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!

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.

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!

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.

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

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.