DAVIS Mark 3 Sextant - Manual (Page 5)

Page 3
Finally, remove the index error. Set the sextant at zero and look at the horizon.
With the sextant still held to your eye, turn the screw that is furthest from the
frame at the back of the index mirror until the two horizons move together and
form one straight line. The index mirror is now parallel to the horizon mirror (Fig. 5).
While you should know how to adjust your sextant for index error, it is not neces-
sary to remove it entirely. It is standard practice to simply note the error and then
correct one’s reading for this amount each time the sextant is used (as much as 6'
index error is allowable). To check for index error, hold the sextant in your right
hand and look at the sea horizon. By moving the index arm, line up the real and
mirror horizons so that both appear as a single straight line. Now, look at the
scale. If it reads zero, there is no index error. If the scale reads anything but zero,
there is an index error which must be added to or subtracted from each reading.
For example, if the scale reads + 6' when the horizons are aligned, the 6’ is sub-
tracted. If the reading is below the zero mark, for example –6', the 6' is added
(Note: for an index error of –6', the scale actually reads 54').
MEASURING THE SUN’S ALTITUDE
When looking at the sun through the sextant, be sure to use a sufficient num-
ber of shades to protect your eyes from the direct rays of the sun. Choose
the combination of index and horizon shades that gives you a clear image of the
sun without glare.
On a correctly adjusted sextant, the real and mirror horizons remain in a single
line when the instrument is rocked from side to side (Fig. 6).
Index mirror not parallel to
horizon mirror.
Index mirror parallel to
horizon mirror.
Figure 5
Figure 6