DAVIS Mark 15, 25 Sextant - Manual (Page 3)

Page 1
USING YOUR DAVIS SEXTANT
This booklet gives the following information about your new Davis Sextant:
• Operating the sextant
• Finding the altitude of the sun using the sextant
• Using sextant readings to calculate location
• Other uses for the sextant
To get the most benefit from your sextant, we suggest you familiarize yourself with
the meridian transit method of navigation. A good basic reference book is
Practical Celestial Navigation by Susan P. Howell (Mystic Seaport Publications,
1987). Further discussion of this method of navigation is beyond the scope of this
booklet.
OPERATING THE SEXTANT
There are three steps to adjusting your sextant: index mirror adjustment, horizon
mirror adjustment, and index error adjustment and calculation. The index arm of
the sextant can move in relation to the body by turning the micrometer drum or by
squeezing the spring-loaded quick release levers. The levers free the fine adjust-
ment screw in the interior of the index arm and allow it to be moved quickly to any
angle. Be sure to squeeze the levers completely so that the screw clears the gear
rack on the underside of the sextant. Release the levers and turn the micrometer
drum at least one full turn to ensure that the screw has meshed fully with the gear
rack. An incorrect reading may be obtained at the drum if this is not done.
Note: Every sextant exhibits some difference in readings when turning toward
higher or lower angles (called backlash error). Always make the final movement of
the knob toward a higher angle.
Reading the Sextant Scales
The Davis Mark 15 and Mark 25 sextants have three scales that give readings to
2 /10 of a minute. The scale on the frame is called the “arc”; each division of the
arc equals one degree.
To read the number of degrees:
Find the lines on the arc that are closest to the index line on the index
arm.
The index line is usually somewhere between two lines. The correct reading is
usually that of the lower value, i.e., the line to the right of the index line.
Note: When the index line is very close to a line on the arc, check the reading
at the micrometer drum to be sure that you have taken the correct whole
degree.