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    Re: Parallactic Retardation
    From: Frank Reed CT
    Date: 2004 Jan 28, 19:55 EST
    George H wrote:
    "But I have a nagging suspicion than perhaps he has also picked up my warnings about the evil effects of parallactic retardation (about which I have since recanted) and immortalised them in his own web pages."

    I think you're right on that.

    Generally, IMHO, your instinct that there was an accuracy issue in lunars due to the Moon's rapidly changing parallax at high altitude was right. It wasn't as serious a problem as you originally thought (and most everyone agreed --except, to his credit, Jan K) but it's still something to consider. You and, I think, Fred exchanged a couple of messages where you shifted the issue to the Moon's altitude, and that's a good way of looking at the problem. You need better altitudes for the Moon when it's at high altitude than when it's low because of the rapidly changing effect of lunar parallax. How would this manifest itself in practice? If you measure the Moon's altitude, it's simple enough: you need more accurate observations than you would lower down. And high altitudes are always dificult to measure so this is a real concern --measure the altitude of the Moon wrong by a quarter of a degree and you'll throw off your longitude by as much as an eight of degree. If instead you calculate the Moon's altitude, there are other problems. If your local time is off by a minute (which might happen when you've set a watch to local time in the middle of the day and you're observing your lunar the following night), then you'll get a resulting error in your longitude. There's the direct error from incorrect local time, but in addition you'll get an indirect error from the calculated altitude of the Moon, as much as 50% of the direct error. Sometimes this will make the resulting longitude worse than it would be when you've measured the Moon at low altitude (not always though).

    The rapidly changing parallax of the Moon at high altitudes does have an effect on the accuracy of lunars. It's not a huge issue, but there's something to it.

    Frank E. Reed
    [X] Mystic, Connecticut
    [ ] Chicago, Illinois
       
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