There are three main errors of marine sextant, which can quite easily be corrected by the mariner. A fourth error, for “collimation”, can also be corrected, with care and attention, but only to an older sextant where telescope collars are fitted with adjusting screws.

1. The first error, of Perpendicularity, is caused by the index mirror not being perpendicular to the plane of the instrument. To check if this error is present, clamp the index arm between a third and half way along the arc, remove the telescope, and look obliquely into the index mirror, observing the true and reflected arcs of the sextant. Hold the sextant horizontal, arc away from the body. If the true and reflected arcs are not in line with each other, then an error of perpendicularity must be considered to exist (Figure 1).

To correct the error, adjust the screw at the rear of the index mirror until the true and reflected arcs are brought together in line.

2. The second error, side error, is caused by the horizon mirror not being perpendicular to the plane of the instrument. There are two ways of checking if this error is present. The first is by observing a star. Hold the sextant in the vertical position with the index arm set at zero, and observe a second magnitude star through the telescope. If the true and reflected stars are side by side, then side error must be considered to exist (Figure 3). It is often the case when checking the instrument for side error that the true and reflected stars are coincident. If this is the case, a small amount of side error may exist, but a minor adjustment of the micrometer should cause the true star to appear below the reflected image. Should, however, the reflected image move to one side rather than move in a vertical motion, side error may be considered to exist.

The second way is by observing the horizon. Set the index arm at zero and hold the sextant just off the horizontal position. Look through the telescope at the true and reflected horizons. If they are misaligned, as indicated in Figure 4, then side error must be considered to exist.

To correct for side error, adjust the centre screw furthest from the plane of the instrument at the back of the horizon mirror, to bring either the star and its image into coincidence or the true and reflected horizons into line.

3. The third error, index error, is caused by the index mirror and the horizon mirror not being out of parallel to each other when the index arm is set at zero. To check whether index error is present by observing a star, look through the telescope when the sextant is set at zero, and if the reflected image of the star is above or below the true image, then index error must be considered to exist. Should the true and reflected images be coincident, then no error will exist. To check by observing the horizon, set the index arm at zero, hold the sextant in the vertical position, and observe the line of the true and reflected horizons; if they are seen as one continuous line, then no error exists, but if the line between the true and reflected horizons is broken, an adjustment needs to be made to remove the error.This adjustment is made by turning the screw nearest to the plane of the instrument. Index error may also be checked by observing the sun. Fit the shaded eye piece to the telescope. Clamp the index arm at about 32′ off the arc and observe the true and reflected images to the position of limb upon limb. Repeat the observation with index arm set at about 32′ on the arc, and note the two readings of both observations. The numerical value of the index error is the difference between the two readings divided by two, and would be called”on the arc” if the “on the arc” reading were the greater of the two, and “off the arc” if the “off the arc” reading were the greater.

Let us consider an example:

Adjust the micrometer to bring the true sun into contact with the reflected sun

Note the reading, for example

Repeat the observation, but with images the other way about.

Note the reading, for example

Take the difference of the two readings and divide by 2.

Index error is (36-27)/2 = 4.5′ off the arc.

This error must be subtracted from the future sextant readings.

The accuracy of the observations may be checked by adding the numerical values of both readings together and dividing the number by four.The resulting value should equal the semi-diameter of the sun for the period at which the observation was taken.

combined. Should this undesirable condition be apparent, the mariner can resolve the problem by removing each error a little at a time, as shown in Figure 5. The correction is made by turning the second and then the third adjustment screws alternately, by a small amount each time, until concidence of image is achieved.

Collimation error

This is an error caused by the axis of the telescope not being parallel to the plane of the instrument.To check whether the error is present, insert the inverting telescope, setting the eyepiece so that one pair of the cross wires are parallel to the plane of the sextant.

To check by observation of two stars (selected about 90° apart), move the index arm to bring the two stars into exact contact with each other resting on the wire nearest to the plane of the sextant. Now tilt the sextant upwards so as to bring them on to the wire which is furthest from the plane of the instrument. Should the images diverge or converge from the top intersections of the wires, it must be assumed that an error of collimation exists, and that the axis of the telescope is not parallel to the plane of the instrument.

This error can be corrected by adjustment of the two screws in the collar or telescope mounting.The screws are moved together, one being tightened, the other slackened, to align the stars on the top intersection which will bring the telescope back to parallel with the sextant frame. (Not all sextants, however, have adjustable collar screws.)

Non-adjustable errors

1.Centering error
This error could be caused by wearing of the pivot on which the index arm moves, perhaps because the index arm is not pivoted at the exact point of the centre of curvature of the arc.
2.Prismatic error
This error is caused by the two faces of the mirror not being parallel to each other.
3.Shade error
This is an error caused by the faces of shades not being parallel to each other. If it is known to exist, the telescope is used in
conjunction with the dark eyepiece.
4.Graduation error
This error may be encountered on the arc itself or on the vernier or micrometer scales. If the micrometer drum is known to be correct, then the first and last graduations on the drum should always be aligned with graduation marks on the arc.

The manufacturer tables all the non-adjustable errors and issues the sextant with a certificate usually secured inside the lid of the case. The combination of the above four errors is known as “Instrument Error”.