Global Positioning System Reference
In-Depth Information
FIGURE 3.7.
A cross-staff. The user lines up the sun along the upper edge of the tran-
som and the horizon along the lower edge. The sun's height is read off a scale on the
main staff. The cross-staff can also be used, perhaps more comfortably, to measure
the angular distance between two stars.
two, but was di≈cult to use.
6
Not least of the problems was the necessity of
looking at the sun directly. Also, a mariner would have to hold the sta√
steady so that it was aligned with the horizon while he switched his gaze to
the sun. Quite apart from this di≈culty, the eye's movement introduced an
error, especially for large angles. (In practice, this limited the angle being
measured to less than 50\.) One advantage of the cross-sta√ compared with
some later instruments was that it was independent of gravity—it did not
require a plumb bob—and therefore swayed less on a pitching deck. In
other words, the angle of the sun was measured relative to the horizon (not
to the vertical), and both sun and horizon pitched in the same way.
QUADRANT
From the fifteenth century, the quadrant consisted of an alidade attached
to a quarter circle (hence the name) graduated in degrees, and a plumb
bob. The instrument was used to estimate the sun's noonday altitude, both
for navigation and early mapmaking. You can see how it works from fig-
ure 3.8. Clearly, like the cross-sta√, it would be di≈cult to use for estimat-
ing latitude because the user would be obliged to look at the sun. The
plumb line was an improvement on land because it provided an absolute
6. Errors in measuring the height of the sun with a cross-sta√ included instrument
parallax, the angular width of the sun, and horizon dip (due to atmospheric refraction).
These errors were addressed by Edward Wright, who published
Certain Errors in Navigation
in 1599.
