Page 190 - The Miracle of Migration in Animals

THE MIRACLE OF MIGRATION IN ANIMALS

Compass
Since the 12th century, ocean-
going explorers have used magnetic
compasses to determine their route. The
first compasses consisted of no more than
magnetized needles suspended on strings to
show the north. Later, these needles were
fixed on a pivot at the center of a dial.

Cross-staff
Above left: This instrument, popular with 16th-century Astrolabe
navigators, was used to measure latitude. One end of Above: Arabian astronomers made
the staff was held at the navigator's eye. The cross- a two-dimensional model of the
piece was then slid forward or back until its upper heavens. The instrument was sus-
edge lined up with the Sun or pole star and the lower pended from a cord so as to hang
edge with the horizon. The point where the cross-piece perpendicular to sea level, while
cut the scale on the staff was noted, to be converted the user could sight a star or the
into degrees according to a table. Sun through two small holes in the
plates on its moveable vane, and
read its altitude from the graduated
scale around the rim.
Sextant
Right: The English navy invented the
sextant in the mid-18th century. It
could measure latitude to a
degree of accuracy of 0.01 by
means of a mirror arrange-
ment. The navigator views
the Sun through the telescope
and reads its angular distance
above the horizon off the scale.
The data is then used to calcu-
late the ship's position.

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