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    Re: Sun Moon Lunars to 155 degrees
    From: Peter Hakel
    Date: 2010 Apr 2, 19:23 -0700
    Thank you for the clarification, Frank.  Something was telling me to double-check my understanding by reposting it, and I'm glad I did.  Also, the Nautical Almanac states:

    "If an error of 0'.2 is significant the expression for the parallax in altitude for the Moon should include a small correction OB for the oblateness of the Earth as follows:" etc.


    Peter Hakel



    From: Frank Reed <FrankReed{at}HistoricalAtlas.com>
    To: NavList@fer3.com
    Sent: Fri, April 2, 2010 2:03:56 PM
    Subject: [NavList] Re: Sun Moon Lunars to 155 degrees

    Peter H, you wrote:
    "Flattening: not quite spherical shape of the Earth
    Oblateness: refraction-induced "apparent flattening" of the observed Sun or Moon disc. All clear now."

    Clear, but backwards. :-) Note that in some other languages, there is no separate word for oblateness. It's just the "flattening of the Earth at the poles" (translated word for word) so some confusion over the terminology should be expected.

    Historically oblateness was usually ignored when clearing lunars even in detailed work. It's a VERY small detail which is largest in terms of minutes of arc of longitude at the poles but, since longitude lines converge at the poles, in terms of actual nautical miles in the observer's position it maxes out in mid-latitudes. Refractional flattening only mattered when the Sun or Moon was quite low. It is a "position-angle-dependent" variation in the Sun or Moon's semi-diameter. Go to google books and download Thomson's "Lunar and Horary Tables" to see a simple example of a table for this modification to SD (linked from the Resources section right at the top of fer3.com/NavList).

    Today, oblateness and refractional flattening are as easy to include as any other aspects in the lunars clearing process so by default they are "on" in the clearing tool on my web site. You can try turning them off with various geometries to assess their relative importance.

    -FER

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