NavList:

Steve Gillion, you wrote:
"I'm just so interested in the workings and observing the correct moons"

Io, the innermost of the Galilean moons, is always preferred. Europa is a close second. Ganymede and Callisto are less useful.

Why Io? The two most important factors are the angular speed of the Moon as it enters Jupiter's shadow or transits Jupiter's disk and the relative frequency of the events. Excluding the outermost, Callisto, the four Galilean moons are in a 1:2:4 resonance of their orbital periods. Over the course of observing Jupiter for a few months, this means we'll see about four Io events and two Europa events for every single Ganymede event. The angular rates are also faster for the inner moons so the eclipses occur more rapidly and are thus more useful for determing the time.

Have you, or anyone else reading this, tried it? Of course we don't have telescopes like the ones from the late 17th century --those comically long, pencil-thin refractors that often had to be slung from ropes at both ends. But common "backyard" amateur telescopes of a few inches aperture and with eyepieces/oculars allowing magnifications of 50-100x are up to the task. Myself, I had not observed an eclipse of a Galilean satellite in years, but last week while composing a reply to this discussion, I happened to look at a calendar of eclipses, and I realized there was an Io eclipse just twenty minutes ahead. Perfect timing! So I ran downstairs, dragged the telescope outside, and watched an eclipse. While the timing was perfect, the season is not. The problem is that we are close to Jupiter's opposition. For about a month around opposition it is difficult to observe eclipses of Jupiter's moons because the shadow of the planet is directly behind the planet at opposition. Even so, I could follow the event well enough and time the eclipse to the about +/- 30 seconds at 60x magnification. Good enough for an adequate longitude by 17th and even 18th century standards... The actual eclipse durations are several minutes, so there has to be some agreed upon standard by which to time it. And telescope size and quality, observer skill, and weather conditions (including "seeing") will modify the observed times.

There's an article by Joe Rao on the S&T website this week discussing some interesting "compact" groupings of the Galilean moons in the months ahead. These are mostly for observer entertainment, but as he notes, the configurations change noticeably in just a few minutes, illustrating the usefulness as well as the limitations of the Galilean moons for determining absolute time.

Frank Reed