Course / Sextant corrections: Hs to Ho

Part I · Foundations · Page 1 of 5

What the sextant reads

The sextant measures the angle between the Sun and the sea horizon. Through the telescope you see the horizon straight ahead in one half, and in the other half a mirror on the moving arm shows the Sun. Swing the arm, then turn the drum, until the bottom edge of the Sun (its lower limb) just touches the horizon. Try it:

directmirror020406080100120Arm turned 0°00.0'
The view through the telescope (the round picture), and the sextant from the side.
Sextant reading
0°00.0'
Arm turned from zero
0°00.0'

The Sun is above the field of view: swing the arm to a larger angle.

That reading, Hs, is not quite the Sun's true altitude. Three kinds of error are removed, always in this order:

  1. Index correction: the sextant's own error.
  2. Dip: your eye is above the sea, so the horizon you see is below true level.
  3. The main correction: refraction, the Sun's radius, and parallax, combined in one table.