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In the era before global positioning system (GPS) instruments, determining longitude was difficult since it required a fairly accurate clock, in addition to a sextant. I was under the impression that one of the clocks Lewis and Clark used was Jupiter’s moon Io, which would have made them pretty sophisticated navigators.

Michael D. Delano
Brooklyn, N.Y.

The explorers had a good chronometer, but researcher Robert E. Criss says that not much more is known about it because it was auctioned off after the trip. He didn’t find any mention in the party’s diaries of Jupiter’s moon, but the team did rely on Earth’s moon for some measurements .–K. Ramsayer