DAVIS Mark 15, 25 Sextant - Manual (Page 14)

Page 12
To locate your position:
Record the local time and the sextant reading when the sun was at the
highest point.
These two readings will serve to locate your position. The time is used to
determine longitude and the sextant reading to determine latitude.
Calculating the Declination of the Sun
Every star and planet, including the sun, has a ground position, i.e., the spot
on the earth directly beneath it. Standing at the sun’s G.P. (ground position),
you would have to look straight up to see the sun; if you were to measure its
altitude with a sextant, you would find the altitude was 90°.
From the earth, the sun seems to move across the sky in an arc from east to
west. During certain times of the year, it is “moving” around the earth directly
above the equator. In other words, the sun’s G.P. is running along the equator.
Declination of the sun at this time is zero. However, the sun’s G.P. does not
stay at the equator throughout the year. It moves north to a maximum of 23.5°
N in the summer of the Northern Hemisphere and south to a maximum of
23.5° S in the winter. The distance of the sun’s G.P. from the equator,
expressed in degrees north or south, in known as the declination of the sun
(Fig. 14).
In like manner, each star has a ground position and a declination. The declina-
tion of Polaris is 89° 05'N; it is nearly directly above the North Pole. In the
Northern Hemisphere, you can find your approximate position by taking a
sight on Polaris. The reading will vary depending upon the time of night, but
will never be more than 55 miles off. This is a useful check each evening; the
altitude of Polaris will be your approximate latitude without adding or subtract-
ing anything. If you were to find the altitude of Polaris in the evening and again
at dawn, your true latitude would be between the two measurements, provid-
ing you did not change latitude between the two sights. It is, of course, possi-
ble to calculate one’s exact latitude from Polaris with the aid of the Nautical
Almanac, but such a discussion is beyond the scope of this booklet.
Figure 14
Declination of the Sun