Global Positioning System Reference
In-Depth Information
FIGURE 3.10.
The backstaff. (a) A simple design showing how the backstaff permits
the user to measure solar height without having to look directly at the sun. (b) The
Davis quadrant, a variant that was easy to use.
tion and the measured instrument altitude. Navigators who used any of the
instruments available during this period (not just the astrolabe) needed to
account for solar declination.
BACKSTAFF
Backsta√
is the term applied to a family of instruments that were in use for
a long period. Among these was the
Davis quadrant
, or
English quadrant
,
which replaced the astrolabe as the instrument of choice for measuring
latitude at sea. The simplest backsta√, which conveys the basic idea, is
shown in figure 3.10a. The
sta√
(an alidade) supported a moveable
arc
(often curved) that slid along it. A
horizon vane
showed the shadow cast by
the arc and had a hole in the center to permit a mariner to see the horizon.
