Jupiter’s Moons and the Longitude Problem

Mercury, May/June 2002 Table of Contents

Courtesy of the Serge A. Sauer Map Library, University of Western Ontario.

by Robert Mentzer

For a period lasting over a century, the most effective way to determine longitude was to observe the Galilean moons of Jupiter.

On July 15, 1806, Captain Zebulon Pike led a party of 23 soldiers and 51 Amerindians westward out of St. Louis, Missouri. One of Captain Pike’s assignments was to escort the Amerindians back to their villages. He was then to continue on and explore the southwestern part of Thomas Jefferson’s new Louisiana Purchase. On this very day, Meriwether Lewis and William Clark were near the Great Falls (Montana) of the Missouri River on the last leg of their return journey, which would end in September. They had been sent to explore the rich fur-trapping areas of the northern part of the purchase. Later in his expedition, Zebulon Pike traveled to eastern Colorado and described a mountain that now bears his name, Pike’s Peak.

On August 23, 1806, Pike camped with the Osage Amerindians in their villages near the Kansas-Missouri border. On that day he wrote in his journal, "Took equal altitudes and a meridional altitude of the Sun, but owing to flying clouds, missed the immersion of Jupiter’s satellites." In the middle of what Pike would later call "the great American Desert," surrounded by hostile Amerindians, hundreds of kilometers from civilization, he was looking through a telescope. Isn’t this rather strange behavior for a rugged adventurer? Actually no, for Pike was doing what explorers had been doing for over 100 years: He was using Jupiter’s moons to determine his longitude.

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