Occultations and Transits Special Interest Group

revised 2005 Feb 21

Occultations of Jupiter's Moons

Jupiter's four largest moons, the Galilean satellites of Io, Europa, Ganymede, and Callisto, are often occulted (and eclipsed) by Jupiter itself. These events can be fun to watch. In the late 1600's, Ole Roemer used eclipses of Io to calculate the speed of light. Today, these events provide little scientific information that amateurs can collect, but they are still fun to watch. Sky & Telescope magazine, the U.S. Naval Observatory's annual Astronomical Almanac, and the Royal Astronomical Society of Canada's annual Observer's Handbook list the times of these events.

On the other hand, every six years or so, the orbits of the Jovian satellites are seen from edge on. During this time, it is possible for one moon to occult another moon (as well as cast a shadow on another moon to create an eclipse). Accurate timing of such mutual events can be used to refine the orbits of the moons. Even with the data provided by the Voyagers and Galileo spacecrafts, long term changes in the orbits of the moons (and other fun stuff) can be determined only by highly accurate, long-term observations such as provided by these mutual events.

Other Occultations of Jupiter's Moons Links:

Project Pluto's Jupiter Satellite Events times of Galilean satellite occultations by Jupiter calculated by a popular computer software.
Sky & Telescope's Moons of Jupiter article describing how to observe Jupiter's moons. Also includes a Java script that displays the moons for any given date and time.


Galilean Satellite Transits

Just as Jupiter's moons can be occulted, they are often seen in transit across the face of Jupiter. In my opinion, these events can be more interesting than the occultations. First, a transit is also accompanied by a transit of the shadow. The dark spot on Jupiter is easy to see. Second, as the moon transits different clouds, its visibility can change. Thus, the challenge is to see the moon throughout the transit.

A number of double satellite transits and double shadow transits will be visible from Pittsburgh this year, at which times two moons (and possibly their shadows) will be seen moving across Jupiter's disk. The table below gives the events that are visible from Pittsburgh.

Favorable Double Transits of Galilean Satellites
from Pittsburgh, Pennsylvania
Date 2005 Satellite Transit (U.T.) Shadow Transit (U.T.)
May 26-27
double shadow transit

Europa ingress (21:56) Europa  ingress (0:05)
Europa egress (0:37)    
Io ingress (0:37) Io ingress 1:40
    Europa egress 2:45
Io egress 2:48 Io egress 3:50
June 3
double satellite & shadow transit

Europa ingress (0:24)    
Io ingress 2:27 Europa ingress 2:42
Europa egress 3:05 Io ingress 3:34
Io egress 4:38 Europa egress 5:22
    Io egress 5:44
July 12
double satellite transit

Io ingress (0:50) Io ingress 2:05
Europa ingress 2:53    
Io egress 3:02 Io egress 4:14
    Europa ingress (5:09)
Europa egress (5:17)    

Times for each event are given chronologically, reading left to right, top to bottom (resulting in occasional blanks in the table). Double transits occur during the interval in bold. Other events (occultations and eclipses) may also occur during the time interval; they are not listed. Events that occur with the Sun above the horizon or Jupiter below the horizon, as seen from Pittsburgh, have their time in parentheses ( ). The times between [ ] is the interval between sunset and Jupiter set, unless noted otherwise; this is convenient for deciding if the sky will be bright or Jupiter too low.

All times are Universal Time (U.T., Greenwich England). Subtract 5 hours from Universal Time (UT) to get Eastern Standard Time (E.S.T.); subtract 4 hours to get Eastern Daylight Time (E.D.T.). The time is expressed in 24-hour military time. Thus, 13:00 is 1 p.m., 14:00 is 2 p.m., and so on. After subtracting the 4 or 5 hours to get Eastern time, a negative time indicates the event occurs on the previous date (add 24 to the negative number in this case to get the appropriate time on the previous date).

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