Demo One THE SEXTANT - A quick overview
Using a Sextant to Determine Latitude
Sextant and the History of
How do navigators use the stars to find their way?
For at least two thousand years, navigators were
able to determine latitude (position north or south
of the equator). At the North Pole, which is 90
degrees latitude, the North Star (Polaris) is
directly overhead at 90 degrees. At the equator,
which is zero degrees latitude, Polaris is on the
horizon at zero degrees.   The angle of Polaris
above the horizon is a direct measurement of
terrestrial latitude between the equator and the
North Pole,
Sir Isaac Newton conceived the principle
of the doubly reflecting navigation but
never published it.  John Hadley ( 1682 -
1744 ), an English mathematician, and
Thomas Godfrey ( 1704 - 1749 ), an
American inventor independently
rediscovered the sextant and replaced the
astrolabe as the main instrument for
18th Century Sextant
navigation.   The sextant was superior to the
astrolabe because celestial objects were
measured relative to the horizon, rather than
relative to the instrument, allowing better
This modern Astra IIIB Deluxe Sextant is
available on the web for about $550.
The sextant  derives it's name from the arc at the
bottom which is one sixth of a circle. The
principles of a sextant are easy to master but
small errors make for huge discrepancies in

To use the sextant focus the telescope on the
celestial body to be shot.  Bring the body down to
the horizon by moving the arm along the arc and
then clamp the arm.  Use the micrometer knob
make small adjustments while gently swaying the
instrument slightly from side to side until the
heavenly body just brushes the horizon.

Instantly make a note of the time, seconds first,
then minutes and hours, then record the name of
the star or planet and its observed altitude.
Remember that every second counts - an error of
4 seconds equates to one nautical mile in
locating position.
Longitude Store Education Center
Sextant: a navigation instrument that is used to establish position by
measuring the height of stars from the horizon.
Index mirror: large polished plate that reflects light.
Telescope: optical instrument made of lens that magnifies objects.
Telescope clamp: reinforcing circle.
Eyepiece: lens the user looks through.
Telescope printing: lens adjustment.
Frame: structure that serves as the base for the different parts of the sextant.
Graduated arc: graduated edge of the arc.
Locking device: apparatus that holds the sextant in place.
Drum: graduated button used to take measurements.
Index arm: type of ruler that determines direction or measures an angle.
Screw to regulate small mirror: piece of metal used to adjust the horizon
Glass filter: colored transparent substance.
Horizon mirror: small polished glass plate that reflects light.
Glass filter: colored transparent substance.
Elements of the Sextant
USN Midshipman reading sextant
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