Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Sun semidiameter
    From: Alexandre Eremenko
    Date: 2007 Apr 14, 19:55 -0400

    The date May 14 is actually close to the date
    May 15/16 where the almanac data changes to 15.8
    The explanation in the Almanac (p. 254) says
    that the almanac data are accurate to 0.1'
    which means that the difference between the tabulated
    value and the true value is less than 0.1'.
    In your example, asuming that the value given by Frank
    is exact, the difference is 0.07' which is
    consistent with the Almanac statement of accuracy.
    But this statement does not imply that the last digit
    is actually correct. So there is no contradiction.
    The reason of these errors is probably the rounding
    subroutine. To get the last digit really true,
    yoiu have to do all computations with one more digit,
    and then apply an exact rounding algorithm, while
    in most cases they just cut off the last digit
    instead. Because nobody really cares about the exactness
    of the
    decimal minute digit.
    But I understand your concern:-)
    When you determine your IC from Sun, and then apply
    the "Soviet 0.4' test" you may be in trouble just
    because of the inaccuracy of the Almanac data.
    Indeed, the true 4SD computed from the Almanac can
    in principle differ from the true value by up to 0.4.
    (In your example it is 0.32' if we assume Frank's value
    to be correct to the last digit).
    Indeed, this makes the "0.4 test" sometimes somewhat
    too rigid. (I don't know to what precision they give
    this value in Soviet almanac, though:-)
    All this shows that in fact nobody (including the
    almanac publishers) is really concerned with higher
    accuracy than about 0.5'.
    (The days of the Lunars are gone, indeed!)
    And they hope that many various
    errors of the order of fractions of a minute will
    in most cases cancel each other. And they are right.
    I can give an example.
    Yesterday I measured a Lunar distance
    Sun-Moon GMT April 13 2007, 16:15:40,
    sextant reading 53d03.1, IC=-0.5.
    Then I reduce with Frank's calculator. The measurement
    turned out to be precise: Frank's calculator gives:
    error in the distance: 0.0', Error in longitude 0.7'.
    Then I reduce the same distance by hand, using the almanac
    (and Frank-computed altitudes). Using the algorithm
    outlined on the Frank web site.
    In the process of
    I notice a lot of differences between Frank's numbers
    and my numbers (I use the almanac). Namely:
    Sun GHA: 63d46.5' (alm) 63d46.4' (Frank)
    Moon GHA: 112d27.3' (alm) 112d27.2' (Frank)
    Moon Decl: S12d37.6' (alm) S12d37.5' (Frank)
    True distance: 53d00.27' (my computation) 53d00.2' (Frank)
    Alt Moon corr'n: -49.4' (my) -49.6' (Frank).
    This is the largest discrepancy; I used the Moon tables
    in the almanac.
    The final result of my computation gives
    cleared distance 53d00.26' which is just
    0.01' off the true value. That is 0.3' in longitude,
    while Frank says 0.7' in longitude.
    So you see all these plus-minus 0.1 errors usually
    cancel each other nicely in the process.
    And you can really use the almanac data for the Lunars
    and obtain very precise results.
    Not speaking of "practical navigation" for which
    the Complete on Board Celestial Navigator with its
    1' rounding is probably adequate.
    On Sat, 14 Apr 2007, Bill wrote:
    > Looking at Frank's site
    > http://www.clockwk.com/lunars/nadata_v5.html
    > and Sun and Polaris Ephemerii�for Land Surveyors
    > http://www.cadastral.com/2007ephs.htm
    > for 14 May 2007 I find agreement in the semidiameter of the sun; 15!83
    > (Frank) and  15' 49.6" = 15!83 (Cadastral).  Yet the nautical almanac lists
    > SD as 15.9.  I see this happening on other dates as well.  I would expect it
    > could happen near a rounding, point, say 15!9 one page and 16!0 the next,
    > but that thought does check out.
    > Why the difference in SD?
    > Bill
    > >
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site