Surveying - The Science of Navigation - page 67

The arc was adjusted so that the top of its shadow just touched the hole in
the horizon vane. The sta√ was graduated to show the height of the sun (in
degrees) for the adjusted arc position. A version of the backsta√ invented
by Captain John Davis in 1594 is shown in figure 3.10b. The two arcs were
fixed, with a common center. A moveable vane slid along each arc so that
the shadow and the horizon could be lined up.
The main advantage of backsta√s for observing solar height was that the
mariner did not have to look directly at the sun. One disadvantage was that
the shadow was cast by the upper limb of the sun, not the center, so an
error was introduced that had to be compensated for. Also, the backsta√
was no good for measuring the altitude of stars.
Early Optical Instruments and Increasing Accuracy
We now move into the eighteenth century and from there to the instru-
ments of yesterday, if not today. You will see how the older instruments, or
components from them, were reused or modified for the new measuring
tools, and how new technology and new ideas led to both improvements in
accuracy and reduction in size.
OCTANT
The octant was another instrument developed to make celestial and hori-
zon measurements so as to estimate latitude. Its basic elements are shown
in figure 3.11. The arc covered 45\, or an eighth of a circle (hence the
name), but because the instrument used mirrors, the angles were doubled
up, which gave rise to an alternative name: the reflecting quadrant . The mir-
rors of early octants were made of polished metal. The octant was devel-
oped in the early eighteenth century by at least four people, independently.
Most credit is given to John Hadley, an English mathematician, and Thomas
Godfrey, an American glazier, who both came up with the idea in 1731.
By this period, optical instruments such as the telescope and micro-
scope were becoming quite refined, with much of the gross distortions of
earlier optics removed through improved glassmaking and lens grinding.
So, surveying and navigating were ripe for the introduction of a sighting
scope, and these first appeared in octants, as shown in the figure. The index
mirror and index arm rotated together about an axis that was the center of
the graduated arc. The clever aspect of octant design was the second, or
horizon, mirror, mounted on the octant frame so that light from the object
being measured (a star, in our illustration) is reflected o√ both mirrors and