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Celestial Navigation
At
least two millenia before the GPS, navigators used
celestial navigation to voyage thousands of miles across
unmarked waters To navigate by the stars, the sailor
sighted an angular measurement between the horizon and
the sun or other celestial object. For this, the
navigator used coordinates from a nautical almanac and a
chronometer to measure time based on the earth’s
rotation.
In
ancient times, navigators knew how to determine their
latitude, or position north or south of the equator, by
the position of Polaris (the North Star). When setting
sail, the navigator would simply measure the altitude of
Polaris as he left port, and when returning after a long
voyage, needed only to sail north or south to bring
Polaris to the altitude of home port, then turn and
“sail down the latitude,” keeping Polaris at a constant
angle.
The
early Arabs used a finger’s width held at arms length to
sight the horizon and Polaris. Later, they used a kamal,
a knotted cord attached to a wooden transom, to make the
observation. The navigator would hold the cord in his
teeth and sight Polaris along the top of the transom and
the horizon along the bottom. As the years passed,
advancements in astronomy and astrology led to the
development of two important astronomical instruments in
the 10th century; the quadrant and the astrolabe.
The
mariner’s quadrant – a quarter of a circle made of wood
or brass – came into widespread use for navigation
around 1450. Like the Arab kamal, the quadrant measured,
in degrees, the altitude of Polaris or the sun to
determine the geographic position. The motion of the
ship made it difficult to keep the quadrant aligned with
the stars, and it was impossible to keep the wind from
blowing the plumb bob off-line. The astrolabe was simply
a circular scale marked in degrees, and a rotating
alidade with sighting pinnules used to sight the star
and read its altitude. The next step in the evolution of
celestial navigation instruments was the cross-staff.
This was replaced in 1711 by the Davis back-staff which
used the shadow of the sun and a sight of the horizon to
determine altitude.
In the
late 1600s and early 1700s, instrument makers started
shifting their focus to optical systems based on mirrors
and prisms. This led to the idea to use two mirrors to
make a doubly reflecting instrument. The sextant was
invented independently, and almost simultaneously, by
John Hadley of England and Thomas Godfrey of
Philadelphia around 1730.
The
sextant (pictured) is an optical system consisting of a
telescope and two mirrors, one fixed and one moveable. A
mounted telescope points toward a fixed mirror, half
transparent and half silvered. An observer looking
through the telescope sees the horizon through the clear
part of the mirror while seeing the reflection of the
star or sun on the mirror’s silvered portion, as
reflected from a movable mirror on the level. By moving
the level, the image on the mirror is brought into
coincidence with the image of the horizon. The angular
distance between the two can then be read on a scale
engraved on the sextant. This scale is an arc of one
sixth of a circle (60°); hence the sextant’s name from
the Latin word “sextans” for one sixth.
Today,
sextants are used only for back-up navigation. They make
ideal nautical gifts for those who love the sea, or are
collectors of historic navigational instruments.
Written
by Sandy Johnson
Printed
January 2008
Web January 2008 |
