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    Re: Exercise Lunar Distance with Mercury
    From: Frank Reed
    Date: 2009 Sep 23, 00:59 -0700

    Brad, you wrote:
    "Since Mercury is an inferior planet and very near to the Sun, Mercury always 
    shows a phase when we can see it.
    Did you see the phase of Mercury when performing the observation?  What power scope?
    When aligning the limb of Mercury to the limb of the Moon, were both curved 
    limbs in alignment or were caused to estimate the tangency, because the 
    phases of both objects were out of alignment."
    
    Er... This is negligible. Mercury has a very small angular semi-diameter 
    --about 5 seconds of arc when favorably placed. Now, in a lunar observation 
    with a planet that has a discernible diameter, you have three choices: place 
    the limb of the planet (only one available) on the Moon's limb, place the 
    geometric center of the planet on the Moon's limb, or place the center of 
    illumination on the Moon's limb. The illuminated limb of Mercury and its 
    geometric center would be separated by about 5 seconds at a favorable 
    appearance, while the center of illumination would fall in between. It would 
    rarely amount to a difference greater than a few seconds of arc. In other 
    words, this difference is safely negligible. So which choice should you 
    actually use if the options are available? The historical recommendation was 
    almost invariably to use the geometric center. A peek through a 
    higher-powered telescope can help here if you're not certain about the phase 
    at low power. In practice for modern lunarians, most will end up using the 
    center of illumination, and for that the positions in the standard Nautical 
    Almanac are appropriate. Personally, I don't recommend using any planet with 
    a really substantial difference between center of illumination, geometric 
    center, and illuminated limb. That rules out only one planet: the planet 
    Venus. Don't shoot Moon-Venus lunars. 
    
    -FER
    
    
    
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