Refraction

Refraction is caused by light bending as it passes from the vacuum of space to the earth's atmosphere, making the sun and the stars look higher in the sky than they are. You see exactly the same phenomena when you poke a stick into water. How much the light bends varies with the amount of air that it passes through. The higher the sun or star, the less air there is. If it is low in the sky you are looking at it through more air. Refraction is always greater for sights taken when the celestial body is low in the sky. Refraction is always negative and subtracted from your observed angle.

The equation for mean refraction at standard temperature and pressure is daunting and given all the other sources of error surrounding readings with home-made sextants it is easier to use the following table:

Altitude

Correction

-34'

5-6°

-9'

6-7°

-8'

7-8°

-7'

8-10°

-6'

10-12°

-5'

12-15°

-4'

15-21°

-3'

21-33°

-2'

33-63°

-1'

63-90°

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