Question 1

The three adjustable sextant errors are corrected in the order…

Accepted Answer

Perpendicularity, side error, index error — Each adjustment disturbs the ones after it: square the index mirror to the frame first, then parallel the horizon glass, and deal with index error LAST.

Question 2

Sweeping the drum through zero on a star, the reflected image passes BESIDE the direct image. That is…

Accepted Answer

Side error — horizon glass not parallel to the index mirror — With side error the two images cannot be made to coincide, only to pass abeam of each other. Index error shows as a vertical offset AT zero.

Question 3

A small leftover index error of 1.5′ is best…

Accepted Answer

RECORDED, and applied to every sight — chasing tiny IE wears the screws and never ends — Find it on the horizon, write it down, correct arithmetically: on the arc take it off, off the arc put it on. Adjust only when it grows large.

The three errors you can fix

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