Watch Jupiter's Moons in Unusual Lineups

It is always fascinating to watch Jupiter’s Galilean satellites change position relative to each other from night to night and even hour to hour. But in the coming months, observers have the opportunity to see some unusual geometries among Jupiter’s biggest moons.

Io, the innermost satellite, takes only 1.8 days to make one revolution around Jupiter. Europa takes twice as long at 3.6 days, while Ganymede circles Jupiter in 7.2 days (exactly four times Io’s period). Callisto is the farthest out from Jupiter of the Galilean moons and takes 16.7 days to complete one orbit around the giant planet.

It is obvious that since they are constantly moving at different speeds, the positions of all four satellites at any particular time, is peculiar to that particular moment.

Occasionally three satellites bunch up in a group, as seen on the sky. When they do, there is usually a moment when they appear to form a geometric pattern such as a straight line or triangle. If such an alignment is 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 Jupiter’s moons are supposed to look.

Four such lineups will be visible from at least part of North America during the current Jupiter apparition, which stretches from November into next April. When two or three satellites pass near to one another, their motion seems particularly fast, so the event will catch your attention. The human eye is quite sensitive to the trueness of a straight line, so these unusual appearances probably will not last more than several minutes at most.

In addition, there will be some interesting interactions of Jupiter and Callisto with a faint background star in Aries on a night in mid-January. You’ll also have a chance to catch the unusual sight of Jupiter accompanied by one lone satellite (again Callisto), also in January.

I cross-checked the times and configurations using different planetarium software (Starry Night Pro 8, Stellarium, Dance of the Planets), but my times may still be off by several minutes because the Galilean satellites have very complex orbital dynamics, and the calculations are still not perfect. Perhaps the best available theory concerning the satellites motion is a 1977 study by Jay H. Lieske of Caltech’s Jet Propulsion Laboratory.1

The listing of the five events are as follows:

Equilateral Triangle

November 30, 2023, 22:43 UT

Europa, Io, and Callisto will gather on Jupiter’s eastern side, forming a triangle with equal sides, each measuring 50 arcseconds in length. Ganymede is the outlier, appearing on the same side of Jupiter as the other three but far to the east.

Triangle Redux

December 18, 2023, 6:13 UT

In an almost perfect mirror image of the configuration on November 30th, the satellites will this time gather on the western side of Jupiter.

Europa, Io, and Callisto again will form a triangle, with the Europa/Io and Io/Callisto sides measuring 48 arcseconds in length, while the Europa/Callisto side appears slightly shorter at 45 arcseconds. Once again, Ganymede is the outlier, this time positioned far to the west of the trio.

Two Satellites and a Star

January 11, 2024, 23:19 UT

Io, Europa, and a fainter background star, HIP 10415, will form a diagonal line on the western side of Jupiter. Io and Europa shine respectively at magnitudes 5.3 and 5.5, but the star is nearly three magnitudes fainter, appearing only about a tenth as bright; tough to see in steadily held 7x35 binoculars, but easier with larger apertures and higher magnifications.

Interestingly, 15 hours earlier, at 09:00 UT, HIP10415 will pass 18 arcseconds north of Jupiter’s upper rim, a sight that will favor parts of Alaska, Hawai‘i, and much of the central and western Pacific late on the night of January10th. And at 16:17 UT, Callisto will pass a scant 7.6 arcseconds south of the star, visible from eastern Europe as well as western and central Asia.

Jupiter with Only One Satellite

January 21, 2024, 4:57 to 5:39 UT

Most of the time when we look at Jupiter, three or four satellites are in view, but sometimes we only see one. According to a survey done by the noted Belgian celestial mechanic, Jean Meeus, in his book Morsels III, 83 such cases took place between 2004 and 2009, occurring on average 14 times per year. In about 81% of these occurrences, the lone satellite that remains in view is Callisto and such will be the case on this particular occasion.

Europa will be transiting the disk of Jupiter, while Io will be hidden in eclipse by Jupiter’s shadow. At 04:57 UT, Ganymede joins Europa in transit, crossing in front of Jupiter, leaving only Callisto in view far to the west of Jupiter. This circumstance will last until 05:29 UT, when Europa’s transit ends and it moves to the west off Jupiter’s disk.

Four-Satellite Zig-Zag

March 12, 2024, 1:26 UT

All four satellites will be positioned to the west of Jupiter, but rather than stretched out in a straight line, they will appear to form a zig-zag, with abrupt alternate right and left turns. Io and Europa will each appear 30 arcseconds from Ganymede — Io to the upper left and Europa to the upper right — forming a sort of stretched-out triangle. Meanwhile, Callisto sits 1.2 arcminutes to Europa’s lower right.

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Joe Rao got his first view of the Galilean satellites through a Gilbert astronomical telescope in the early 1960s and he has been following their merry dance around Jupiter ever since.