Triple Eclipses on Jupiter

A rare planetary viewing event occurs this week when three of Jupiter’s largest moons cast their shadows simultaneously on the planet below them: three solar eclipses at the same time.

Three of Jupiter’s moons cast their shadows simultaneously on the planet beneath them on Friday night and Saturday morning, October 11 and 12. Picture the sun coming over your left shoulder, causing the three moons on the left to cast their shadows on the planet to the right, causing eclipses of the sun in three different locations on Jupiter. Credit: Starry Night Software

An eclipse of the sun happens when a moon casts its shadow on the planet below it. For observers located where the shadow falls, the sun appears to be completely blocked by the moon, and they are able to see the prominences and corona around the occluding moon.

In the Earth-moon system, the Earth is a fairly small target and its moon is far away, so it’s rare for the moon’s shadow to fall on the Earth. On average it happens about twice a year, and the diameter of the shadow on the Earth’s surface is quite small, only a couple of hundred miles in diameter. To see a solar eclipse, you need to be in exactly the right place, the place where the moon’s shadow falls.

Because our moon’s orbit is tilted with respect to the Earth, most of the time the moon’s shadow passes above or below the Earth, and no eclipse occurs. Only about twice a year do the three bodies line up exactly so that the moon’s shadow touches Earth. This next happens on November 3.

With Jupiter, the situation is different. Jupiter has four large moons: Io, Europa, Ganymede, and Callisto. These are relatively close to Jupiter and Jupiter is much larger than Earth. As a result, the shadows of Jupiter’s moons cross its face very frequently. The innermost moon Io causes an eclipse on Jupiter once every 1.8 days (42 hours). Even the outermost moon, Callisto, traveling much more slowly, should cause an eclipse every 17 days. In fact it does so less frequently because, like our moon, sometimes its shadow passes above or below Jupiter.

So, if eclipses on Jupiter happen very often, why don’t we see more of them? The timing has to be exactly right. Io may cause an eclipse on Jupiter every 42 hours, but the eclipse itself lasts only a little over 2 hours. Also, Io’s shadow is very small; you need a telescope with at least 90mm aperture to see it. If you aren’t looking for it, you probably wouldn’t see it at all, it’s that small. Also, because of the Earth’s rotation, Jupiter is below the horizon half the time, and often lost in the daylight sky.

If the shadows of its moons fall so often on Jupiter, what are the chances of two shadows falling simultaneously? Pretty good, it turns out. This is especially true because there is a resonance between the orbits of Jupiter’s satellites. Europa’s period of revolution is almost exactly twice that of Io, and Ganymede’s almost exactly four times. Only Callisto doesn’t keep step with the inner satellites.

As a result, double shadow transits usually happen in a group. The current group started with a double transit of Io and Europa’s shadows on September 28 and will continue every few days until November 13.

The rarest of all shadow transit events is when three shadows cross Jupiter’s face simultaneously, and this will happen this coming Friday night, October 11, stretching on into Saturday morning, October 12. Because of Jupiter’s present location, this event is mainly visible in the eastern part of North America. The event is in progress when Jupiter rises around midnight on the east coast, but is completely over by the time Jupiter rises on the west coast. Here are the times of the events to look for in Eastern Daylight Time—some events occur before Jupiter rises:

11:12 p.m. Callisto’s shadow enters

11:24 p.m. Europa’s shadow enters

12:32 a.m. Io’s shadow enters: all three shadows visible

1:37 a.m. Callisto’s shadow leaves

2:01 a.m. Europa’s shadow leaves

2:44 a.m. Io’s shadow leaves

Keep an eye on the moons themselves, because they will also begin to cross Jupiter’s disk: Io at 1:48 a.m. and Europa at 2:02 a.m. There’s an added bonus in the Great Red Spot also transiting at this time. It helps to visualize the moons’ movements in three dimensions, with the sun seeming to come from over your left shoulder. Sometimes you get an almost three-dimensional effect with the moons casting their shadows on the planet beneath.

If you miss this event, there will be three double shadow transits later this month visible over most of North America, on October 16/17, 18/19, and 25/26. In each case, the events mostly occur after Jupiter rises around midnight on the first date, so that’s the night you should mark on your calendar. Remember that the date changes at midnight.