Catch the Last Best Antics of Jupiter’s Moons

Jupiter's four brightest moons continue to eclipse and occult one another, but time is running out. Only a few easy-to-see events remain. See them soon or wait six more years!

Mutual satellite events can occur in any of six ways, depending on slight differences in the moon's angular sizes and relative positions. Sky & Telescope

Mutual satellite events can occur in any of six ways, depending on slight differences in the moon's angular sizes and relative positions.
Sky & Telescope

Maybe you've been fortunate and seen some of the more dramatic mutual occultations and eclipses of Jupiter's moons this season. Or maybe not. The vagaries of weather and event times have limited my own opportunities to fewer than a handful.

Jupiter's moons will continue to occult and eclipse one another until the planet is lost in the solar glare this summer, but the average depth of the events diminishes with every week that goes by. April is really the last good month to catch the easy-to-see ones.

Mutual event seasons occur every six years when the orbital plane of the four Galilean moons is edge-on with that of the Sun and Earth. The current run began last September and ends in July. Now, as the four Galilean moons slowly break their neat alignment with us, deep events are becoming few and far between.

The eye can detect a drop of 0.5 magnitude during an eclipse or merger of two moons in a mutual occultation, but experience shows this is a lower limit. Since the events last only a few minutes at most, the change is subtle. You have to pay close attention and carefully compare the fading satellite's brightness with another nearby moon to convince yourself you're seeing it. A dip of at least a magnitude is much easier to detect and far more dramatic.

Watching eclipses far, far from home

Simulation of the March 9th deep eclipse of Europa by Ganymede.

Amateur astronomer Giorgio Rizzarelli of Italy was caught off-guard by the depth of an eclipse of Europa by Ganymede on the evening of March 9th:

"I saw an Europa total eclipse by Ganymede tonight, (a) remarkable event with the intense phase happening in a couple of minutes, including Europa completely disappearing." Europa, normally magnitude +5.3, faded by more than four magnitudes to +9.6 under Ganymede's shadow.

It was the deepest event of the entire season.

A very nice eclipse of Europa by Io during the last mutual event series in September 2009.

A week later on March 16th I was thrilled to see Ganymede pick on Europa again. The eclipsed moon began at normal brightness, dropped a very obvious 2.4 magnitudes at deepest partial eclipse, and returned to normal all in the span of four minutes!

What's really happening during mutual events

Simulations of three fine upcoming mutual events. From left: Io eclipsing Europa April 13th; Callisto occulting Ganymede April 17th; and another Io eclipse on April 20th. See below for details.

Make sure your clock is set correctly and begin observing a good 5 minutes before an eclipse begins or 10 minutes before an occultation, so you can watch the two moons merge. You can judge brightness changes by comparing the moon undergoing eclipse or occultation to one that's not. That probably won't be necessary with the events I've listed below because they all involve fades of one full magnitude or more. They're the best easy-to-see events for the remainder of Jupiter's apparition. After May, it's a long 6 years to the next season opener.

What to expect on April 13th

View of Jupiter and moons during the eclipse of Europa by Io on April 13th. I = Io, II = Europa, III = Ganymede and IV = Callisto.

For a complete list of all remaining eclipses and occultations (even the impossible ones), stop by the British Astronomical Association's (BAA) Jovian moons mutual event pages — one for eclipses and another for occultations. There are several tables out there, but I've found these to be the most reliable.

I've included two diagrams showing Jupiter and its moons at the time of an event, but to determine which moon is which for the others, click on the Sky & Telescope Jupiter's Moons site.

Times below are EDT. To convert to CDT, subtract one hour; MDT subtract 2 hours; PDT subtract 3 hours; AKDT subtract 4 hours; and for UT, add 4 hours. Or use this handy Time Converter. Events visible in the Americas are highlighted. All eclipses of Europa occur when that moon is close to the planet.

April 10 / Io eclipses Europa / 8:22 – 8:25 a.m. / Mag. drop = 2.5 / Hawaii
April 13 / Io eclipses Europa / 9:29 – 9:33 p.m. / Mag. drop = 2.3 / U.S., Europe
April 17 / Callisto occults Ganymede / 9:27 – 9:37 p.m. / Mag. drop = 1.0 / U.S, Europe
April 17 / Io eclipses Europa / 10:36 – 10:40 a.m. / Mag. drop = 2.1 / Asia
April 20 / Io eclipses Europa / 11:44 – 11:47 p.m. / Mag. drop  = 1.8 / U.S., Western Europe
April 24 / Io eclipses Europa / 12:51 – 12:55 p.m. / Mag. drop = 1.6 / Middle East, Asia
April 28 / Io eclipses Europa / 1:59 – 2:02 a.m. / Mag. drop = 1.4 / Western half of U.S.
May 1 / Io eclipses Europa / 3:06 – 3:10 p.m. / Mag. drop = 1.2 / Europe, Middle East
May 5 / Io eclipses Europa / 4:14 – 4:18 a.m. / Mag. drop = 1.0 / Western U.S., Hawaii
May 22 / Callisto occults Ganymede / 10:18 – 10:40 a.m. / Mag. drop = 1.0 / Asia

Merging moons ahead!

Occultations can last a little longer than eclipses, especially if they're nearly central like the Callisto-Ganymede event on April 17th.

The fast pace of the eclipses as the moons' shadows cover and uncover each other's disks will almost take your breath away. We're used to the leisurely several hours it takes our own Moon to pass through Earth's shadow.

Then there's the fun challenge of separating merging moons during occultations. When do two become one and then two again?

But what really dings the bell is the realization that in the smallest of changes — the mere dimming of a satellite —  we witness the interplay of little moons more than 450 million miles away.

Track Jupiter's four largest moons with the Sky & Telescope JupiterMoons app.

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Bob King

About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, just published and available at Amazon and BN.

10 thoughts on “Catch the Last Best Antics of Jupiter’s Moons

    1. Bob KingBob King Post author

      Hi Joseph,
      There are several tables out there for mutual events, and I’ve learned they differ from one another on predictions. Some are accurate, others not as much. I selected the BAA table because it’s been dead-on (so far) on predictions. The BAA predicts the event as happening on April 14 (Tues.) at 1:29 UT. Converting to EDT, we get 21:29 on Monday evening April 13. Perhaps RASC didn’t go backwards a day in making the conversion from UT to EDT.

  1. Havalook

    Bob, Some years ago, in the minute or so before an eclipse of two of J’s moons, they took on an almost Albireo-like color difference. At the time I was doing “street” telescoping, and some of my lookers confirmed what I was seeing. Fast forward to mid-evening 4/01/15 and the eclipse of Io and Europa.
    For the minute or two before the event, to me Io definitely looked orangish while Europa may have had a mild, blue tint. Normally in my 8″ SCT at 80X the moons don’t have noticeable discs nor colors. Bob, how about asking your readers to start looking for color effects when observing the moons’ mutual events while they last. I’d like to know if others can see what I believe I see!
    Herman Heyn, Baltimore, Md.

    1. Bob KingBob King Post author

      Dear Herman,
      Excellent suggestion. I wonder if Io’s natural orange color is enhanced when compared to a nearby moon that’s dimming in eclipse. I like your comparison with Albireo. The striking color difference may be another example of enhanced color contrast between two objects very close to one another.

  2. Clark

    Excellent information. I was able to observe the April 13 Io eclipse from my home. The drop in magnitude was quick and Io stayed very dim for about 1.5 min. before getting brighter again. I was surprised at how obvious the change was. Could it have been more than a 2.1 magnitude drop? The other factor could be that since Io was fairly close to Jupiter it may have enhanced the appearance of Io’s dimming.

    1. Bob KingBob King Post author

      Hi Clark – Happy to hear you saw this fine eclipse. It happened to be clear here, too. The drop was dramatic as you described, perhaps even more than 2 magnitudes. Like you, I thought the Europa’s proximity to Jupiter might have made its fading that much more striking.

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