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    Re: Lunars and pixels
    From: Frank Reed
    Date: 2010 Dec 11, 19:06 -0800

    John H, you wrote:
    "One of the bigger issues was pixel bleeding - in order to get a decent image of Regulus, the disk of the Moon became overexposed and quite fuzzy."

    You didn't try Greg Rudzinski's suggestion of holding up a neutral density filter in front of the Moon? For better results, you could keep the stars so under-exposed that they're impossible to detect by quick inspection and then use some of the various tricks that amateur astronomers use to pull out faint stars in digital images --the most important of these is using a "raw" image format since the standard jpeg compression will introduce noise that washes out faint star images.

    And:
    "They only did as well as 5 degrees in longitude using this"

    Even for this basic approach without careful work, this is pretty bad. It corresponds to about a ten minute of arc error in the Moon's position. Did they clear the lunars?? Did you happen to save any of these images?

    And:
    "their conclusion was that navigators "back in the day" had to have considerable patience to do lunars."

    Yes, digital camera saturation, jpeg compression, and pixel-bleeding spelled the end of the traditional lunar distance observation. ;-) Seriously, there's little connection between a modern observation done with a camera and a traditional one with a sextant except at the theoretical level. Fun, yes, and potentially useful for some sort of automated system, but "digital camera lunars" can't tell a student anything interesting about the standard 18th/19th century practice of lunars.

    In your list of student projects, you ended with:
    "Latitude and longitude from satellite dishes"

    Ok. Do tell. What is this?

    Of the sunstone tests:
    "They were systematically off in their measurements, and were perplexed until I pointed out that the sun was (is) setting south of due west this time of year. When they put this in, the measurements were spot-on."

    Very cool. How "spot-on"? Within five degrees? One degree? I would imagine it's hard to judge the orientation of a handheld piece of calcite to better than five degrees so I'm gonna guess that's the range of accuracy we're talking about. Close?

    From the perspective of useful navigational information, this doesn't seem all that practical --in the scenario you've described. They can see the Sun setting, and knowing its azimuth, they can then use a sunstone to find a spot 90 degrees away. What good is that? Why not just say, "there's the Sun... it sets ten degrees south of west this time of year in this latitude."

    -FER


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