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    Re: Lunar trouble, need help
    From: Frank Reed
    Date: 2008 Jul 02, 13:08 -0400

    Kent, you wrote:
    "Sun SD: 15m 45s
    Moon SD: 15m 25s
    Moon HP: 56m 36s"
    These are fine. Incidentally, you can also check this sort of thing on my
    web site (see below).
    "For my GMT 06-25-51 the values according to Umland shall be:
    Sun SD: 15m 45,1s (USNO 15m 48s)
    Moon SD: 15m 25,5s (USNO 15m 36s)
    Moon HP: 56m 36,5s."
    Who is Umland, incidentally? I notice that you have gotten in the habit of
    quoting things to a FRACTION of a SECOND of arc. You don't need anywhere
    near that level of detail, especially since you say you're trying to do
    things the way a 19th century navigator would have done things.
    Andyou wrote:
    "My augmentation in the moon's altitude reduction was -12,96s. Minus because
    the UL was observed."
    Well, now, that right there could explain a whole lot. The augmentation is
    ALWAYS positive. It's true that you would subtract that quantity from an
    upper limb altitude sight, but that's because it's adding onto the Moon's
    semi-diameter, and the fact that it was an upper limb obs is really
    irrelevant to the rest.
    And you wrote:
    "This figure also includes a small correction for refraction of +0,34s."
    There is no reason to include such miniscule effects. Clearing lunars is
    EASY. I think you're getting carried away with minor details.
    "My moon parallax incl. corrections for earth flattening was 26m 50,39s. It
    is not clear to me how George corrects for earth flattening."
    The discrepancy between your clearing and George's (and everybody else's!)
    has nothing to do with the oblateness of the Earth (earth flattening). This
    is always a very minor factor. It can be ignored in lunar distance problems,
    and when it is, on average it will lead to an error in position of only
    about one nautical mile, occasionally as much as three nautical miles.
    That's all.
    "The angles between the distance line and the verticals were not calculated
    by me"
    Yes, but we KNOW that those angles were close to zero because Jeremy
    described his observation and said that the Sun and Moon were on opposite
    sides of the sky. And as I noted in another post, this simple alignment
    means that the whole lunar observation can be cleared without using any trig
    at all. Lunars are easy.
    "and therefore I did not put in any corrections for:- the decrease of SD's
    in my distance reduction. The altitude of the moon is high and does not
    require any correction. The sun can be corrected with -1s."
    In case anyone else is following along and trying to understand what Kent is
    describing, this is "refractional flattening" of the visible shape of the
    Sun or Moon in the sky. Basically this is a correction to the object's
    apparent semi-diameter. But it is exceedingly small unless the Sun or Moon
    are below about 12 degrees altitude. It can be ignored without any serious
    consequences (except when the object is very low in the sky).
    And you wrote:
    "the moon's parallax in azimuth. If I do a calculation of these angles this
    correction is -0,74s (moon in E and sun to the right
    seen by the observer). These angles are small (8 and 13 degr respectively).
    Anyway, the exclusion of these two small corrections will have minor impact
    on the final LD."
    If you really want to deal with the small changes in the Moon's position due
    to oblateness, I recommend you use the method in Chauvenet. Then you can
    drop all talk of "parallax in azimuth". The corrections are simple. But
    again, just as you say, this is a trivial matter and it would not change
    anything in the case under consideration.
    Kent, have you tried the lunars clearing calculator on my web site?
    Click on "Clear a Lunar Online" You can easily experiment with different
    lunar observations. There are checkboxes that let you include or ignore the
    corrections for oblateness of the Earth and refractional flattening. By
    toggling these, you can see very quickly that they are usually minor
    corrections. Nothing you need to fuss over. And in addition, it is highly
    unlikely that any real navigator at sea would have applied the oblateness
    correction. This is an "armchair navigator's" detail.
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To , email NavList-@fer3.com

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