Jupiter's Moon Dances

By: Jean Meeus July 31, 2003 0

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.

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 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	–