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
Date
2002
Start
(UT)
End
(UT)
Event
type
Mag.
(%)
Duration
(A or T)
Oct. 28 01:08 01:14 2o1 81
Oct. 31 14:18 14:24 2o1A 86 27s
Nov.   3 18:23 18:33 1o4 18

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
Date
2003
Start
(UT)
End
(UT)
Event
type
Mag.
(%)
Duration
(A or T)
May 30 22:00 22:05 1o2 83
May 31 12:05 12:09 4o1 48
May 31 20:08 20:14 3e2 69
June   1 00:06 00:13 3e1 40
June   1 13:30 13:52 3e1T 77s
June   1 21:56 22:08 3e1 54
June   3 11:11 11:16 1o2 74
June   5 05:46 05:50 2o3 21
June   5 17:35 17:36 2o1 07
June   7 00:22 00:27 1o2 65
June   7 23:30 23:36 3e2 80
June   8 03:55 04:07 3e1 44
June   8 11:49 12:11 3e1 94
June   8 13:27 13:30 3o4 07
June   8 14:12 14:18 1o4 75
June   8 17:06 17:21 3e1 07
June   8 23:05 23:12 2o4A 65 74s
June   9 01:48 01:54 3e1 21
June   9 06:44 06:46 2o1 15
June 10 13:34 13:39 1o2 55
June 12 09:12 09:17 2o3 33
June 12 19:54 19:57 2o1 24
June 14 02:46 02:51 1o2 46
June 15 02:54 03:01 3e2 89
June 15 03:50 03:57 301 16
June 15 14:31 14:44 3o1 21
June 16 01:57 02:03 3o1 11
June 16 09:04 09:07 2o1 34
June 16 17:44 17:50 4o1 69
June 17 14:29 15:05 4o1 89
June 17 15:59 16:03 1o2 36
June 17 16:09 16:14 4o2 31
June 17 18:13 18:46 4o1T 163s
June 18 07:09 07:18 4o3 37
June 19 12:40 12:45 2o3 48
June 19 22:14 22:17 2o1 44
June 21 05:12 05:16 1o2 27
June 22 06:20 06:27 3e2 97
June 22 09:18 11:46 3o1 38
June 23 05:40 05:47 3o1 30
June 23 11:24 11:28 2o1 54
June 24 02:04 02:16 3o4 41
June 24 18:26 18:29 1o2 17
June 24 22:27 22:37 1o4 09
June 25 03:59 04:13 1o4 13
June 26 00:02 00:08 1o4 45
June 26 07:13 07:17 1o3 25
June 26 16:08 16:14 2o3A 59 60s
June 27 00:34 00:38 2o1 66
June 28 07:40 07:42 1o2 8
June 29 09:48 09:56 3e2T 14s
June 30 09:03 09:10 3o1 55
June 30 13:45 13:45 2o1 77


Mutual Events to July 24, 2003

Mutual Events of Jupiter's Satellites
Date
2003
Start
(UT)
End
(UT)
Event
type
Mag.
(%)
Duration
(A or T)
June 30 09:03 09:10 3o1 55
June 30 13:45 13:48 2o1 77
July   3 01:03 01:17 4o2T 61s
July   3 10:09 10:14 1o3 48
July   3 19:37 19:43 2o3A 59 90s
July   4 02:55 02:59 2o1A 86 15s
July   6 13:19 13:27 3e2T   8s
July   7 12:16 12:22 3o1 86
July   7 16:06 16:09 2o1 83
July 10 13:07 13:13 1o3A 69 35s
July 10 23:07 23:13 2o3A 55
July 11 05:06 05:20 2o1 70
July 13 11:29 11:37 3o4 30
July 13 16:54 17:03 3e2 100
July 14 15:23 15:29 3o1T 69s
July 14 18:27 18:31 2o1 56
July 17 16:08 16:14 1o3A 69 49s
July 18 07:38 07:41 2o1 42
July 20 20:32 20:43 3e2T 23s
July 21 18:26 18:31 3o1 85
July 21 20:49 20:52 2o1 27
July 24 19:14 19:21 1o3 51

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