Question 1

Index error “2′ on the arc” means you…

Accepted Answer

Subtract 2′ — “on the arc, take it off” — The instrument reads high by 2′ at zero, so every Hs gives back 2′. Off the arc, add it on.

Question 2

Dip is applied because…

Accepted Answer

Your eye is above the sea, depressing the visible horizon below the true horizontal — Dip = 1.76′ × √(height of eye in m), always subtracted. Refraction and semi-diameter are the other rungs of the ladder.

Question 3

Low-altitude sights are avoided because…

Accepted Answer

Refraction is huge and unstable near the horizon (~34′ at 0°) — The correction collapses from ~34′ at the horizon to ~1′ at 45° — and the low-air value wobbles with conditions. Shoot high.

The sextant and its honest errors

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