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    Re: Planet rising
    From: R.H. van Gent
    Date: 2002 Feb 2, 00:25 +0100

    "daveweilacher---.net" wrote:
    
    > So.  How do you compute the time a planet rises.
    
    The same way as you do for the Sun (note that this depends both on your
    latitude and on the declination of the Sun or planet). The correction
    for the semi-diameter you can neglect, only the correction for
    refraction matters. This is about 34 minutes of arc, so you determine
    when its zenith distance is 90 degrees and 34 minutes.
    
    > I've got the meridian passage of the planet from the daily pages of the nautical almanac.  Good.
    >
    > It's true rise should occur at 90 degrees before then.  Arc to time has this 
    convert to 6 hours earlier.
    
    Only when the planet's declination is near to 0 degrees (i.e. is near
    the celestial equator. Depending on the planet's declination and your
    latitude this interval could be significantly smaller or larger than 6
    hours.
    
    The time difference between planet rise to meridian passage (H) is given
    by the relation:
    
      cos H = (cos 90� 34' - sin decl * sin glat)/(cos decl * cos glat)
    
    Convert the angular measure H into a time interval (hours) by dividing
    it by 15.
    
    When you neglect the correction for refraction, the equation simplifies
    to:
    
      cos H = - tan decl * tan glat
    
    > Its apparent rise should have some factor for refraction.  (by way of 
    example, the sun has 14 minutes in its 50' adjustment).
    
    See above. The 50 arc minute correction for the Sun is the sum of 16 arc
    minutes for the semi-diameter and 34 arc minutes for the refraction.
    
    Regards,
    
    =======================================================
    * Robert H. van Gent                                  *
    * E-mail: r.h.vangent{at}astro.uu.nl                     *
    * Homepage: http://www.phys.uu.nl/~vgent/homepage.htm *
    =======================================================
    
    
    

       
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