Global Positioning System Reference
In-Depth Information
FIGURE 7.8. The Galilean moons of Jupiter. The four largest moons are, from the
inside outward, Io, Europa, Ganymede, and Callisto. There are dozens of smaller
moons that are visible only with a high-magnification telescope, not available in
Galileo's day. Galileo realized that the relative position of these moons constituted a
universal clock. He did not win a longitude prize with this method because it involved
accurate measurement of the moons' positions, which was impractical at sea. Photo
by Jan Sandberg.
fixed and were constrained to lie in a plane, but neither of these prerequi-
sites applies in practice, so the idea was stillborn.
Somewhat earlier, in 1610, Galileo thought he might win the Spanish
longitude prize with his idea of measuring time by observing the moons of
Jupiter. He had discovered the four biggest Jovian moons (now called the
''Galilean moons'') with his newly developed telescope and realized that
their regular orbits constituted a celestial clock that could be read by
anyone with a telescope. It was not di≈cult to see these four moons even
with the poor-quality telescopes of Galileo's time. (The moons are shown in
fig. 7.8; they are readily observed through a modern telescope of modest
magnification.) Giovanni Cassini, a century after Galileo, got the Jovian
moon method to work well. By then, the orbits were known with su≈cient
accuracy to be able to predict them. Therefore, the positions of the moons
relative to Jupiter and each other could indeed be used to tell the time and
reveal the observer's longitude.
The method worked fine on land and was used well into the nineteenth
century. An observer would take measurements and consult tables that
showed the position of the moons at a given time in Greenwich, say. He
would then check his local time, and from the di√erence he estimated his
longitude. The trouble with this method was in making accurate measure-
ments of the four moons while on the moving deck of a ship at sea. This prob-
lem proved intractable, and the method was therefore not adopted at sea.
 
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