DAVIS Mark 3 Sextant - Manual (Page 6)

Page 4
To measure the sun’s altitude, stand facing the sun with the sextant in your right
hand. With your left hand on the index arm, look through the eye piece at the hori-
zon and move the index arm until the sun is visible through the two mirrors and
index shades. Rock the entire sextant from side to side so that the sun’s image
travels in a half-arc. Now, adjust the index arm to bring the sun’s image down to
just touch the horizon (Fig. 7).
Being careful not to disturb the setting, read the sun’s altitude from the scales on
the sextant. 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 correc-
tion, shown later.
HEIGHT OF EYE
When measuring the altitude of the sun, we want to measure the angle formed by
a ray from the sun and a plane tangent to the earth at the point where the observ-
er is standing. Due to the height of the eye of the observer, however, the visible
horizon actually falls below this theoretical plane (Fig. 8).
Figure 7
Figure 8
The sun’s image travels
in a short arc which just
touches the horizon.
Due to the height of the eye of the observer, the
visible horizon (H) falls below the plane (P) tan-
gent to the earth at the point where the observer
is standing.
To correct for the height of
the eye, one must apply a
“dip correction.” Dip correc-
tion increases as the eye is
raised further above the sur-
face of the water (Table 1)
and must always be subtract-
ed from the sextant reading.
Table 1
Height of Eye Correction
Feet Meters Dip
5 1.5 2'
10 3.0 3'
15 4.5 4'
25 7.5 5'
40 12.0 6'