Global Positioning System Reference
In-Depth Information
matical instruments from the north of England, constructed a dividing
engine that very accurately graduated linear and circular scales. The design
of Ramsden's machine was influenced by earlier work, but his dividing
engine set new standards of accuracy and was widely copied. It is no
coincidence that the great improvement in accuracy of octants and sex-
tants happened during this period.
SEXTANT
Sextants operate on the same principle as octants. They were an improve-
ment because they cover a wider range of angles: 1 6 of a circle instead of 1 8 .
This became important in the last decades of the eighteenth century be-
cause by then navigators had figured out a way to estimate longitude via the
lunar-distance method, as we will see. From the relative separation of sun
and moon, navigators and geodetic surveyors could consult the Nautical
Almanac, determine local time (relative to Greenwich Mean Time, which
applied at the prime meridian), and thus estimate their longitude. Some-
times the moon and sun could be separated by more than 90\, however,
and so the sextant was developed. The doubling of angles due to mirrors
meant that sextants could be used for separations as great as 120\.
Astronomical sextants have been around since the sixteenth century,
but sextants that were suitable for use on board ships were invented much
later, in 1757 by Captain John Campbell, with the help of a London instru-
ment maker named John Bird. As manufacturing processes improved, the
sextant became more sophisticated and more accurate. Silvered glass re-
placed polished metal mirrors. Filters were added so that the navigator or
surveyor could sight the sun. Sextants are still used as backup instruments
today (fig. 3.13): improved optics and reduced machining tolerances make
modern sextants accurate to less than a tenth of a minute (an error of one
minute of arc translates into a position error of one nautical mile). 8
Sextants are delicate instruments and easily thrown out of alignment—
for example, by being dropped. For this reason they are treated carefully
and are provided with a protective case. As a consequence, many old in-
struments survive in good condition. The National Maritime Museum in
London has an extensive collection of old navigation instruments, and it is
interesting to note, from their dimensions and from the graduation marks,
the potential accuracy and the size of each type of instrument. Two trends
8. For the early navigation and surveying instruments (mariner's astrolabe, cross-sta√,
quadrant, octant, sextant) see Boorstin (1983), Kemp (1976), Mörzer Bruyns (1994),
Turner (1988), and Turner (1998, pp. 39-80).
 
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