Jupiter’s Moons Dance for You!

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 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.
© 2009 Christopher Go
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 explained
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%