The sextant
As formidable a piece of ironmongery as one would wish to encounter.
In actual fact it is merely an instrument that measures the angle a heavenly
body (star, planet, sun, moon) makes with the visible horizon. It 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 its use requires some
skill and practice. Small errors make for large discrepancies in one's
position.
Parts of the sextant
The sextant basically consists of a telescope, a half silvered horizontal
mirror which the telescope "looks" through and a moving arm on which the
index mirror is fixed. By manipulating this arm a star or other celestial
body can be made to appear on the horizon. Accurate adjustments are made
by means of a micrometer knob. The angle can then be read off the arc and
micrometer. The shades are to use when the object being looked at is bright
 such as the sun.
The trick is to make the celestial body just brush the horizon  and herein
lies somewhat of a knack.
Principle of the sextant
The sextant relies on the optical principle that if a ray of light is
reflected from two mirrors in succession then the angle between the first
and last direction of the ray is twice the angle between the mirrors. And
this angle can then be read off the arc.
To use the sextant the telescope must be focused on the horizon.
The celestial body to be shot, found and the sextant aimed at it. Bring
the body down to the horizon by moving the arm along the arc and then clamp
the arm. Using the micrometer knob make small adjustments while gently
swaying the instrument slightly from side to side until the heavenly body
just brushes the horizon.
When this is achieved instantly make a note of the time, seconds first,
then minutes and hours, then the name of the body and its observed altitude.
Every second of time counts  an error of 4 seconds equates to an error of
a nautical mile in the position.
Errors and adjustments of the sextant
The sextant is subject to a number of errors and adjustments. To find
the true altitude of a celestial body from the observed these must be
allowed and adjusted for.
Briefly these are:
 Index Error
 Dip
 Refraction
 Parallax
 Semidiameter
Index error is an instrumental error. When looking through a sextant
at the horizon the exact level horizon will seldom be seen to be at 0°.
Sextant set at 0°  horizon split.
Before every sextant session the Index error should be determined.
Index error corrected for  horizon level.
If the error is less than 0° it should be added
to whatever reading is obtained  if more subtracted. Hint: remember Noah,
if off the Ark  add, if on the Ark  take off.
Dip is an adjustment made for the height of the eye above sea level.
In practice this is usually taken as 0.98 times the square root of the height
of the eye in metres above sea level multiplied by 3.28.
Refraction is extracted from the Nautical Almanac. It allows for the "bending"
of light rays as they travel through successive layers of varying density
air.
Parallax corrections are needed if the observed body is a planet, the
sun or the moon. From the Almanac.
Semidiameter correction is needed if the observed body is the sun
or the moon. In this case either the top or bottom of the celestial object
(known as upper or lower limb) is made to touch the horizon. To obtain
the centre of the body this correction is applied  from the Almanac.
Once all the corrections are applied we have the true altitude. And this
subtracted from 90 gives us the zenithal distance to the substellar point.
Which means we know exactly how far we are from that elusive point on the
earth which is at right angles to our observed celestial body!
The Position Circle.
When an observer measures the altitude of
a heavenly body he obtains from it, by correction and subtraction
from 90°, a true zenith distance  ZX in the figure. Z, therefore,
might be any point on a small circle of radius ZX and centre X.
On the Earth the observer's position, z, lies on the circumference
of a small circle, the centre of which is the heavenly body's
geographical position. The radius of this circle is also the true
zenith distance, zx, and since it is now measured on the surface of
the Earth, it can be expressed in nautical miles.
This small circle is known as a position circle.
The astronomical position line is the small arc of this position
circle on which the observer or navigator discovers his position to be.
If zx is very small, some twenty miles or so, the geographical
position can be plotted on the chart and the actual circle drawn
without loss of accuracy, but in general zx will be large,
of the order 1,000 miles, and the geographical position will seldom
be on the chart that the navigator is using for keeping his reckoning.
The part of the position circle that concerns the navigator must
therefore be found by methods that confine the plotting they involve to
the neighbourhood of the ship's actual position.
The method in common use is the Marcq St. Hilaire or 'intercept' method.
By kind permission of Mr.Eugene Griessel
Our
collection
of Fine Solid Brass hand crafted Sextants are probably the finest
reproductions
of the traditional Nautical sextants, which have been used in celestial
navigation
since 1757.Our Sextants are based an a design bought in by Captain
Cambell, but the original Octant from which the modern sextant came was
made by John Hadley about 1731.The sextants are workable but not meant
to be used for serious navigation.
They make ideal nautical gifts for those who love the sea, or are
collectors of historic navigational instruments.
CLICK ON ONE OF THE SEXTANTS BELOW TO TAKE YOU TO OUR SEXTANT PAGE.
