Before using the sextant you must note the index error. All sextants have an index error, especially plastic instruments which have to be checked regularly. When you have achieved an unbroken line, note the index error. Remember that index error is what you have to add or subtract in order to get 0.0’. The index error is the same for any angle. It can change – check the index error before any observation.

Alidade angle

Find a secure place to stand. Hold the sextant in your right hand and move the alidade until you see the sun in the right half of the viewer. Move the alidade until you see the horizon as well.

Fine-tune the alidade with the micrometer screw until the sun ‘touches’ the horizon. It can be difficult to find the sun to begin with, but with practice, preferably in varying conditions, you will soon get the hang of it.

Important note: Be sure to use correct filters in front of the mirrors. Unfiltered sunlight may damage your eyes.

Sextant angle

If you hold the sextant at a slight angle you can easily get an exaggerated value. Allow the sextant to tilt from side to side. The sun will now trace a curve which should ‘touch’ the horizon. If it dips below the sea or doesn’t quite reach down, you must adjust the micrometer screw.

Note the exact time you have the correct adjustment. You can now note the altitude at your own leisure. This is called observing the sun’s lower limb. Important note: In a rough sea you must take your shot while on top of a wave.

Noting the time

The time must be noted at the exact moment you get the sun to touch the horizon. The ideal situation is for someone else to note the time as you call it out. Take several shots and use the one you are most comfortable with.

Altitude vs azimuth

The altitude of a celestial body is its angle above the horizon, in degrees. Even if we focus primarily on navigation using the sun, the majority of the sextant principles can be used with other celestial bodies.

The azimuth angle in the northern hemisphere is the angle along the horizon from true north to a vertical line through the chosen celestial body.

In the southern hemisphere it is the angle along the horizon from true south to a vertical line through the chosen celestial body.

Noon sight

The sun is at its highest at midday. If you measure the altitude at the precise moment the sun is at its highest, you can find your latitude the easy way.

After finding the declination from the almanac you can work out the latitude. This is the simplest form of celestial navigation.

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