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    Re: Moonrise video
    From: Frank Reed
    Date: 2013 Apr 24, 15:20 -0700

    Brad, you wrote:
    "regarding extinction and large angle lunars.
    Since extinction will very likely limit the maximum possible angle at which the sun and moon are both visible, what is the consensus opinion as the the maximum feasible lunar distance."

    I do not speak for a consensus, of course. :)

    But as we've noted, you can now and then see the Full Moon when it's right on the horizon. At least its center would be. And the Sun's upper limb could be right on the horizon at the same time, too. Certainly there are cases where people have watched the Moon rising partially eclipsed. Here's an example: http://www.racingshadow.com/LEclipse/2006Sep7/LunarEclipse_2006Sep07.html
    This was not a case of the Moon rising over a sea horizon, but instead some distant hills. Given the significant flattening, however, its apparent altitude was quite low. In this specific example, I would guess that the Sun had already set since the Moon is at the lower edge of the Earth's umbra. Can you find a photo of the Moon rising partially eclipsed where it is in an orientation that would imply the Sun is still up?

    The theoretical maximum angle for Sun-Moon lunars is probably very close to 180 degrees, as far as visibility and extinction are concerned. I will go out on a limb and say that 175 degrees would be "no problem" for visibility. I would be more concerned about the instrument's ability to measure such angles with any useful accuracy. There is an issue of sextant accuracy that I don't think we've gotten into recently (I don't know if it also applies to reflecting circles). Telescope collimation introduces an error which is proportional to tan(LD/2). It is of "some concern" for lunars around 90 degrees and more so near 120 degrees. What happens with higher angles? Obviously, this error equation blows up at 180 degreees. I suppose for backsights, the error equation is different.

    You added:
    "We know Cook did 155 degrees. "

    Speaking of useful accuracy... For those unfamiliar with the story, Cook's team of astronomers experimented with a number of exotic sighting techniques out there in the Pacific including backsight lunars using a special sextant attachment. Very long lunars like those and that attachment were never heard from again after the observations from the voyage were analyzed back home. Either the observations proved inaccurate or the process was just too much trouble. Or there was a conspiracy to crush the "long lunar" among the merciless minions and/or nattering nabobs at the Royal Observatory... :)


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