NavList:

Robin S, you wrote:
"At present Jupiter is near opposition meaning that eclipses occur close to the planet's limb and are difficult to observe."

I wrote nearly the same thing in my post. It may have looked repetitive, but it's due to a funny coincidence: our posts both arrived within sixty seconds of 17:00 UT yesterday.

In your post you noted:
"The average duration of eclipses is for Io (501) 3.78 min; Europa (502) 4.54 min"

It's worth mentioning, I think, that the difference is not terribly important between these two. An eclipse lasts "about four minutes" for either Io or Europa, only 20% longer for Europa, so they're both worth watching on that score alone. Europa is often less useful, not so much because eclipses are slower, but because they are less frequent. Eclipses of Europa occur about half as often as eclipses of Io. On the other hand, when Jupiter is in the general vicinity of 90° elongation from the Sun, it's sometimes (*) possible to see both immersion (passing into the planet's shadow) and emersion (coming out of the shadow). That could make Europa a superior choice. Did anyone in the 17th and 18th centuries know this and understand it for practical longitude observations? Hindsight gives insight, but it may be beyond the pale of history.

Frank Reed

* For an example of the "sometimes", simulate the Jovian system in Stellarium for 6 March 2025 at 18:10 UT. This is close to the date when Jupiter is 90° from the Sun which implies that we are looking at Jupiter, its shadow, and the moons from "off to the side" somewhat, compared to its current (Dec 2024) state, near opposition. At Europa's distance from Jupiter the planet's shadow sticks out far enough to the side that we are, just barely, able to see Europa enter the shadow immediately after it emerges from behind the planet's disk. And then about two and a half hours later it emerges from the shadow. But wow, take a look at that immersion event! Europa peeks out from behind Jupiter and then slips right into the shadow. Tough to observe and difficult to time it. Probably good fun, too.