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How to Adjust a Sextant for Seaman

Seaman Must Know How to Adjust a Sextant 

Long before GPS, the Coast Guard used sextants to position buoys and lights
to a tolerance of +/−10 yards. With a bit of practice, it’s easy to use a sextant
for finding your position or your distance off an object in piloting waters. 

Out at sea, celestial navigation remains the primary backup to GPS. The sextant
requires periodic maintenance, like any fine instrument.

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 calculations,
you could convert this angle to a bearing line (a line of position or LOP).

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).


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. 

Now let’s check the sextant to make sure it’s properly adjusted and ready
to use.

The Parts of a Sextant

A sextant has eight standard parts. 

How to Adjust a Sextant for Seaman

The weblike structure that holds all other parts. The frame approximates 1/6th of a circle, thus the name sextant.

The curved piece of metal or plastic running along the bottom of the frame. Lines engraved on the
arc show whole degrees.

Index Arm
The arm attached to the top of the frame. Squeezing two clamps at the bottom of the arm enables it to
move up and down the arc.

Micrometer Drum
The wheel near the index-arm clamps, with marks showing minutes. This is used to fine-tune an observation.

Index Mirror
A small mirror attached to the top of the index arm that captures the reflected image of an observed

Horizon Glass 
Attached to the front of the frame, one half is clear and one half is a mirror.

Shade Filters 
Both the index mirror and horizon glass have shade glasses to protect the eye from water surface glare or direct sunlight.

The scope power should not exceed 5X. More powerful scopes have too small a field of view, making the observed image difficult to view.

How to Adjust a Sextant for Seaman

Hold the micrometer drum and spin it one way or the other to perfectly align three things at once the index-arm mark, the zero-degree mark on the arc, and the zero-minutes mark on the micrometer drum.

How to Adjust a Sextant for Seaman

Turn the micrometer drum away from you about a quarter turn. You’ll notice the minutes increase. 

When adjusting or checking for error, this is called on the arc. Subtract the minutes to correct an observation.

Three Steps to Adjust Your Sextant

If the error is more than 2 minutes on or off the arc, you’ll need to go through the adjustment process shown below. Read other article Learning Cargo Ship Handling

Follow each step consecutively to reduce or eliminate the index error. First, check the type of screws on the back of the index mirror and horizon glass. 

Read the sextant manufacturer’s recommendation for adjustment tools.

1. Adjust the index mirror perpendicular to the frame. Remove the telescope. Set the micrometer drum to zero. Set the index arm to 35 degrees. Turn the sextant around so that you’re looking at the index mirror. 

Hold the sextant up to your eye and look directly into the mirror.

You may need a jeweler’s screwdriver or Allen wrench insert the tool into the screw and hold it. 

Bring the sextant back up to your eye and once again rotate it to get the actual and refl ected image of the arc in the mirror. Turn the screw until you have a straight line. Th en go to Step 2.

2. Remove side error. Insert the telescope. Set the micrometer drum and index arm to zero. Find a distant vertical object such as a fl agpole or water tank.

Sight the object through the horizon glass. Now, keeping the sextant vertical, move it slightly to the left and right. You’ll see two images, the object itself and its refl ected image. 

As you move the sextant from side to side, they should stay lined up on top of one another. If not, you need to adjust the horizon glass.

The horizon glass has two screws. Use the horizon glass screw farthest from the frame. 

Insert your tool in the screw and then sight the vertical object again. 

Adjust the screw incrementally up or down to vertically align the actual and reflected object. Check the adjustment again when you are done. Then go to Step 3.

3. Remove index error. Set the micrometer drum and index arm to zero. Look through the scope at the horizon or a distant horizontal surface. 

If you see a broken line, you must make this adjustment. Set your tool into the horizon glass screw closest to the sextant frame. 

Now look at the horizon again and slowly turn the setscrew up or down until you see a straight line. 

Go back and check all three adjustments again, and if necessary, repeat steps 1 through 3. Remember, you must go through all three adjustment steps in the order shown.

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