NavList:

Might I be a wee bit pedantic George?
Just to make it clear in case of confusion. You say:

"Add the appropriate refraction correction, to get the apparent altitude".
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Refraction is a minus quantity always subtracted from the Apparent direction (altitude) of the light ray to give the True direction (altitude) of the light ray. (i.e. the apparent altitude is always greater than the true altitude).

The statement above about adding the corection is correct if refraction is algebraically added as a minus quantity.
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Then find difference in hour angles (RA's) of the two stars and work out angular distance between them with spherical trig as George says.
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Terminology is usually:

_Sextant altitude_: corrected for index error gives:
_Observed altitude_: Corrected for Dip gives
_Apparent altitude_: Corrected for refraction, semi-diam, irradiation, and parallax in altitude (as necessary) gives -
_True altitude_.

Douglas Denny.
Chichester England.
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