Observe Jupiter’s Moons in Peculiar Alignments

In the upcoming months, there will be five chances for you to witness Jupiter’s moons in fascinating configurations.

Seeing the Galilean satellites of Jupiter shift positions in relation to one another from night to night and even hour to hour is always intriguing. However, in the upcoming months, watchers will be able to see some odd geometries among Jupiter’s largest moons.

The innermost satellite, Io, completes a full rotation of Jupiter in a just 1.8 days. Ganymede spends 7.2 days around Jupiter, which is precisely four times the period of Io, whereas Europa takes twice as long—3.6 days. Of the Galilean moons, Callisto is the farthest from Jupiter and its circle around the massive planet takes 16.7 days to complete.

It is evident that the locations of all four satellites at any given time are unique to that specific instant since they are continuously traveling at various rates.

Occasionally, three satellites may gather together to form a group in the sky. When they do, there’s generally a split second when they seem to form a triangle or other geometric design. The image through your eyepiece will seem very different from the common perception of Jupiter’s moons if the alignment is at a sharp angle to the plane of their orbits.

During the current Jupiter apparition, which runs from November into April of next year, four of these lineups will be visible from at least a portion of North America. You will notice the occurrence when two or three satellites pass close to one another because of how quickly they appear to be moving. Given how sensitive the human eye is to the veracity of a straight line, these strange looks most likely won’t persist longer than a few minutes.

Additionally, there will be some fascinating interactions between Jupiter, Callisto, and a dim background star in Aries on a particular evening in mid-January. In January, there’s also an opportunity to witness the uncommon phenomenon of Jupiter accompanied by only one single satellite, Callisto.

Because the Galilean satellites have extremely complicated orbital dynamics and the calculations are still not perfect, my times may still be incorrect by several minutes even though I double-checked the configurations and times using numerous planetarium programs (Starry Night Pro 8, Stellarium, Dance of the Planets). A 1977 research by Caltech’s Jet Propulsion Laboratory’s Jay H. Lieske is perhaps the best hypothesis now available about the satellite’s velocity.

The following is a list of the five events:


Thanksgiving 30, 2023, 11:43 UT

On the eastern side of Jupiter, Europa, Io, and Callisto will assemble to create a triangle with equal sides that are each 50 arcseconds long. The exception is Ganymede, which is located far to the east on the same side of Jupiter as the other three planets.


At 6:13 UT on December 18, 2023

The satellites will assemble once again on Jupiter’s western flank, in a nearly exact mirror copy of the arrangement on November 30.

Europe/Io and Io/Callisto will once more form a triangle with Europa, Callisto, and Io; the sides of the triangle are 48 arcseconds long, while the Europa/Callisto side is 45 arcseconds long. Ganymede is the anomaly once more, but this time it is located far to the west of the other two.


UT on January 11, 2024, at 23:19

On Jupiter’s western side, Io, Europa, and HIP 10415—a fainter background star—will create a diagonal line. The brightness of Io and Europa are 5.3 and 5.5, respectively, but the star itself is about three times fainter, looking barely a tenth as brilliant; it is difficult to detect via stably held 7×35 binoculars, but easier at wider apertures and higher magnifications.

Interestingly, HIP10415 will pass 18 arcseconds north of Jupiter’s top rim 15 hours earlier at 09:00 UT. This sighting would favor regions of Alaska, Hawaii, and most of the central and western Pacific late on January 10th. Additionally, Callisto will be visible from eastern Europe as well as western and central Asia around 16:17 UT, traveling 7.6 arcseconds to the southeastern direction of the star.

Jupiter having a single satellite

2024 January 21, 4:57–5:39 UT

When we gaze at Jupiter, we usually see three or four of its satellites, but occasionally we only see one. In his book Morsels III, renowned Belgian celestial mechanic Jean Meeus conducted a survey which revealed that between 2004 and 2009, there were 83 similar incidents, with an average of 14 per year. Callisto is the only satellite that is still visible in around 81% of these instances, and that is what will happen this time as too.

When Jupiter’s shadow obscures Io during an eclipse, Europa will be traveling across Jupiter’s disk. Only Callisto is visible far to the west of Jupiter as Ganymede and Europa collide at 04:57 UT and pass in front of Jupiter. This situation will last until 05:29 UT, at which point Europa will move off Jupiter’s disk to the west as its transit comes to a close.


March 12, 2024, 1:26 UT

Each of the four satellites will be located west of Jupiter, but instead of appearing to form a straight line, they will zigzag, making sudden twists to the left and right. The positions of Io and Europa, which form a type of extended triangle, will be 30 arcseconds apart from Ganymede, with Io to the higher left and Europa to the upper right. Callisto is located 1.2 arcminutes to Europa’s lower right in the meanwhile.

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