital periods almost exactly in the ratio 1:2:4 but in fact their positions in their orbits are also locked together. This has the following consequences:-

1) It is not possible for all three inner Galileans to be in transit, occulted or eclipsed simultaneously.

2) If the two inner satellites are in transit or occulted, the third cannot be.

3) If satellites two and three are in transit, number one must be occulted (and vice versa).

4) If satellites one and three are in transit/occulted, number two will be occulted/in transit just one time in three.

Of course it is still possible for three moons to be in transit at the same time, but one of them has to be the outermost Galilean (Callisto). However, as this moon moves slowly, and sometimes even misses the disc of Jupiter completely, such events are actually rather less common than JWS events themselves: probably not what you would have expected. In the 60yrs from 1981 to 2040 there are only 10 such events, an average of about 17 per century as compared to about 23 per century for JWS events. There's a brief "3T" event visible from the UK at 7:10am on 24th January 2015 but the next events visible at night in the UK aren't until 2026 and 2032, so you've plenty of time to prepare!

As well as the moons themselves transiting Jupiter, their shadows can also do so: this is the Jovian equivalent of a solar eclipse. Depending on the alignments, the moon; its shadow, or both objects can be in transit: one type of transit can also follow another, of course. Triple Transits involving shadows as well as moons are more common than those involving moons alone as the number of possible combinations is much greater. In the years 1981 to 2040 there are 21 such events (excluding the 10 mentioned above): 35 per century. The next one visible from the UK (4:30am on 12th October 2013) is a triple shadow transit.