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: Precomputed lunar distances
    From: Bill B
    Date: 2005 Apr 20, 00:25 -0500

    > Bill you wrote:
    > "In fact that is not what raised a red flag for me.  I had drilled  down too
    > far and done a scatter graph with Excel, so every or hundredth or  thousandth
    > was magnified."
    Frank responded:
    > Yeah, I wondered if that was part of the problem. Remember, if your input
    > data is accurate to the nearest tenth of a minute of arc, generally you should
    > quote your output data to the nearest tenth as well. Anything beyond that is
    > just random garbage.
    Point well taken.  If I recall the concept of "significant digits" is
    centuries old (not that I am up to that math level yet ;-)  Determining when
    to round, and how many places to carry forward when using a handheld
    calculator or computer application for computations is a work in progress
    for me.  If I recall correctly, Alex informed me that a significant digit
    can be added in division of a ten figure average--but that does not apply in
    this case.
    All I can accurately state; when someone I respect sends me refraction
    correction figures to 5-or-so places I respond in kind. Much like the story
    of the daughter that asked her mother why she always cut the end off the
    roast before putting it in the roasting pan/oven.  Mom replied, "That's the
    way I learned it from my mom." So they phoned the daughter's grandmother and
    asked her.  Grandma replied, "because my pan was always too short for the
    Bill wrote:
    > And:
    > "Regarding my question, "Another hypothetical scenario.  If I take the  same
    > two stars, calculate true separation of 34d 27.7', they have identical  Hc's
    > of 1d 36.8', and  hypothetical refraction is -88d, what separation  might I
    > expect to measure with a sextant?"
    Frank responded:
    > I didn't respond to this before because I cannot for the life of me figure
    > out what you're getting at. If you have two stars with an unrefracted distance
    > of 34 deg 27.7' and you observe them down at 1.5 degrees altitude, then the
    > measured distance will be very close to 34d 27.7'. What's this "-88d"  number?
    Perhaps I misstated.  Their true/calculated, unrefracted altitudes are
    nominally 1.5d above the terrestrial horizon.  (A value I chose as the
    center of the sun can be almost -50' true/Hc altitude and still have the
    upper limb visible, and if I recall list postings stars extinguish near the
    terrestrial horizon, how near I do not recall--so left a little leeway as I
    did not want to muddy the waters with technicalities.) The theoretical
    refraction figure of -88d was proposed to test my understanding of movement
    up the triangle sides, as well as limits of the refraction-separation
    formula. (I acknowledge this refraction value is *way*  outside physical
    reality as Earthlings experience it -- but it is sometimes useful for me to
    reduce an argument to the extreme/absurd.)  Sorry for any confusion.
    My assumption was that given the above *theoretical* scenario, the stars
    would be lifted up and towards the zenith by 88d (up the triangle sides,
    straight or arcs ;-) and therefore be *observed* as being close to 0d apart.

    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