From - Sky & Telescope
By - Joe Rao
Edited by - Amal Udawatta
In the coming weeks, Jupiter’s four Galilean satellites will arrange themselves in some unusual geometric patterns that will be fascinating to observe.
Jupiter’s four Galilean satellites are always fun to watch as they change position relative to each other from night to night and even hour to hour. Galileo originally assigned Roman numerals to these moons, based on the amount of time each took to revolve around Jupiter. However, the mythological names that we use today were chosen by Galileo’s rival, Simon Marius, who claimed that he discovered the four moons about a month before Galileo did.
Io (I), the innermost of this quartet, takes only 1.8 days to make one revolution around Jupiter. Europa (II) takes twice as long at 3.6 days, while Ganymede (III) circles Jupiter in 7.2 days — exactly four times Io’s period! Callisto (IV), the farthest out, takes 16.7 days to complete one orbit around the big planet.
Since they are all constantly moving at different speeds, the positions of all four satellites at any particular time, is unique and peculiar to that particular moment.
António Cidadão
Occasionally three satellites bunch up in a group (as seen from Earth). When they do, they briefly appear to form a geometric pattern like a straight line or a triangle. If such an alignment appears at a high angle to the plane of their orbits, the view in your eyepiece will be quite at odds with the typical idea of how they’re supposed to look (appearing along a straight line).
Five such lineups will be visible from most of North America during the current Jupiter apparition that stretches from December into late next March. When two or three satellites pass near to one another, their motion seems particularly fast and the eye is quite sensitive to their deviation from a straight line. However, these unusual appearances will not last more than several minutes at most.
I cross-checked the times and configurations using different planetarium software (Starry Night Pro 8, Stellarium, and Dance of the Planets). However, my times may still be off by several minutes because the Galilean satellites have very complex orbital dynamics that are still not perfectly understood. Perhaps the best available theory concerning the satellites motion is a 1977 study by Jay H. Lieske of Caltech’s Jet Propulsion Laboratory.
With that caution in mind, mark your calendars (and check your watch) — don’t wait until the times listed below to start observing. Instead, plan to start ahead of time to savor the celestial dances as they take place. Note that these moons won't appear equally bright. Ganymede is 2½× brighter than Callisto and 1.9× brighter than Europa. And will you be able to tell the 40% difference between Ganymede and slightly dimmer Io?
With that caution in mind, mark your calendars (and check your watch)! Don’t wait until the times listed below to start observing. Instead, plan to start ahead of time to savor the celestial dances as they take place. Note that these moons won’t appear equally bright. Ganymede is 2½× brighter than Callisto and 1.9× brighter than Europa. And will you be able to tell the 40% difference between Ganymede and slightly dimmer Io?
The views below are shown north up. Abbreviations are used for Io (I), Europa (E), Ganymede (G), and Callisto (C).
DEC. 20, 2024 @ 4:49 UNIVERSAL TIME (UT): AN ISOSCELES TRIANGLE
Europa, Ganymede, and Callisto will gather on Jupiter’s eastern (trailing) side, forming a wide triangle with equal sides each measuring 1½ arcminutes. Io is the odd moon out, appearing on the opposite side of Jupiter and well to its west.
DEC. 28, 2024 @ 23:34 UT: A TRIPLE LINEUP
Callisto and Europa join Io on the western (leading) side of Jupiter along a line that spans 1¾ arcminutes. At its closest Callisto sits only about 17 arcseconds from Jupiter’s upper limb. Meanwhile, Ganymede is the “solitary one,” positioned far to the east of Jupiter near greatest elongation (not shown). This configuration favors U.S. regions east of the Mississippi; farther west viewing is unfortunately precluded either by bright twilight or daytime conditions.
JAN. 6, 2025 @ 7:01 UT: TRIPLE LINEUP REDUX
As was the case on December 28th, once again you’ll see a lineup of three satellites, though the trio of satellites in question (Io, Ganymede, Callisto) are east of Jupiter and are more closely spaced together, spanning 0.9 arcminute. Callisto is again closest to Jupiter, just 28 arcseconds from its limb. This time Europa is the outlier, also located east of “Big Jupe” though farther away.
FEB. 26, 2025 @ 5:39 UT: A COMPACT TRIANGLE
While Io is left all by its lonesome to the east of Jupiter, on the opposite side Europa, Ganymede and, Callisto form a rather striking compact triangle; the trio will fit inside of a circle only about 30 arcseconds wide. Also, using a medium-to-large telescope at 100×, see if you can glimpse a 12thmagnitude star (TYC1279-220-1) appearing roughly midway between Europa and Ganymede. The larger the aperture of your scope, the easier it will be to see this star.
MAR. 23, 2025 @ 2:27 UT: THREE’S A CROWD!
Callisto, Ganymede, and Io bunch together in a strikingly close alignment — just 25 arcseconds in length — to the east of Jupiter. Meanwhile, Europa is out of the loop, so to speak, sitting alone to the planet’s west.
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