Surveying - The Science of Navigation - page 64

reference for measuring the sun's height, and on land the horizon might
not be visible, thus rendering a cross-sta√ useless. The plumb line was a
disadvantage at sea, however, because it meant that the angle being mea-
sured would change as the ship pitched.
MARINER'S ASTROLABE
Much simpler than the planispheric astrolabe, from which it was adapted in
the fourteenth century, the mariner's astrolabe consisted of a circle that was
graduated in degrees and a movable alidade that rotated about the center
(fig. 3.9a). The alidade was aligned with the noon sun so that latitude could
be determined. The instrument was typically made of brass, was 7-8
inches (20 cm) in diameter, and had a thumb ring at the top by which it was
suspended. It was made heavy, particularly the lower part, so that it would
not swing much in a wind. As you might imagine, this instrument was
di≈cult to use at sea, so mariners who were known to use it, such as Francis
Drake, may have obtained more accurate sightings while on land. Also, the
mariner's astrolabe accuracy increased with size because angles could be
marked more accurately on a larger circle. Drake may have suspended a
large astrolabe from a tripod while on land to obtain more accurate read-
ings than were possible on deck. Despite its clumsiness, over time the
mariner's astrolabe largely replaced the cross-sta√ and the quadrant.
The angle measured by a mariner's astrolabe, the instrumental altitude
(angle a of fig. 3.9b) was converted to altitude by consulting an astro-
FIGURE 3.8. A quadrant. No reference to the horizon is needed, so this instrument can
be used in places where the horizon is not visible. The sun's height in the sky is the
angle a .