Longitude. by Jove! – navigating with Jupiter’s moons

Long before John Harrison perfected a chronometer sufficiently accurate to be invaluable to mariners in determining longitude at sea, learned scholars had made a number of attempts to find longitude using nature’s universal timepiece — the daily parade of celestial objects like the sun, moon and stars across the sky.

By 1610, Pisa-born genius Galileo Galilei had perfected an earlier Dutch optical invention known as the refracting telescope. His refinement of this long-distance viewing device and the celestial wonders he uncovered with it eventually earned for him a guest appearance before the Catholic Inquisition, and a bit of jail time, but not before he observed the heavens in more detail than had ever before been possible. Galileo saw for the first time sunspots, the cratered surface of Earth’s moon, and took special note of Jupiter and the four previously unknown bodies orbiting with precise regularity about it — the captive moons Io, Callisto, Europa and Ganymede.

Galileo recorded eclipses and periods of the jovian moons, eventually concluding that their clocklike motions about the mother planet might be of particular use to astronomers for timing celestial events, and might even be used by sailors attempting to find their longitude at sea. Galileo’s refined telescope was quickly hailed by astronomers, but his idea for using the movements of Jupiter’s satellites to find longitude at sea was shelved for the next 150 years.

Interestingly, it was a British churchman and astronomer, Dr. Nevil Maskelyne (the gentleman responsible for the first Nautical Almanac in 1767) that resurrected Galileo’s idea. Maskelyne attacked Harrison’s atempts to create an accurate mechanical timepiece for navigators as not worthy of serious consideration, especially since God had already so thoughtfully supplied humanity with the master clockwork of Jupiter’s satellites. While he was an intelligent and gifted scholar, Maskelyne was no sailor. He failed to appreciate the impracticality of aiming and keeping a 3-foot telescope trained on Jupiter in anything but a flat-calm sea. When that tiny flaw was pointed out to him, he proposed an unsuccessful gimballed “marine chair” for shipboard observations, which was supposed to allow the observer and telescope to remain horizontally stable in much the same way a gimballed ship’s compass negotiates the yawing, pitching and rolling motions of a vessel at sea.

As elegant and appealing as scientists and astronomers found this idea for solving the age-old problem of longitude at sea, the true value of the exercise was to be found ashore. Capt. James Cook used jovian moon observations to find Greenwich time and determine his longitude on various coasts during one of his round-the-world voyages of exploration, making his observations on solid, newfound terra firma. A special navigational benefit of Cook’s observations was that he had an accurate point of departure from which to begin the next leg of his voyage.

The New Practical Navigator (the nineteenth edition published by John Hamilton Moore during the War of 1812) offered navigators the following instructions for determining longitude accurately using observations of eclipses of Jupiter’s moons:

“On the day preceding the evening on which it is proposed to observe an eclipse, look for the time when it will happen at Greenwich, in page 3d of the month in the Ephemeris. Find the difference of longitude either by a good map, sea chart, or dead reckoning.

“Let the watch be regulated by the sun with all possible exactness to the apparent time. Turn the difference of longitude into time, according as it is east or west of Greenwich, the sum or difference will be nearly the time when the eclipse is to be looked for in that place. But as the longitude is uncertain, it will be proper to begin 20 or 30 minutes before.

“Observe the hours, minutes, and seconds of the beginning of the eclipse, called immersion, that is, the very instant that the satellite appears to enter into the shadow of Jupiter; or the emersion, that is, when it appears to come out of the same. The difference of time between the observed immersion, or emersion, and that set down in the Nautical Almanack, being turned into degrees, will give the difference of longitude between Greenwich and the place of observation.”

While the Nautical Almanac and Astronomical Ephemeris for many years continued offering detailed information on the eclipses of Jupiter’s satellites, finding longitude by this method was rarely practiced by sailors at sea. Less expensive and more seaworthy knockoffs of Harrison’s ingenious marine chronometer made certain of that.

J. Gregory Dill


By Ocean Navigator