62 seamanship secrets
A sextant measures the angle between two objects. For instance, you could
measure the vertical angle from the horizon to the sun, and with a few calcula-
tions, you could convert this angle to a bearing line (a line of position or LOP).
When you cross that LOP with another obtained by some other means, you’ll
have fi xed your position at the intersection of the two. Or, you could measure
the angle from the top of a tall lighthouse down to its base at the water’s edge,
enter a table with that angle, and (knowing the height of the lighthouse, which
will be noted on the chart or in the Light List) fi nd your distance off the light-
house. You can also measure the horizontal angle between two charted objects,
such as a tank and church spire. Th e angle between them will give you a line
of position.
Note: any line of position based on an angular measurement—including the three
examples just noted—is in fact a circle of position or COP. A COP is a specialized
instance of an LOP, but the distinction is illusory, as Chapter 4 will make clear.
Every sextant comes with eight standard parts (see sidebar).
HOW TO READ A SEXTANT
Before beginning adjustments, let’s do some practice setting and reading the index
arm. Squeeze the levers and slide the arm up and down the arc. Notice an engraved
mark on the index arm. Move the index arm until it’s close to the 0 on the arc, and
release the levers. Just get it close to the zero, not directly on it. Turn the micrometer
drum and line up two things: the index arm at zero degrees and the micrometer
drum at zero minutes. Next, fi nd out if there is any error in the sextant.
On the arc. Turn the micrometer drum away from you. You’ll notice the
minutes increase. Th is is called on the arc.
Off the arc. Turn the micrometer drum toward you. You’ll notice the minutes
decrease. Th is is called off the arc.
Now let’s check the sextant to make sure it’s properly adjusted and ready
to use.
SEA-CRET TIP
X Some sextants have a vernier scale next to the micrometer drum.
It reads in tenths of a minute or in seconds. Look for an unbroken
line formed by the vernier and micrometer scales. Add the vernier
scale reading to the micrometer minutes. Many navigators choose
to ignore the vernier reading and estimate tenths by eye. This will
not affect your accuracy.