Global Positioning System Reference
In-Depth Information
FIGURE 6.11. Determining latitude from the sun or the Pole Star. From elementary
geometry, an observer's latitude is measured by ''shooting'' the sun or the Pole Star.
(a) At equinox, if the sun is due south at noon, a measured altitude of 60 \ (with the
sun 30 \ from zenith) means that the latitude of the observer is 30 \ . (b) The measured
altitude of the Pole Star equals the observer's latitude, apart from a correction for the
deviation of Polaris from true north.
improved the mariner's astrolabe, and it became quite accurate. 12 The
improved mariner's astrolabe was used to ''shoot the sun''—to measure its
angular altitude for the purpose of estimating latitude. The instrument was
suspended at waist height, and its alidade adjusted so that sunlight passed
through a small aperture at the upper end and lit up the lower end, thus
ensuring that the alidade was pointing to the sun. The solar elevation angle
or altitude was read o√ the scale, with an error of less than a degree—
unless the navigator was on board a pitching or rolling ship. The mariner
needed to compensate for certain errors of measurement, and that entailed
consulting tables or performing mathematical calculations. For example, if
the measurements were made at night and the altitude of Polaris was
measured, compensation had to be made for the star's small movements
(these details of celestial wanderings were appreciated at the time of Co-
lumbus). But such tables and calculations were not always required. While
the sun is at its noon zenith during an equinox, its altitude is simply related
to the ship's latitude, as you can see from figure 6.11. In addition, a naviga-
tor fortunate enough to have a clear sky and a calm sea, so that he could
12. Rabbi Zacuto left Spain following the expulsion of Jews in 1492 and found sanctuary
in the Portuguese court of John II, successor to Henry the Navigator. Here, he developed the
improved astrolabe.
 
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