DAVIS Mark 3 Sextant - Manual (Page 8)

Page 6
DECLINATION
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 ground position (G.P.), 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 or, 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 1 / 2 ° N in the
summer of the northern hemisphere, and south to a maximum of 23 1 / 2 ° S in the
winter. The distance of the sun’s G.P. from the equator, expressed in degrees
north or south, is known as the declination of the sun (Fig. 11).
In like manner, each star has a ground position and a declination. The decli-
nation 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 subtracting anything. If you
were to find the altitude of Polaris in the evening and again at dawn, your true lati-
tude would be between the two measurements,
providing you did not change latitude between the
two sights. It is, of course, possible 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.
To find POlaris, locate the pointers of the Big
Dipper (Fig. 12). Find a point in line with the point-
ers and five times the distance between them.
There, shining alone, is Polaris. The Big Dipper
revolves around Polaris, so be prepared to see the
diagram in any position.
Figure 12
Figure 11