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
    Re: Quicker Ex-Meridian Formula for Slide Rule or Calculator
    From: Gary LaPook
    Date: 2016 Dec 2, 01:55 +0000
    If you do the normal sight reduction using HO 229, HO 249, HO 214 and HO 216 with the LHA of zero you can use any sight within plus and minus 2 minutes of noon.

    gl



    From: Frank Reed <NoReply_FrankReed@fer3.com>
    To: garylapook---.net
    Sent: Thursday, December 1, 2016 10:53 AM
    Subject: [NavList] Re: Quicker Ex-Meridian Formula for Slide Rule or Calculator

    Greg Rudzinski, you wrote:
    "Cos of Altitude is used in place of Cos (Latitude +/- Declination).  This is a good alternative method but doesn't allow the navigator to precalculate [...]"
    I think you meant cos(altitude) in place of sin(lat +/- dec)? Note that you can still precalculate, so long as the instructions are clear. The altitude that should be used in the calculation is the true noon altitude. That formulation also makes it easier to understand the +/- choice and usually means that you don't have to think about that choice. The result is not very sensitive to an error in this altitude for "typical" noon observations so you can use a predicted noon altitude based on the DR, and it's also possible to use the altitude as observed close to noon, which would, as you say, imply that you would have to do the calculation after taking the sight (not really a significant cost). For noon observations where the altitude is high, roughly above 60°, the error from an error in the estimated altitude increases linearly with altitude. Of course, when in doubt it's possible to re-run the calculation with the better estimate of the noon altitude after the first pass through the ex-meridian computation. The real uncertainty in these calculations is the time from local apparent noon. That error obviously increases quadratically with time from noon. Given that the calculation is short, it seems to me that a user should always be advised to run this twice, varying the input parameters by reasonable estimates of the errors in those quantities, especially the time from noon.
    It's also important to remember with ex-meridian calculations that this is just a calculation method. Ultimately, every altitude sight always yields a line of position whether we get around to plotting it or not. A sight near noon corresponds to a line of position that runs nearly east-west, skewed a bit. Inputting a time from noon in the ex-meridian calculation, is equivalent to drawing a longitude through that slightly skewed line of position and picking off the corresponding latitude where they cross. Just as with any case where we cross two LOPs, the error in the resulting latitude will depend on the crossing angle between the lines, and in the case of a sight near noon, that depends on the actual azimuth of the Sun. 
    Frank Reed



       
    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