Global Positioning System Reference
In-Depth Information
compiling navigational almanacs. The sextant had been invented largely to
improve the ease and accuracy of the necessary celestial measurements.
But making suitably accurate measurements at sea was di≈cult. Also,
many corrections were required to account for the diameter of the moon,
atmospheric refraction, and parallax e√ects. In the days before computers,
many hours of mathematical calculation were still required to reduce the
observations and obtain an estimate of GMT—and all this measurement
and calculation presupposed a clear sky so that moon and stars could be
seen. Despite these di≈culties, by the end of the eighteenth century, both
almanac data and sextant accuracy were good enough to provide a reason-
able estimate of longitude at sea, in fair weather.
FINDING LATITUDE AND LONGITUDE BY
THE ALTITUDE INTERCEPT METHOD
A sextant was used for celestial navigation, of course, as well as for coastal
positioning. Earlier, we applied a sextant to measure bearing angles; here,
we turn it 90\ and measure altitudes. By the mid-1800s the accuracy of
almanacs and of sextants enabled position-fixing anywhere on the globe, at
sea or on land, via the
altitude intercept
method. That is to say, the knowl-
edge of star positions in the night sky and the technology of sextant manu-
facture were both su≈ciently well developed for a navigator to determine
his latitude and longitude with a couple of celestial readings with his
sextant (assuming he had a clear sky and an almanac). Even today, the
altitude intercept method is used as a backup to satellite navigation; before
GPS, it was a standard technique. This procedure was superior to, and
replaced, the older lunar distance method because it did not require shoot-
ing the moon: with the lunar distance technique, the angular size of the
moon rendered measurements error-prone.
A modern navigator's almanac lists the positions of 57 ''navigation stars''
for every hour GMT of every day of the year. The chosen stars are bright;
and they spread over the northern, equatorial, and southern skies so that,
given clear visibility, a navigator will always be able to find a few navigation
stars above his horizon. For example, in the northern sky are Capello,
Draco, and Vega. In equatorial skies you will find Betelgeuse, Pollux, and
Sirius. In the southern hemisphere there are Canopus, Harad, and Rigil
Kentaurus. A navigator shoots a few stars with her sextant. She then car-
ries out ''sight reduction calculations''—nowadays greatly simplified by
software—to correct for observational errors due, for example, to the ob-
server's altitude above sea level or to atmospheric refraction (particularly
