Sextants - Latitude & Longitude

Demo One THE SEXTANT - A quick overview

Using a Sextant to Determine Latitude

Sextant and the History of
Latitude

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 precision.

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 position.

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
mirror.
Glass filter: colored transparent substance.
Horizon mirror: small polished glass plate that reflects light.
Glass filter: colored transparent substance.

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