DAVIS Mark 15, 25 Sextant - Manual (Page 4)

Page 2
To read fractions of a degree:
Use the two scales involving the micrometer drum at the side of the
index arm.
The outer revolving drum scale indicates minutes of arc (one minute equals
1/60 of a degree), while the stationary vernier reads to 2/10 of a minute.
To read the number of minutes:
Find the single LONG line at the top of the vernier.
The line on the drum scale that is opposite this line gives the number of min-
utes. If the line on the vernier is between two lines on the drum, choose the
line of lower value.
To read fractions of a minute:
1. Find the SHORT line of the vernier that is opposite to a line on the
drum.
2. Count the number of spaces this line is away from the long line at the
top of the vernier. Each one equals 2/10 of a minute.
Note: The micrometer drum scale and its screw mechanism, not the arc, deter-
mine the accuracy of your sextant. The arc is stamped with sufficient accuracy to
ensure that you are never reading the incorrect whole degree; full accuracy in min-
utes of arc depends exclusively on the drum scale. For example, when the sextant
reads 0° 00', the drum scale will be set precisely at zero, while the index line and
the zero on the arc may be slightly out of alignment. As you are concerned only
with reading whole degrees on the arc, this difference is not significant.
Figure 1
In this diagram (Fig. 1), the line on the vernier
that is opposite to a line on the drum is two
spaces away from the long line at the top of the
vernier. The sextant reads 45°16.40'.