DAVIS Mark 3 Sextant - Manual (Page 3)

Page 1
HOW TO FIND YOUR POSITION WITH A SEXTANT
This booklet has been written as an introduction to your new Davis sextant. By
studying its pages, you will learn how to operate your sextant, how to find the alti-
tude of the sun, and how to use your readings to calculate location. The meridian
transit method of navigation described is both easily learned and simply
applied. When you finish reading, the mystery surrounding celestial navigation
and sextant use should disappear. Before becoming an accomplished navigator,
however, you will need to study those aspects of navigation which are beyond the
scope of this booklet.
HOW TO READ THE VERNIER
There are two scales on the sextant. The scale on the frame is called the “arc,”
while the scale on the index arm is the “vernier.” Each division of the arc equals
one degree. Each division of the vernier equals two minutes (2'). To read the num-
ber of degrees, find the lines on the arc which are closest to the zero mark on the
vernier. The zero mark is usually somewhere between two lines. The correct arc
reading is always that of the lower value, i.e., the line to the right of the zero mark.
To read fractions of a degree, find the division of the vernier which is in alignment
with a division of the arc.
To get a clear picture of how this works, set the zero on the vernier exactly
beneath any whole degree mark on the arc—let’s say 30°. Now move the index
arm very slightly to the left until the first vernier mark to the right of the zero lines
up exactly with a mark on the arc. Since the marks on the vernier are 2' apart,
you have actually moved the index arm 2' beyond 30°; your sextant reads 30° 02'.
Now, move the index arm slightly further to the left so that the next division of the
vernier comes into alignment with a division of the arc. Your sextant now reads
30° 04' (Fig. 1).
As you continue moving the index arm, successive divisions of the vernier will
come into alignment with a division of the arc. When the last mark on the vernier
(60') is in alignment with a division of the arc, the sextant will read 31°. In figure 2
below, the sextant reads 43° 26'.
Figure 2 Figure 1