Getting the most out of a plastic sextant

Although plastic sextants are cheaper, less susceptible to corrosion and more durable than precision metal sextants, their precision of approximately five to 10 minutes of arc doesn’t meet sight folder requirements. However, with some simple precautions, you can increase their precision to one to three minutes of arc.

Inaccuracies in plastic sextants arise from index error drift with temperature and backlash in the tangent screw/micrometer drum mechanism.

Index error drift

When a plastic sextant is taken out of an air-conditioned car in the summer or a warm car in the winter, it might undergo a temperature change of 50 degrees Fahrenheit or more. Plastic is dimensionally unstable with temperature, and as a result, the index error will drift until the sextant temperature stabilizes.

The drift can be seen by recording index error measurements every five minutes; it could take up to 30 minutes for the sextant temperature to stabilize. Before taking any sights, the sextant should be allowed to reach ambient temperature.

As a further precaution, four or more index error measurements should be taken both before and after each run of sights, and the average values should be calculated. The spread of the measurements and the difference between the averages will indicate the degree of drift and give an indication of the sights’ precision. Provided the difference between the averages does not exceed three minutes of arc, the mean of the before and after averages should be used for the sight reduction.

Tangent screw backlash

Any axial slop in the tangent screw mechanism will give rise to backlash in the micrometer drum rotation so that a measurement will depend on the drum’s rotation direction. To eliminate backlash error, you should approach the alignment of the body with the horizon or the double horizon index error measurement from the same direction. The direction doesn’t matter as long as it’s the same for all measurements.

By taking these precautions, I can easily and consistently meet the five-nautical-mile distance requirement between the line of position and the known position without repeating sights until it “just happens.” It takes extra time, but it’s worth knowing that you haven’t cheated.