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    Re: October Lunar
    From: Frank Reed
    Date: 2008 Oct 04, 20:20 -0400

    Jeremy, you wrote:
    "Admittedly, this was a poor time to shoot lunars.  Both bodies were well
    below 10 degrees of altitude, and the observed lunar distances were just
    below 12 degrees, which does not usually help with accuracy, but I wanted to
    practice, so I proceeded."
    
    The low altitudes may be a problem for some lunars but probably not in this
    case. The short distance is not an issue (two reasons: the altitudes are
    calculated so their accuracy is not a problem, and also we don't have to
    interpolate between geocentric distances three hours apart, which was an
    issue historically but not today).
    
    I cleared your sights a couple of different ways and agree that they seem
    very consistent as a group but show an error of about 1 minute of arc. So
    what could explain that?
    
    You suggested:
    "Is it strange refraction due to low altitudes?"
    
    Because the two bodies were at nearly equal altitudes, we can safely rule
    this out. First of all, if there were any unusual refraction, it would
    probably affect both objects by nearly the same amount. More significantly,
    since the lunar distance was nearly horizontal, the corrections to that arc
    due to the altitude corrections (which are entirely vertical) is much
    reduced. You can experiment with this issue by trying different values for
    barometric pressure on your sights. Though these lead to substantial changes
    in refraction, they don't change the cleared lunar distance much at all.
    
    And you wrote:
    "Perhaps is an error in my sextant calibration"
    
    Always possible.
    
    "or perhaps just a personal observation error."
    
    A minute of arc at 7x magnification is big. Unless you had very poor
    observing conditions, you would see this very clearly.
    
    By the way, one issue that sometimes comes up with Venus is its large
    angular diameter and also its phase. Right now, that's not a concern since
    it's still relatively far away.
    
     -FER
    
    
    
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