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
    or...
       
    Reply
    It Works.
    From: Bruce Stark
    Date: 2002 Apr 2, 13:10 EST

    The procedure I posted March 30, for calculating altitudes when both GMT and
    longitude are wildly uncertain, works fine.
    
    It's the same system navigators from James Cook to Joshua Slocum used to find
    local hour angles. The only difference is it uses sidereal hour angle instead
    of right ascension, so things are measured from east to west.
    
    I've found it's less confusing if I first take only the GHAs of Aries and the
    sun from the Almanac with the guessed-at GMT. That way, fewer numbers are
    lying around to stumble over. Only after finding SHA meridian do I take the
    other numbers from the Almanac, with the same guessed-at GMT of course.
    
    The acid test for the method was the lunar Chuck Griffiths posted March 21.
    Here both bodies are near prime vertical, so change in hour angle affects the
    altitudes nearly 100%. What makes it exceptional is that the moon, Venus, and
    the observer are all in nearly the same plane, so the distance is affected
    nearly 100% by refraction and parallax. Total correction in altitude for
    parallax and refraction was 45.'6, of which 44.'9 showed up in the difference
    between the apparent and cleared distances. It would be hard to find a
    situation where the distance could be more sensitive to changes in the GMT
    used to calculate the altitudes. It couldn't happen except in low latitudes.
    
    Since Chuck neglected to take an observation for local apparent time I did,
    in this case, have to know his longitude and accurate GMT to get his LAT. I
    did so by applying the equation of time to GMT to get GAT, and his longitude
    to GAT to get LAT. LAT was March 18th, 18:36:35. That's 6:36:35 past noon.
    
    I pretended Chuck had found his LAT by observation before the sun went down.
    I also pretended he'd so egregiously screwed up his dead reckoning that it
    put him seven and a half degrees, nearly 400 nautical miles at his latitude,
    east of where he was. This made his estimated GMT half an hour less than the
    true.
    
    The first try got a GMT of 00:20:19, March 19th.
    
    That's nearly half an hour from the GMT used to calculate the altitudes, so a
    repeat was called for.
    
    The second try got GMT of 00:18:33.
    
    Obviously nothing much to be gained by going further. Half an hour off in the
    GMT used to calculate the altitudes affected the result less than two
    minutes. Two minutes off can be ignored. This may ease George's concerns
    about the use of calculated altitudes for clearing distances.
    
    Bruce
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    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:

    Email Settings

    Posting 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