absent from the sky or overshadowed by the sun’s light. Nighttime observations could be carried on for only part of the year, and then only when skies were clear.
In spite of these obvious difficulties, Galileo had designed a special navigation helmet for finding longitude with the Jovian satellites. The headgear—the celatone—has been compared to a brass gas mask in appearance, with a telescope attached to one of the eyeholes. Through the empty eyehole, the observer’s naked eye could locate the steady light of Jupiter in the sky. The telescope afforded the other eye a look at the planet’s moons.
An inveterate experimenter, Galileo took the contraption out on the harbor of Livorno to demonstrate its practicability. He also dispatched one of his students to make test runs aboard a ship, but the method never gained adherents. Galileo himself conceded that, even on land, the pounding of one’s heart could cause the whole of Jupiter to jump out of the telescope’s field of view.
Nevertheless, Galileo tried to peddle his method to the Tuscan government and to officials in the Netherlands, where other prize money lay unclaimed. He did not collect any of these funds, although the Dutch gave him a gold chain for his efforts at cracking the longitude problem.
Galileo stuck to his moons (now rightly called the Galilean satellites) the rest of his life, following them faithfully until he was too old and too blind to see them any longer. When Galileo died in 1642, interest in the satellites of Jupiter lived on. Galileo’s method for finding longitude at last became generally accepted after 1650—but only on land. Surveyors and cartographers used Galileo’s technique to redraw the world. And it was in the arena of mapmaking that the ability to determine longitude won its first great victory. Earlier maps had underestimated the distances to other continents and exaggerated the outlines of individual nations. Now global dimensions could be set, with authority, by the celestial spheres. Indeed, King Louis XIV of France, confronted with a revised map of his domain based on accurate longitude measurements, reportedly complained that he was losing more territory to his astronomers than to his enemies.
The success of Galileo’s method had mapmakers clamoring for further refinements in predicting eclipses of the Jovian satellites. Greater precision in the timing of these events would permit greater exactitude in charting. With the borders of kingdoms hanging in the balance, numerous astronomers found gainful employment observing the moons and improving the accuracy of the printed tables. In 1668, Giovanni Domenico Cassini, a professor of astronomy at the University of Bologna, published the best set yet, based on the most numerous and most carefully conducted observations. Cassini’s well-wrought ephemerides won him an invitation to Paris to the court of the Sun King.
Louis XIV, despite any disgruntlement about his diminishing domain, showed a soft spot for science. He had given his blessing to the founding, in 1666, of the French Academie Royale des Sciences, the brainchild of his chief minister, Jean Colbert. Also at Colbert’s urging, and under the ever-increasing pressure to solve the longitude problem, King Louis approved the building of an astronomical observatory in Paris. Colbert then lured famous foreign scientists to France to fill the ranks of the Académie and man the observatory. He imported Christiaan Huygens as charter member of the former, and Cassini as director of the latter. (Huygens went home to Holland eventually and traveled several times to England in relation to his work on longitude, but Cassini grew roots in France and never left. Having become a French citizen in 1673, he is remembered as a French astronomer, so that his name today is given as Jean Dominique as often as Giovanni Domenico.)
From his post at the new observatory, Cassini sent envoys to Denmark, to the ruins of Uraniborg, the “heavenly castle” built by Tycho Brahe, the greatest naked-eye astronomer of all time. Using observations of Jupiter’s satellites taken at these two sites, Paris and Uraniborg, Cassini confirmed the latitude and longitude of both. Cassini also called on observers in Poland and Germany to cooperate in an international task force devoted to longitude measurements, as gauged by the motions of Jupiter’s moons.
It was during this ferment of activity at the Paris Observatory that visiting Danish astronomer Ole Roemer made a startling discovery: The eclipses of all four Jovian satellites would occur ahead of schedule when the Earth came closest to Jupiter in its orbit around the sun. Similarly, the eclipses fell behind the predicted schedules by several minutes when the Earth moved farthest from Jupiter. Roemer concluded, correctly, that the explanation lay in the velocity of light. The eclipses surely occurred with sidereal regularity, as astronomers claimed. But the time that those eclipses could be observed on Earth depended on the distance that the light from Jupiter’s moons had to travel across space.
Until this realization, light was thought to get from place to place in a twinkling, with no finite velocity that could be measured by man. Roemer now recognized that earlier attempts to clock the speed of light had failed because the distances tested were too short. Galileo, for example, had tried in vain to time a light signal traveling from a lantern on one Italian hilltop to an observer on another. He never detected any difference in speed, no matter how far apart the hills he and his assistants climbed. But in Roemer’s present, albeit inadvertent, experiment, Earthbound astronomers were watching for the light of a moon to reemerge from the shadow of another world. Across these immense interplanetary distances, significant differences in the arrival times of light signals showed up. Roemer used the departures from predicted eclipse times to measure the speed of light for the first time in 1676. (He slightly underestimated the accepted modern value of 300,000 kilometers per second.)
In England, by this time, a royal commission was embarked on a wild goose chase—a feasibility study of finding longitude by the dip of the magnetic compass needle on seagoing vessels. King Charles II, head of the largest merchant fleet in the world, felt the urgency of the longitude problem acutely, and desperately hoped the solution would sprout from his soil. Charles must have been pleased when his mistress, a young Frenchwoman named Louise de Keroualle, reported this bit of news: One of her countrymen had arrived at a method for finding longitude and had himself recently arrived from across the Channel to request an audience with His Majesty. Charles agreed to hear the man out.
The Frenchman, the sieur de St. Pierre, frowned on the moons of Jupiter as a means of determining longitude, though they were all the rage in Paris. He put his personal faith in the guiding powers of Earth’s moon, he said. He proposed to find longitude by the position of the moon and some select stars—much as Johannes Werner had suggested one hundred sixty years previously. The King found the idea intriguing, so he redirected the efforts of his royal commissioners, who included Robert Hooke, a polymath equally at home behind a telescope or a microscope, and Christopher Wren, architect of St. Paul’s Cathedral.
For the appraisal of St. Pierre’s theory, the commissioners called in the expert testimony of John Flamsteed, a twenty-seven-year-old astronomer. Flamsteed’s report judged the method to be sound in theory but impractical in the extreme. Although some passing fair observing instruments had been developed over the years, thanks to Galileo’s influence, there was still no good map of the stars and no known route for the moon.
Flamsteed, with youth and pluck on his side, suggested that the king might remedy this situation by establishing an observatory with a staff to carry out the necessary work. The king complied. He also appointed Flamsteed his first personal “astronomical observator”—a title later changed to astronomer royal. In his warrant establishing the Observatory at Greenwich, the king charged Flamsteed to apply “the most exact Care and Diligence to rectifying the Tables of the Motions of the Heavens, and the Places of the fixed Stars, so as to find out the so-much desired Longitude at Sea, for perfecting the art of Navigation.”
In Flamsteed’s own later account of the turn of these events, he wrote that King Charles “certainly did not want his ship-owners and sailors to be deprived of any help the Heavens could supply, whereby navigation could be made safer.”
Thus the founding philosophy of the Royal Observatory, like that of the Paris Observatory before it, viewed astronomy as
