Welcome to the NavList Message Boards.

NavList:

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

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    Name or NavList Code:
    Email:
       
    Reply
    Re: Error in taking lunar distance
    From: Frank Reed
    Date: 2008 Feb 11, 23:06 -0500

    Jim, you wrote:
    "I have begun to notice an issue when taking lunar distances between
    the sun and moon when the moon is very faint i.e. in the order of 10%
    illuminated."
    
    Oh definitely. At 10% illumination you're talking about a Sun-Moon lunar
    distance of 36 degrees or so [fraction illuminated = haversine(LD) = (1 -
    cos(LD))/2]. I almost never shoot Sun-Moon lunars when the angle is less
    than 45 degrees. It just doesn't work very well.
    
    When you look at old logbooks, lunars were usually taken when the Sun-Moon
    angle was closer to 90 degrees --closer meaning roughly 70-110 degrees.
    Every two weeks or so, for a few days around First and Last Quarter they
    would shoot lunars. That's really all that was necessary since dead
    reckoning for longitude was "good enough" for ten days or more (lunars were
    a check on the DR longitude through 1835 +/-10 years). Again, judging by the
    logbooks, Sun-Moon lunars were greatly preferred over Sun-star lunars.
    Celestial navigation back then was a daytime activity.
    
    There are two other reasons for preferring lunars close to 90 degrees (these
    were not mentioned in most of the navigation manuals but may have been
    "common knowledge"). First, if you're doing a series solution, like all of
    the methods in Bowditch, then you can frequently ignore the quadratic
    correction since it is negligible at 90 degrees. Additionally, the
    calculation depends on the altitudes of the Sun and the Moon, but when the
    lunar distance is near 90, the altitude of the Moon doesn't matter much
    anymore. You can get the Moon's altitude wrong by a degree and it will not
    affect the final result. These two points apply to all lunars, not just
    Sun-Moon lunars.
    
    You mentioned that your lunars are all too short by about 1.5 minutes of arc
    when the Moon is a faint crescent. Oddly enough, I find that mine are
    generally too long by a minute or arc or more. The explanation is, I think,
    the obvious one. The Moon is dramatically fainter when it's a crescent (it's
    much fainter than most people guess --it's not just proportional to the
    percent illumination) and the limb "blends in" with the daylight sky. The
    contrast is simply far too low. There's one exception: sometimes the sky is
    clear as crystal, when there's low humidity at high altitudes. On those days
    when the sky is a very dark blue, there's enough contrast to get good
    results.
    
    And you wrote:
    "As soon as the moon gets to 25% illuminated or better or the sighting
    is done with a star or planet, I obtain results that are typically
    within 0.3 minutes. Again this would be the average of 5 sights."
    
    What power telescope do you use? The unaided human eye has a (corrected)
    resolution of one minute of arc. If you're using a 7 power scope, you can
    resolve about 0.14 minutes of arc on each sight. Let's double that to be
    generous. An average of five sights should give you an error smaller than
    0.2 minutes, closer to 0.12 minutes actually.
    
     -FER
    
    
    --~--~---------~--~----~------------~-------~--~----~
    To post to this group, send email to NavList@fer3.com
    To , send email to NavList-@fer3.com
    -~----------~----~----~----~------~----~------~--~---
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Get a NavList ID Code

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

    A NavList ID Code guarantees your identity in NavList posts and allows faster posting of messages.

    Retrieve a NavList ID Code

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

    Email Settings

    NavList ID Code:

    Custom Index

    Subject:
    Author:
    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