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Re: Parallactic retardation - don't give up so easily.
From: George Huxtable
Date: 2004 Jan 10, 23:05 +0000
From: George Huxtable
Date: 2004 Jan 10, 23:05 +0000
Bruce Stark said- >Try this perspective: > >Put the moon on the equator, or south of it, and yourself in fairly high >north latitude. All right; then the Moon doesn't attain a high altitude. > >During meridian passage the moon's altitude is hardly changing, I agree. >and what >change there is will have slight effect on her refraction and parallax >correction. I disagree here. Although the AMOUNT of the parallax (and also refraction too, but let's not bother too much about refraction now) is hardly changing, its DIRECTION is changing rapidly. Remember, the parallax displaces the Moon's position such as to reduce its apparent altitude, acting along a line between the zenith and the Moon. When the Moon is on the meridian, it is due South of the Zenith, so the displacement due to parallax is along the azimuth of 180 deg. Before meridian passage, the line between the zenith and the Moon (along the direction of the Moon's azimuth) is somewhat to the East of South, so has an Eastward component. After meridian passage, it will have an increasing Westerly component. It's these Easterly and Westerly components that are acting to displace the Moon along its path, and the rate at which the Moon is displaced along its path is a maximum when the Moon's on the meridian. At that point, the displacement of the Moon caused by parallax is exactly at right-angles to its path, so the actual displacement along its path caused by parallax is exactly zero, although the rate-of-change of that quantity is a maximum. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================