Jupiter's Moons Dance for You!

By: Alan MacRobert August 20, 2009 0

Right now the planet Jupiter is oriented such that its equator and the orbits of its four big moons are almost exactly edge-on to the Sun and Earth. This alignment happens every six years, on opposite sides of Jupiter's 12-year orbit around the Sun.

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.

At such times its four Galilean satellites undergo mutual phenomena: they often get occulted and eclipsed not just by big Jupiter and its shadow but also by one another. Amateur Christopher Go recently recorded a striking example of the latter.

During a mutual occultation, for example, you can watch two satellites appear to merge and, in the middle of the merger, slightly dim. During an eclipse, a lone moon fades and rebrightens as it's crossed by the shadow of one of its siblings.

Although these little encounters are great fun simply to watch, anyone who records their light curves accurately with a photometer can help to refine the satellites' orbits.

Listed below are the mutual occultations and moderately deep eclipses through the end of 2009, when you'll find Jupiter conveniently placed for viewing in the evening sky. (Thanks to Belgian astro-calculator Jean Meeus for providing the list!)

Mutual events among satellites can occur six different ways, depending on differences in the moons' sizes and relative positions.

S&T illustration

Events are listed by their date and Universal Time, which is 4 hours ahead of Eastern Daylight Time. The next column tells which satellite occults (o) or eclipses (e) another. 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) casts its shadow onto satellite II (Europa). Sometime these notations are followed by A for annular or T for total; otherwise the event is partial. The six possibilities are shown schematically here.

Event size indicates how much of the more distant satellite's diameter is obscured during an occultation, measured as the percentage drop in its light. The table lists only those events for which the dropoff is at least 25% (0.3 magnitude).

Although this table lists only those events easily observable from North America, Sky & Telescope has prepared a comprehensive tabulation of all pairings for the entire year. So find the events that occur at night for you when Jupiter is up, and mark your calendar!

Mutual Events of Jupiter's Satellites for North America
Date (UT)	Start (UT)	End (UT)	Event type	Event size
Aug. 7	5:12	5:19	1e2	53%
7	5:33	5:43	1o2	79%
12	1:46	2:01	3e2T	100%
12	2:05	2:17	3o2	31%
14	7:49	8:01	1o2	92%
14	7:49	7:58	1e2	55%
15	0:26	1:04	1o2	31%
15	23:50	23:56	1e3	27%
19	5:32	5:48	3o2	41%
19	6:04	6:22	3e2	99%
21	10:11	10:25	1o2T	100%
21	10:38	10:49	1e2	52%
22	4:00	4:17	1o2	47%
23	3:19	3:27	1e3	26%
24	23:24	23:40	1o2T	100%
25	0:08	0:22	1e2	47%
26	9:16	9:37	3o2	48%
26	11:00	11:25	3e2	82%
29	6:42	6:55	1o2	53%
29	7:38	7:46	1e2	28%
30	5:44	5:51	1o3	7%
30	7:47	8:09	1e3	30%
Sep. 1	2:04	2:26	1o2	81%
1	3:56	4:29	1e2	25%
5	9:10	9:21	1o2	55%
5	10:21	10:29	1e2	50%
16	0:41	0:51	1o2	53%
16	2:12	2:19	1e2	86%
23	3:01	3:08	1o2	51%
23	4:41	4:47	1e2	96%
30	5:18	5:25	1o2	49%
30	7:07	7:13	1e2	81%
Oct. 7	7:35	7:41	1o2	47%
17	23:01	23:07	1o2	46%
22	23:12	23:17	3o1	40%
24	0:31	0:40	3o2	61%
25	1:19	1:24	1o2	47%
30	2:04	2:10	3o1	49%
31	3:55	4:03	3o2	68%
Nov. 1	3:38	3:43	1o2	51%
6	5:06	5:13	3o1	56%
8	5:57	6:02	1o2	56%
13	2:04	2:08	2o1A	86%
17	1:19	1:26	2o3A	59%
20	4:19	4:23	2o1A	86%
24	4:39	4:46	2o3A	59%
25	23:47	23:52	1o2	83%
Dec. 3	2:09	2:13	1o2	99%
15	0:26	0:30	2o1	54%
22	2:48	2:52	2o1	41%
27	22:28	22:31	1o2	35%

About Alan MacRobert

Alan M. MacRobert became an avid Sky & Telescope subscriber in 1966 at age 14, joined the editorial staff in 1982, and is now a senior contributing editor, semi-retired. He played a role in practically every part of the magazine and the company's other products for more than a generation, both on the amateur-observing side and the science-reporting side. In 1994 a book collection of his observing how-tos and telescopic sky tours was published as Star Hopping for Backyard Astronomers. He has produced This Week's Sky at a Glance online every week since 1989.