DAVIS Mark 3 Sextant - Manual (Page 14)

Page 12
THE SEXTANT AS A PELORUS
Your sextant may also be used to find your position by sighting known land
objects such as lighthouses, small harbors, or any other land features that are
clearly recognizable on the chart. Pick out three features on the land. With the
sextant held horizontally, measure the angle between the center feature and one
of the other features, and note the angle on a piece of paper. As quickly as you
can, measure the angle between the center feature and the third feature. Lay out
the three angles on a piece of tracing paper so that the angles have a common
center point. Move the tracing paper around on the chart until the lines are posi-
tioned so as to run through the three features. The point of intersection of the
three angles is your position (Fig. 17).
Note: Since the sextant reading made with an artificial horizon must be halved,
the maximum altitude that you can observe with the artificial horizon is equal to
one-half the maximum arc graduation on your sextant. There may be several
hours around noon during which the sun is too high to take a sextant reading with
the artificial horizon, so plan sights for the morning or evening hours.
Figure 16
4. Line the two suns up by continuing to move the index arm. For a lower limb
observation, bring the bottom of the mirror image into coincidence with the top
of the image on the liquid.
5. After the observation has been made, apply the index correction.
6. Halve the remaining angle and apply all other corrections (except for Dip or
height of eye correction, which is not applicable) to find the altitude of the sun.