The Southwestern Historical Quarterly, Volume 29, July 1925 - April, 1926 Page: 80

Southwestern IHisiorical Quarterly

The cross-staff consisted of two pieces of wood set at right angles
to each other, like a cross; yet free to slide, one upon the other.
With one hand the observer held the proper end of the horizontal
stick, or staff, to his eye; with the other hand he moved the vertical
cross piece back and forth until he sighted over its upper end the
sun, and under the lower end the sea's horizon. The angle at the
eye between these two lines of vision, an approximate measure of
the sun's altitude, was read on the graduations marked along the
horizontal bar.
The astrolabe in its elemental form was a disk of metal graduated
circumferentially to 360 degrees. At the center of the disk a
pointer was pivoted. The disk was suspended in a vertical plane
by a string attached to a point in its periphery. Therefore a line
across this disk at right angles to the direction of gravity was an
artificial but true horizon. This horizontal line was easily found
by joining two points on the circumference of the disk, lying 90
degrees each way from the place of suspension. The pointer there-
fore indicated on the graduations the angular distance, above or
below the horizon, of any object sighted. The astrolabe was intro-
duced into Europe about 700 A. D. by the Moors. Their as-
tronomers and engineers had used it for astronomical observation,
for ascertaining heights of mountains and even for establishing the
gradients of irrigation and water-power canals.
The astrolabe could be used only on land, because the motion of
the ship's deck disturbed the artificial horizon. The cross-staff
was difficult to use, because it was necessary for an observer to look
in two directions almost at once: at the sun, and also at the sea's
horizon. Each instrument was useful in its own field. Columbus
carried them both on his voyages, as all navigators did who fol-
lowed him for some two hundred and forty years.
The latitude of a position on the earth's surface was readily as-
certainable within practical limits; but degrees of longitude were
extremely difficult to determine, because the ship's chronometer
had not been invented. The Moorish astronomers had obtained
fair results in longitude measurements by observing eclipses of the
moon from different places. The difference between the local
times of this phenomenon was of course a measure of the difference
in longitude. They had also measured considerable arcs of longi-
tude by triangulation and chaining, so urgent was it for the fol-