Part I · Foundations · Page 2 of 5
Index correction and dip give Ha
Index error read on the arc is subtracted; off the arc it is added ("on the arc, take it off"). Dip comes from your height of eye in the dip table, and is always subtracted. The result is the apparent altitude, Ha.
Finding the index error
Set the sextant near zero and look at the horizon. If the reflected horizon does not line up with the real one, the sextant has an index error. Turn the drum until they make one line, and read it:
- Reading
- 0.0′ (zero)
- Horizons
- 2.4′ apart
- Index correction
- —
Readings below zero are "off the arc"; the drum runs backwards past the zero mark.
From the globe to your boat: why there is dip
Here is the same Earth again, sliced through your position and seen side-on. Zoom in to your boat. Your eye is above the sea, so you look slightly down to see the horizon. The sextant measures from that lowered horizon, so every reading is a little too large. Raise your eye and the dip grows.
The slider for the Sun shows the next correction too: its light bends as it comes down through the air, so you see it a little higher than it really is. That is refraction, part of the main correction on the next page.
Predict You climb the mast, higher above the sea. What happens to the dip correction?
- Height of eye
- 10 ft (3.0 m)
- Dip
- −3.1'
- Sun as you see it (Ha)
- 8°00.0'
- Refraction
- −6.6'
- Sun as it really is
- 7°53.4'
Near the horizon the light crosses much more air, so it bends a lot, and the bending changes fast. That is why low sights are less reliable.
Hs 48°34.5', index error 1.1′ off the arc, height of eye 10 ft:
48°34.5' +1.1' −3.1' = Ha 48°32.5'
No sea horizon where you are? On land you can use a tray of liquid as an artificial horizon: no dip, but the reading is halved.