DAVIS Mark 15, 25 Sextant - Manual (Page 19)

Page 17
Calculating Position Using Celestial Navigation
The method already described for calculating your position is the oldest
method used since the introduction of the chronometer. Please note;
■ Latitude can be determined at noon if you know the corrected altitude
of the sun and its declination. You don’t need to know the time. The
accuracy of your calculation is limited only by the accuracy of measure-
ment of the sun’s altitude and by the accuracy of the declination tables.
■ Longitude can be determined if you know both the time of observa-
tion and the equation of time. While your sextant gives highly accurate
measurements, practical difficulties inherent in this method normally pre-
clude accuracy of more that 10' of longitude.
A generalized system of position determination which enables you to use
observation of the sun and other celestial bodies made at time other than
noon requires knowledge of the navigation triangle, circles of equal altitude,
assumed position, and associated navigation tables such as the Nautical
Almanac and Sight Reduction Tables. These systems of celestial navigation
are thoroughly studied and extensively used by serious navigators throughout
the world.
Note: Work forms and sight reduction tables are available from publishers,
typically with step-by-step instructions. Nearly all navigators use work forms
such as these to prevent errors and omissions in the calculation of celestial
navigation problems.
Working with an Artificial Horizon
At times, it is not possible to see the natural horizon. Sun or moon shots may
still be taken, with the aid of an artificial horizon—a simple device containing
water or oil shielded from the wind (Fig. 18). It may be used by individuals
exploring inland far from the sea, or by students or experienced navigators to
practice celestial navigation without traveling to large bodies of water.
To use the artificial horizon:
1. Position the artificial horizon on level ground or other steady place.
One end of the artificial horizon should face directly into the sun so that a
shadow is cast at the opposite end. The sides and end facing the sun
should be shadow-free.
2. Looking into the center of the liquid, move your head about so that
you can see the sun reflected on the liquid surface.
3. Bring the sextant to your eye and move the index arm of the sextant
until you see two suns—on reflected on the liquid and a double-
reflected image on the mirrors.