Question 1

The arc of a sextant spans about 60° of metal yet reads to 120° because…

Accepted Answer

Double reflection doubles the angle — the arm moves HALF the angle measured — Light folding off two mirrors turns through twice the mirror rotation, so the arc is engraved at twice its geometric sweep. (And the name: the frame is a sixth of a circle.)

Question 2

Because of the half-angle principle, a 0.1° wobble of the index arm…

Accepted Answer

Reads as 0.2° of measured angle — arm precision matters double — Every arm movement is doubled in the optics. That is why the fine control is a micrometer drum, not your thumb.

The double-reflection trick

This lesson is part of a paid course