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Frank,

You asked:

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?

I'm attaching the advice given by Cassini in 1693 taken from "Longitude and the Satellites of Jupiter" https://archive.org/details/questforlongitud0000long that Robert van Gent drew our attention to in https://navlist.net/Time-Longitude-Jupiters-moons-RobertH.vanGent-nov-2024-g56512

I should point out that the filters in the spreadsheet I posted here were set up with the idea that they could be used explore the properties of the eclipses. I'm reattaching a version with filters applied to show the eclipses of Europa. It's easy to see that the sequence of EC.D - EC.R pairs changes to just EC.R on 27 February 2024.

For reference, in the spreadsheet, the column D(r) is

Distance from the satellite undergoing the phenomenon to the limb of the planet, referred to the apparent radius of the planet in the direction of the satellite at the time of the maximum.

and column S(") is

Angular distance between the two bodies involved at the time of the maximum.

Regards,
Robin

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