DAVIS Mark 3 Sextant - Manual (Page 11)

Page 9
FINDING LATITUDE
The altitude of the sun at local noon may also be used to calculate latitude. First,
the measured altitude must be corrected for index error, height of eye, refraction,
and semi-diameter. Refraction correction is negligible for altitudes above 25°,
while the 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 the sun’s diameter). After the cor-
rections are made, determine the declination of the sun from the Nautical
Almanac or from the approximate declination values at the end of this booklet.
Finally, 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.
See figure 14 for a diagram based on this example.