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    Re: Star - Star Observations
    From: George Huxtable
    Date: 2010 Mar 10, 00:39 -0000

    Douglas Denny wrote-
    
    "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."
    
    No, that wasn't as I said; but perhaps Douglas will elaborate further on how
    it's to be done that way.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    ----- Original Message -----
    From: "Douglas Denny" 
    To: 
    Sent: Wednesday, March 10, 2010 12:17 AM
    Subject: [NavList] Re: Star - Star Observations
    
    
    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".
    ----------
    
    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.
    --------
    
    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.
    -------
    
    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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