Jupiter is usually the one occulting and eclipsing the Galilean moons, but this observing season the moons also eclipse and occult one another.

Europa eclipses then occults Io
In this 11.4-minute sequence taken with a 14-inch Schmidt-Cassegrain telescope, Europa first occults and then partially eclipses Io on February 7, 2015, during the last mutual events season. Here's the animated version.
John Sussenbach

After their spectacular Great Conjunction last December, Jupiter and Saturn overwintered in the solar glare, lost from sight. They've now returned to the morning sky to begin another observing season, and amateurs are eager for a look.

Jupiter and Saturn return at dawn
Both Jupiter and Saturn are now easily visible low in the southeastern sky at dawn.
Bob King

Unlike some planets that hold their details close, Jupiter lets it all hang out. With an apparent diameter nearly double that of Mars at its closest opposition even a modest telescope reveals a colorful cloudscape of shifting belts and zones. The four Galilean moons are constantly on the move, bunching together and spreading apart like ballet dancers in the Nutcracker. If that wasn't enough, you can watch the planet occult and eclipse those satellites or catch a transit, when a moon casts its pinpoint shadow on the gas giant's cloud tops.

The Feeling Is Mutual

But there's something else happening that makes this Jupiter apparition special. Every six years the orbital plane of the four Galilean moons is edge-on with the Sun and Earth, and we enter the season of mutual phenomena, when the moons occult and eclipse one another. The season began on January 3rd when Europa partially eclipsed Io and finishes up on November 16th with a Ganymede-Io occultation. Although Jupiter is still low at dawn it inches higher every morning and will soon be in good view.

Io and Ganymede dance
Io and its shadow overtake larger Ganymede in this 14-image sequence taken every few minutes from 16:38 to 17:39 Universal Time on August 16, 2009. Christopher Go

In total 242 events occur, of which 192 are observable. An occultation happens when one moon passes in front of another, blocking a portion of the background moon's disk. During an eclipse a foreground moon casts its shadow on a background moon. Both eclipses and occultations can be partial, total or annular.

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

Depending on the nature of the occultation event, the two moons may appear to barely touch or even completely merge. When closest, the spacing and orientation of the two moons changes rapidly, an exciting sight you can see unfold in near-real time. Magnitude differences between the satellites also make for noticeable brightness changes during an occultation. This is especially true for Jupiter's opposition on August 20th, when the dimmest of the Galilean moons, 5.5-magnitude Callisto fully occults 4.4-magnitude Ganymede, resulting in a dramatic drop in brightness. Similar circumstances occur with Io (magnitude 4.9) and to a lesser extent with Europa (magnitude 5.1).

Io eclipses Ganymede
Io eclipses Ganymede on March 13, 2015, as seen photographed through a 14-inch Schmidt-Cassegrain telescope.
John Sussenbach

In an eclipse, the dip in brightness varies according to the depth of the eclipse. During a total eclipse a moon may briefly disappear from view in a multi-magnitude plunge. I've included a selective table below, but you can peruse a list of all events at the Institute of Celestial Mechanics and Ephemeris Computing (IMCCE) site.

Jupiter's Ganymede occults then eclipses Io
Ganymede occults and then eclipses Io on February 9, 2015.
Marc Delcroix

Table of Portents

Date Time (UT)EventMag. (%)
April 113:17 — 13:23Ganymede occults Io94
April 611:46 — 11:51Europa eclipses Io47
April 1110:00 — 10:11Io eclipses Callisto69
April 1211:50 — 12:12Io eclipses Callisto80
April 1510:07 — 10:16Ganymede eclipses Europa95
April 189:52 — 9:57 Io eclipses Europa87
April 2213:23 — 13:32Ganymede eclipses Europa81
April 2512:05 — 12:11Io eclipses Europa98
April 2912:12 — 12:27 Ganymede eclipses Callisto75
This table highlights events visible from the Americas in April 2021. Maximum occultation or eclipse occurs at the middle of the time interval. The magnitude of an event is the percentage of the more distant satellite's diameter that is covered in an occultation or obscured during an eclipse. The greater the number the more dramatic the light change in an eclipse and the more central the occultation. 50 percent or more should be easily visible.

To use the IMCCE site, click the Show button for the complete list of events. Times for the beginning and end of the event are in Universal Time, so remember to deduct 4 hours for Eastern Daylight; 5 for Central; 6 for Mountain and 7 for Pacific or use this converter. The next column tells you which satellite eclipses (e) or occults (o) another. For example, 1E4 means that satellite I (Io) casts its shadow on satellite IV (Callisto). In 3O2, satellite III (Ganymede) covers II (Europa).

Two mutual events visible from the Americas in April are depicted here in detail. UT times are shown in green below each stage. During the Callisto eclipse, the fuzzy gray-black disk is Io's shadow.
Created with the free WinJUPOS program

The columns that follow give the duration of the event in minutes; the impact factor ( "0" indicates the satellite passes centrally through the shadow or behind the apparent disk of the other satellite, while "1" is a grazing event, and negative values indicate the action occurs in front of or behind Jupiter's disk); M is the combined magnitude of the satellites; Δm is change in magnitude of the two moons before and at the time of the event; limb is the apparent distance of the involved satellite from the planet's limb (in arcseconds); and dist the distance between the two satellites (in arcseconds).

For a wonderful sampling of photos and animations from previous mutual events seasons, be sure to check out the Mutual Events site maintained by avid planetary imager Marc Delcroix.

Mutual phenomena are dynamic events, just the thing to spice up your observing this season. Pull up a telescope and dance the night away with Jupiter's moons.

Playing Hide-and-Seek with 44 Capricorni

Jupiter's Io and 44 Cap appulse
Jupiter's moon Io (magnitude 5.9) passes extremely close to 44 Capricorni at dawn on April 2nd.

A different sort of occultation happens at dawn on April 2nd when the 5.9-magnitude star 44 Capricorni has close encounters of several kinds at Jupiter. As seen from western and central South America, Jupiter will occult the star. Observers in eastern North America will witness the star reappear at the planet's western limb in twilight, at around 6 a.m. EDT. Since Jupiter will be low in the southeastern sky at the time (~12° from 40°N), make sure you find a spot with an unobstructed view.

Observers in the Midwest will miss the occultation but will have front-row seats when Io passes just 0.5″ south of 44 Capricorni around 5:21 a.m. CDT (6:21 EDT for the eastern Midwest). The two equally bright objects will appear to nearly merge and then separate over the span of about 10 minutes. From some locations in Central America and northwestern South America Io will occult 44 Capricorni. For example, in San José, Costa Rica, Io covers the star between 4:18 and 4:20 a.m.


Image of jacobus-nl


April 1, 2021 at 4:49 am

Hi Bob, in the fourth paragraph you mention that “Occultations can be partial or total, while eclipses are partial, total, or annular.” According to the illustration below the paragraph occultations can be annular too!

Anyway, very interesting events to try to observe. Great opportunities to try out my first telescope that should arrive tomorrow! 🙂

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Image of Bob King

Bob King

April 1, 2021 at 2:55 pm

Hi Jacobus,

Of course! Thank you for pointing that out 🙂

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