Jupiter’s Moon Dances

Jupiter with Io and Europa in the foreground
In this false-color close-up view from the Voyager 1 spacecraft on February 13, 1979, Jupiter’s moon Io hovers above the planet’s Great Red Spot while Europa hangs in the foreground to its right. The moons’ disks are not as clearly seen from Earth. Nevertheless, a host of unusual observations can be made whenever two moons line up precisely with Earth or the Sun — something they do often until June 2003.
Courtesy NASA / JPL.
Jupiter’s four large moons travel in orbits that are tilted only slightly to the plane of the ecliptic, and a popular observing project is to watch in a telescope as they pass in front of or behind the giant planet’s disk. But twice in every revolution of Jupiter around the Sun, or roughly every six years, the satellites’ orbits are presented so nearly edge on to our view that the satellites can be seen going behind or in front of each other. The geometry favoring these "mutual events" lasts for a number of months, and a new season is well underway.

Here we list all the mutual occultations and eclipses of the satellites of Jupiter visible from Earth until the conclusion of this series of mututal events in mid-July.

Because these events normally last just a few minutes, careful timings of them can serve as a valuable check on the mathematical expressions used to describe the moons’ motions over many centuries. (To contribute to such efforts, visit the PHEMU03 campaign Web site.) Even when two moons appear too close to be distinguished in a small telescope, the drop in their combined light can be quite noticeable. Historically, before visits to the Jovian system by the Pioneer and Voyager spacecraft, Earth-based observations of mutual events provided our most reliable clues concerning the surface characteristics of the individual moons.

Partial occultation of Io by Ganymede
This 19-frame animation shows Io (the small, moving object) being partially occulted by Ganymede on January 16, 2003. Hong Kong amateur Yan Chi-keung used a CCD imager attached to his 250-mm, f/20 Maksutov-Cassegrain telescope to capture this 36-minute-long sequence. The actual occultation (a 17-percent partial) took place between 18:56 and 19:00 UT.
These events also offer extraordinary challenges for testing a telescope’s resolution and image quality. Each satellite presents a disk only about 1 arcsecond across, so even a weak partial occultation provides a close "double star" whose orientation and spacing change rapidly before our eyes. Today’s digital and video imaging techniques, which need less exposure time than film photography, should be capable of capturing these events with unparalleled clarity.

An Explanation of the Mutual Events Table

In our tables on the following pages (see the sample below), all events are listed by date and Universal Time. The pairs of times immediately after the date give the beginning and end of the partial phase (exterior contacts) of the phenomenon as seen from Earth. These include the travel time needed for the satellite’s light to reach us.

Sample Listing of Mutual Events of Jupiter's Satellites
(A or T)
Oct. 2801:0801:142o181
Oct. 3114:1814:242o1A8627s
Nov.   318:2318:331o418

Mutual event types
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 illustration.
The next column ("Event type") tells, in shorthand notation, which satellite occults or eclipses which other. For example, 4o3 means that satellite IV (Callisto) occults satellite III (Ganymede) by passing in front of it. Similarly, 1e2 means that satellite I (Io) eclipses satellite II (Europa). In the case of an eclipse, the satellites themselves are usually well separated as seen from Earth, but one satellite fades and then brightens again as it passes through the shadow of the other. Sometime these notations are followed by A for annular or T for total; otherwise the event is partial. The six possibilities are shown schematically (left). Eclipses of a satellite in the penumbra (outer fringe) of another satellite’s shadow are not considered here; we list eclipses by the umbra (shadow core) only.

The "magnitude" of an event is expressed by the percentage of the more distant satellite’s diameter that is obscured. In the case of an annular occultation, the magnitude is simply the ratio of the diameters of the two bodies. Finally, the last column in the table gives the duration of the annular or total phase (if any) in seconds. This phase generally occurs near the middle of the time interval specified for the partial phase.

Of course, only some of these events can be seen from a given geographical location. The sole requirement is that Jupiter be above the horizon in a clear, dark sky at the time of an event.

Mutual Events to July 1, 2003

June 8th (second 3e1). Io is in transit over Jupiter when eclipsed by Ganymede (which is also transiting).

June 15th (second 3o1). Both satellites are in transit over Jupiter.

June 22nd (3o1). During this partial occultation, the angular separation between the satellites’ centers undergoes two minima. The first is at 9:45 (with a magnitude of 38 percent), and the second is at 11:17 (37 percent).

Mutual Events of Jupiter's Satellites
(A or T)
May 3022:0022:051o283
May 3112:0512:094o148
May 3120:0820:143e269
June   100:0600:133e140
June   113:3013:523e1T77s
June   121:5622:083e154
June   311:1111:161o274
June   505:4605:502o321
June   517:3517:362o107
June   700:2200:271o265
June   723:3023:363e280
June   803:5504:073e144
June   811:4912:113e194
June   813:2713:303o407
June   814:1214:181o475
June   817:0617:213e107
June   823:0523:122o4A6574s
June   901:4801:543e121
June   906:4406:462o115
June 1013:3413:391o255
June 1209:1209:172o333
June 1219:5419:572o124
June 1402:4602:511o246
June 1502:5403:013e289
June 1503:5003:5730116
June 1514:3114:443o121
June 1601:5702:033o111
June 1609:0409:072o134
June 1617:4417:504o169
June 1714:2915:054o189
June 1715:5916:031o236
June 1716:0916:144o231
June 1718:1318:464o1T163s
June 1807:0907:184o337
June 1912:4012:452o348
June 1922:1422:172o144
June 2105:1205:161o227
June 2206:2006:273e297
June 2209:1811:463o138
June 2305:4005:473o130
June 2311:2411:282o154
June 2402:0402:163o441
June 2418:2618:291o217
June 2422:2722:371o409
June 2503:5904:131o413
June 2600:0200:081o445
June 2607:1307:171o325
June 2616:0816:142o3A5960s
June 2700:3400:382o166
June 2807:4007:421o28
June 2909:4809:563e2T14s
June 3009:0309:103o155
June 3013:4513:452o177

Mutual Events to July 24, 2003

Mutual Events of Jupiter's Satellites
(A or T)
June 3009:0309:103o155
June 3013:4513:482o177
July   301:0301:174o2T61s
July   310:0910:141o348
July   319:3719:432o3A5990s
July   402:5502:592o1A8615s
July   613:1913:273e2T  8s
July   712:1612:223o186
July   716:0616:092o183
July 1013:0713:131o3A6935s
July 1023:0723:132o3A55
July 1105:0605:202o170
July 1311:2911:373o430
July 1316:5417:033e2100
July 1415:2315:293o1T69s
July 1418:2718:312o156
July 1716:0816:141o3A6949s
July 1807:3807:412o142
July 2020:3220:433e2T23s
July 2118:2618:313o185
July 2120:4920:522o127
July 2419:1419:211o351