DAVIS Mark 15, 25 Sextant - Manual (Page 7)

Page 5
To calculate the index error:
1. Hold the sextant in your right hand and look at the sea horizon.
2. By moving the index arm and the micrometer drum, line up the real and
mirror horizons so that both appear as a single straight line.
3. Read the sextant scales.
If the sextant reads 0° 00', there is no index error. If the sextant reads anything
but zero, there is an index error, which must be added to or subtracted from
each subsequent sight.
For example:
If the sextant reads 6', the 6' is subtracted; if the sextant reads –6', the 6' is
added. For an index error of –6', the micrometer drum will read 54'.
3. To be certain that the sextant is now correctly adjusted, check to see
that the sextant is still set at 0° 00' and the real and reflected horizons
remain in a single line when the instrument is rocked or inclined from
side to side (Fig. 5).
On a correctly adjusted sextant, the real and mirror horizons remain in a sin-
gle line when the instrument is rocked from side to side.
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 readings for this amount each time the sextant is used (6' or so of
index error is allowable).
Figure 5