Global Positioning System Reference
In-Depth Information
FIGURE 3.11.
The octant. Because it used mirrors, thereby doubling angles, the octant
covered the same range of angles as the earlier quadrant. It also achieved the same
accuracy as earlier instruments that were twice the size. Like the backstaff, the octant
measured a relative angle (between horizon and star, here) and so was quite easy to
use on board a ship. Unlike the backstaff, it could sight stars as well as the sun.
into the eyepiece of the sighting scope. The navigator or surveyor could see
the horizon directly and the star alongside it. This meant that the eye need
not be moved during observation: the index mirror angle was simply ad-
justed until the star and horizon were aligned.
The double mirror made the octant much easier to use than previous
instruments for estimating latitude. It ascertained latitude from the height
of point-like stars, rather than from the sun, thus reducing a major source
of measurement error. High-quality octants were constructed of brass; less
expensive, lower-quality instruments were made from wood and ivory. An
octant was half the size of a Davis quadrant, but with similar accuracy.
When a Vernier scale was added, the octant was accurate to within one
minute of arc. The octant and its successor, the sextant, almost eliminated
the earlier solar-height instruments by 1780.
VERNIER SCALE AND INSTRUMENT ACCURACY
In the 1630s the French mathematician Pierre Vernier developed a very
simple and successful supplementary scale that greatly improved the ac-
curacy of reading that can be made from a measuring instrument. (Ver-
nier's scale was based on earlier work by a Portuguese mathematician,
Pedro Nunes, whom we will meet again.) So successful was his idea that it
is everywhere today: there are hundreds of examples in your local hard-
ware store.
