DAVIS Mark 15, 25 Sextant - Manual (Page 17)

Page 15
Finding Latitude
The altitude of the sun at local noon may also be used to calculate latitude.
To calculate latitude:
1. Correct the measured altitude for index error, height of eye, refraction,
and semi-diameter.
Refraction correction is negligible for altitudes above 25°.
Semi-diameter correction averages + 0" 16' (semi-diameter correction adjusts
the sextant reading from an observation of the lower limb of the sun to one of
the center of the sun; 16' equals one-half of the sun’s diameter).
2. After the corrections are made, determine the declination of the sun
from the Nautical Almanac or from the approximate declination values at the
end of this booklet.
3. Calculate latitude by combining the altitude of the sun at local noon with
the declination of the sun from the navigation tables. Assuming you are north
of the sun, the following formula is used in northern latitudes:
Latitude = 90° – Corrected Altitude ± Declination of the Sun
When the sun is north of the equator, ADD the declination; when it is south of
the equator, SUBTRACT the declination.