Global Positioning System Reference
In-Depth Information
chronometers H4 and H5 were very expensive—they were hand-crafted,
idiosyncratic masterpieces—most ship's captains and navigators could not
a√ord to pay for the construction of one of them. It fell to other watch-
makers to develop chronometers that were a√ordable enough to be used
by most ships. It was another half century before every ship in the Royal
Navy was equipped with a marine chronometer. In the interval, the less-
expensive lunar distance method was used. Indeed, many navigators who
could a√ord a chronometer also used the lunar distance method during
this period, so that each method could act as a check on the accuracy of the
other.
In practice, the early chronometers were handled with kid gloves. They
were kept in gimbals in a dry room near the center of a ship to minimize
the e√ects of varying conditions. One of the navigator's key tasks (that's a
pun) was winding the chronometer. When making an observation to esti-
mate longitude (a celestial altitude measurement, for example), he would
use a good pocket watch to record the time, not the marine chronometer,
which was never taken outside, so as to avoid exposure to wind or sea. The
pocket watch time would be set by reference to the chronometer.
FINDING LONGITUDE BY THE LUNAR DISTANCE METHOD
Advocates of the lunar distance method of ascertaining longitude were
doubtful of the e≈cacy of the rival chronometer method. Newton ob-
served that a chronometer, should it go wrong during a long voyage, would
render all subsequent estimates of longitude incorrect; the timekeeping
error could not be fixed by subsequent observations. The lunar distance
measurement, in contrast, was robust in this regard. One bad measure-
ment could be ignored because a subsequent good measurement would
bring a ship back on course.
The idea of using the moon as a clock to estimate longitude is quite old;
it was first proposed in 1514 by Johann Werner. In those days, however, the
means to implement the method were lacking. For one thing, the cross-
sta√ instrument was incapable of making measurements with su≈cient
accuracy; for another, the theory of lunar motion was not developed su≈-
ciently to describe the moon's orbit.
How does this method work? The moon's movement across the night sky
means that it has a di√erent background of stars at di√erent times. If the
orbit is known accurately, an astronomer can predict the (angular) distance
between the moon and a specified star and so could say, ''At 22:00 hours
GMT on April 21, 1750, the moon will be 23\ 31% 10& distant from Sirius and
 
Search WWH ::




Custom Search