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    Re: 60° Pendulum Astrolabe
    From: Paul Hirose
    Date: 2016 Jul 21, 02:18 -0700

    On 2016-07-19 11:03, Phil Sadler wrote:
    > I will need a table of the time of 60° altitude passage for stars brighter 
    than mag 4 for every 1° of latitude to the 1/100 of a second
    
    Before you start work on that table, it would be a good idea to do a
    basic functional test on the astrolabe with the computational tools of
    conventional celestial navigation.
    
    I doubt your table specification can be attained unless custom generated
    for a given set of conditions. For example, annual aberration causes the
    coordinates of every star to vary by tenths of a minute of arc
    throughout the year. This is easily visible at Nautical Almanac
    precision. Stars separated by a small angle are similarly affected, but
    you'll be measuring stars tens of degrees apart.
    
    If the astrolabe works as well as you say, deflection of the vertical
    (often several seconds of arc) should be taken into account, so the
    table will be specific to the observing site.
    
    Then there's precession, which is about 50 arc seconds per year, and
    nutation, which superimposes a wobble of about 20 seconds. And Earth's
    variable rotation rate ... maybe you need to account for polar motion as
    well ... it gets ugly to crystallize all this in a table. The parameters
    keep changing.
    
    Getting the parameters is not a problem. For example, the National
    Geodetic Survey has an online calculator for the deflection of the
    vertical at any point in US territory. IERS Bulletin A at the US Naval
    Observatory site has daily values for Earth orientation (UT1-UTC and the
    polar motion angles). Existing models predict precession and nutation to
    better than a millisecond of arc, years in advance. And so on.
    
    

       
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