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Using the Sextant
General
The sextant is the instrument of chief importance in celestial navigation.
It is used to measure the altitude of a heavenly body above the visible
horizon. Sextant altitude is corrected for various factors to determine
the body’s true (or corrected) altitude above the celestial horizon.
Techniques
Here are some techniques commonly used to take sights with the marine
sextant. It will always be necessary to find any index error prior to
taking sights; refer to chapter 8 to find index error.
Use the following step action table for the general steps to take sights on
the Sun. The steps for stars and planets are basically the same, except
you would omit steps 2 and 4.
Step
Action
1.
Hold the sextant level with the horizon and determine index
error.
2.
CAUTION: Set shade filters in place now, or eye burns
may result.
3.
Aim the sextant to a point on the horizion directly below the
Sun.
4.
IF...
THEN...
the Sun is rising
Move the index arm slowly outward
from the 0° position until the Sun’s
lower limb is just below the horizon.
the Sun is setting
Move the index arm slowly outward
from the 0° position until the Sun’s
lower limb is just above the horizon.
5.
6.
Swing the arc.
This means to gently move your hand grasping
the sextant handle in a small upward arcing motion. Up to the
left, then back to the right. You will see the reflected image of
the Sun arc back and forth.
Give the recorder a standby to mark (marking the exact time of
the sight). Continue swinging the arc while turning the
micrometer drum slightly until the lower limb of the Sun
touches the horizon. At that exact moment, mark the time of
the sight and record the sextant altitude.
9-18
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