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    Re: With your help Lunars are getting better
    From: Frank Reed
    Date: 2013 May 14, 10:52 -0700

    Randall,

    The first two sets are obviously excellent. The third is okay. But your last set with an error of 1.9' averaged from five observations should worry you.

    Consider this: how big is Jupiter? You can see it as a "dot", right? It doesn't appear as a twinkling point of light, like a star. Instead you can just detect its visible disk with that 7x scope. We can work it out. Jupiter is about 80,000 miles in diameter. Its mean distance from the Sun is about 500 million miles but since it's heading around to the far side of the Sun right now (must be since we see it just after sunset), its distance is closer to 600 million miles. The angular diameter, just as well for a distant planet as a nearby lighthouse, is the physical size divided by the distance and multiplied by 3438 to convert to minutes of arc. That yields just about 0.5 minutes of arc. So an error of 1.9 minutes, especially after averaging, is actually huge. As you suggested, it MAY be fatigue. But I would look for some other cause.

    As an experiment, the next time you shoot a Jupiter-Moon lunar, get it lined up perfectly, and then manually adjust the sextant away from your perfect alignment by 1.9 minutes of arc and look again. The actual distance changes slowly enough (about 0.1' in 12 seconds for the true distance, a bit more or less than that for the apparent distance depending on the changing parallax) so that you can do this "live": line it up, shift by 1.9', and look again. You should find that you can easily see that 1.9' is very far out of alignment. At a magnification of 7, that is a plainly visible difference, even if you're a bit fatigued.

    So if it's not fatigue, then what is it? This is where things get interesting!

    -FER


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