Jupiter through binoculars

Trying to replicate Galileo’s observations is harder than it sounds.
By | Published: September 1, 2021 | Last updated on May 18, 2023
A 200mm (effective focal length [EFL] 320mm) telephoto lens image of Jupiter on the morning of June 10, 2020, showing Io and Europa immediately to the left (east) of Jupiter. These moons initially appeared as a single object to the author, who was using 8×42 binoculars. North is up.
Stephen James O’Meara

Jupiter is entering our night skies again after sunset — just in time for a project you can undertake in the coming months. All that’s required are a pair of handheld binoculars and some patience.

Before we start, I have to admit that after more than a half century of observing Jupiter, I always took for granted one simple statement: that Jupiter’s four Galilean satellites can be seen through binoculars. While that statement is true in a general sense, I never gave thought to the nuances involved in detecting these moons as their positions shift relative to one another and to their parent planet.

A 200mm (EFL 320mm) telephoto lens image of Jupiter on the night of July 4, 2020, showing Ganymede at far left (east), Callisto at right (west), and Europa just east of the planet — too close to be resolved through 8×42 binoculars; Io was behind the planet. North is up.
Stephen James O’Meara

The challenge

The awakening, for me, came the morning of June 10, 2020, when I trained a pair of 8×42 binoculars on Jupiter at 4:21 UT from Maun, Botswana, shortly after the start of astronomical twilight. A waning gibbous Moon was nearby, which also helped illuminate the background sky and cut down on contrast. With a glance, I saw three moons. And had I put down the binoculars, I would have left it at that — assuming that the fourth moon was either behind or in front of the planet, or very close to its edges.

But as I fiddled with the focus, I suddenly saw the moon closest to Jupiter take on an egg-shaped appearance. When I braced the binoculars against the hood of my car, I resolved the “egg” into two close moons.

On July 4, 2020 at 18:47 UT, I used the same binoculars to observe Jupiter. This time, a prolonged study revealed two moons, Ganymede and Callisto, on opposite sides of the planet. I then used a telephoto lens to image Jupiter. To my surprise, the image showed another moon (Europa) only about 16″ east of Jupiter’s disk; Io was occulted by the planet.

Try as I might, I could not see Europa through the 8x42s, nor in 10x40s. The moon was, however, just visible through 10x50s, though both stabilized binoculars and patience were required to pull it out from the planet’s glare.

Two weeks later, on July 18 (18:00 UT), three moons were visible (Callisto, Europa, and Ganymede) through 8×42 binoculars. Europa, however, was only seen with difficulty, even though it was roughly one Jupiter diameter (47.5″) to the west of the planet. I found this the most surprising sighting of all, as I had expected that moon to be clearly visible at this apparent separation.

A photo-illustration of the way Jupiter’s moons appeared on the night of Jan. 17, 1610 (top), compared to a reproduction of Galileo’s sketch of the two “stars” he observed (bottom) that night.
Stephen James O’Meara

Thoughts on Galileo

On Jan. 7, 1610, in the first hour after twilight, Italian astronomer Galileo Galilei observed, and drew the positions of, what he believed were three fixed stars near Jupiter. These turned out to be moons — a fact revealed to him over the course of months. Notice, however, that I do not say that “he discovered three of the four Galilean moons.”

Using Fifth Star Labs’ Sky Guide app, I checked the positions of the moons for that date and time against Galileo’s original drawing. I was surprised to find that one of the moons he saw was actually the combined light of Europa and Io, which were passing close to one another during the first hour of nightfall. Had he observed a couple of hours later, the moons would have been further apart and identifiable as four distinct objects.

Similar situations occurred over the course of Galileo’s later observations, where he could not resolve close pairings of moons. But I was impressed by his accurate deduction of the events on the night of Jan. 17, 1610, when, at about 30 minutes after sunset, he saw, initially, only two “stars”:

The easterly star [the combined light of Ganymede and Europa] was distant from … Jupiter by 3′. The westerly [Callisto] by 11′. The easterly seemed twice greater [in brightness] than the other [westerly star]. No more than these two stars were visible. But … on the 5th hour, a third star began to appear which, as I conjecture, was joined with the easterly one and such was their appearance.

A fourth moon (Io) was also visible that night, but Galileo did not resolve it — even though the moon was a little more than a Jupiter-width to the west.

Galileo sometimes sketched his initial thoughts and observations on scratch paper before entering them into his logbook, as in this example. This top portion of this page contains a rough draft of a letter Galileo was writing to the doge of Venice. On the rest of the page, he scribbled down observations of Jupiter Jan. 7–15, 1610, noting how the “stars” appeared to move around it.
University of Michigan Library (Special Collections Library)
What I gained from these experiences attempting to reproduce Galileo’s observations was a crisp understanding of the challenges he had to overcome in order to puzzle out the mysterious motions of these new worlds. I also gained an increased admiration for the man whose observations with an inferior instrument missed nothing that a modern observer today would have seen through binoculars. As always, send your observations and thoughts to sjomeara31@gmail.com.