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Old 19-05-2009, 02:45 PM
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Robh (Rob)
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Join Date: Feb 2009
Location: Blue Mountains, Australia
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Quote:
Originally Posted by Wavytone View Post
f) atmospheric refraction (I often wonder whether these mounts compensate for it) which can be half a degree near the horizon.
This is something we quite often don't think of. Refraction at the horizon is about 34' of arc. The apparent diameter of the sun is about 32' of arc. As the sun begins to set and its disk appears to be sitting on top of the horizon, it is actually below the horizon. Refraction also accounts for the flattening of the sun's disk at the horizon.
Atmospheric refraction is about 1' at 45 degrees and 5' at 10 degrees above horizon. At the height of the South Celestial Pole it is about 2'.

Rob
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