DAVIS Mark 15, 25 Sextant - Manual (Page 10)

Page 8
FINDING THE ALTITUDE OF THE SUN
USING THE SEXTANT
Before looking at the sun through your sextant, be sure to position a sufficient
number of index shades (the large set of four shades) between the two mirrors to
protect your eyes from the direct rays of the sun. Choose whatever combination of
shades gives you a clear image of the sun without glare.
6. Read the sun’s altitude from the scales on the sextant, being careful not
to disturb the setting.
Since all calculations in the Navigation Tables use the center of the sun or
moon, this lower limb reading must be adjusted for semi-diameter correction,
as shown later.
Figure 10
The sun’s image travels in an arc that just touches the horizon.
Note: For comparison purposes, the sun’s image and horizon are also illustrated
as viewed using a Beam Converger, instead of a half-silvered horizon mirror.
Viewed through a
Beam Converger
Viewed through a
split horizon mirror
To Measure the Sun’s Altitude:
1. Use index shades to protect your eyes, as discussed above.
2. Use the horizon shades to darken the clear section of the horizon mirror
so that it acts as a semi-mirror.
The horizon will still be visible through it, but the sun’s image will be reflected.
3. Stand facing the sun with the sextant in your right hand.
4. With your left hand on the quick release levers of the index arm, look
through the eyepiece at the horizon and move the index arm until the
sun is visible through the two mirrors and index shades.
5. Release the levers and, while slowly rocking the entire sextant from side
to side, use the fine adjustment drum to bring the sun’s image down to
just touch the horizon with its lower edge (lower limb).
The sun’s image should travel a short arc that is made to touch the horizon
(Fig. 10).