It was a pleasure to read Fokke Tuinstra’s letter (Physics Today, December 2004, page 16) on Ole Rømer’s proof that the speed of light is finite. Perhaps an account of the reasons that led him and other scientists to study so precisely the moons of Jupiter (the system Rømer used for his result) would help complete that nice piece of history.
After Christopher Columbus’s 1492 trip to America, ships, mostly Spanish and Portuguese, crossed the Atlantic Ocean in increasing numbers. That migration forced a qualitative change in coastal navigation techniques, since mariners sailed without visible land or fixed points of reference. The determination of geographical longitude became an urgent need—at sea for navigation, and on land for more precise cartography and settlement of territorial disputes.
The need led Spanish King Felipe II (1527-98) to offer a substantial reward to the inventor of a method to “find longitude.” The reward amount was increased years later by his successors, and greater rewards were promised by other countries as they began their own oceanic navigations. The British government, for example, offered £20 000 in 1714 to whoever could provide a satisfactory method of finding a ship’s position to within half a degree.
What has this circumstance to do with the abrupt and almost obsessive dedication to the study of Jupiter’s moons?
When Galileo discovered Jupiter’s first four satellites in 1610, he realized that comparing their eclipse times with local times at a ship’s position could be a key component in determining longitude. In 1612, and on three occasions thereafter, Galileo tried unsuccessfully to convince the Spanish monarchy of the usefulness of his method. Although his idea proved to be impractical at sea, it did eventually work on land. For example, the difference in longitude between Paris and Uranienborg, Denmark, was calculated on the basis of the eclipse times taken by Giovanni Cassini, Jean Picard, and Ole Rømer in 1671.
Clockmaker John Harrison won the British prize by developing, between 1735 and 1764, increasingly precise and practical mechanical chronometers, including the one used by James Cook in some of his expeditions.
Rømer’s work is probably the first measured Doppler effect; that is, he discovered that the value observed on Earth for the period of Jupiter’s moons depends on Earth’s velocity relative to Jupiter. It is also noteworthy that today’s global positioning system solution to the old problem of finding longitude requires the use of general-relativistic corrections to attain maximum accuracy. If Rømer’s studies are an excellent example of how technologically driven research may provide fundamental basic science results, the GPS application of general relativity also nicely demonstrates complementary feedback in the science—technology system.
