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I had a recent query on my website and thought it would be a good topic for wider discussion.

Most of us have heard of the principle of observing the eclipses of Jupiter's large moons, numbered historically as I, II, III, IV, but known to us, since the names became popular in the 19th century, as Io, Europa, Ganymede, and Callisto. Io and Europa, closer to Jupiter and thus moving at higher speed, frequently and rapidly pass into Jupiter's shadow. These eclipses can be observed with a good telescope, and they occur at fixed "absolute" times, effectively providing Greenwich time or equivalent without adjustment. The observer watches the moon disappear, and as it vanishes, records the local time, based on the altitudes of stars typically. The difference between absolute time, from the distant moon's eclipse, and local time then yields a difference in longitude, as usual.

Observing Jupiter's moons at sea was never practical. Even on land the method seems to have been more popular in principle than in practice. Yet some famous key longitudes were found by observing the moons of Jupiter, especially under the guidance of the Italian-born French astronomer, Cassini, in the latter part of the 17th century.

I'm attaching below a large map below. It's a copy of Cassini's world map, or "planisphere terrestre" from about 1680, published in this form in 1696. Dozens of longitudes derived from the "eclipse of the satellites of Jupiter" as noted in the border around the map were compiled to generate the map. How well do they hold up? How many of them can you find on the map? Note that the original from which I generated this version can be zoomed to slightly higher resolution and may be viewed here: https://exhibits.stanford.edu/ruderman/catalog/rc195gv8122. Look for the locations with a "star" or asterisk after the name. For example, Quebec (the city) has a star. Also in France, not just Paris but also Nantes. Or in what is now southernmost modern Turkey, there's Alexandretta. There are a suprising (to me) number of observations in southern and eastern Asia including an observation in Louvo, modern Lopburi, which was apparently a royal capital in Thailand for just a few decades in the late 17th century.

How was it done in practice? What observing guidebooks might have spelled it all out in the era? What were the limitations of the method? Was it replaced by "lunars" and lunar occultations? And how did results from eclipses of the moons of Jupiter compare with common lunar eclipses --that is, eclipses of the Earth's moon, eclipses of the Moon? How did these different astronomical methods measure up?

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA

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