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: True Distance vs. Observed Distance
    From: Marcel Tschudin
    Date: 2012 Sep 17, 22:09 +0300
    Greg,

    I like your drawing skills, especially the ship ;-)

    Your star positions are likely given in astrometric coordinates (without atmosphere). These positions are then converted  to air-free local coordinates of the observer. Refraction is applied to these local coordinates by deducting its value from the ZD (or added to its altitude) for each of the two bodies depending their air-free ZD (or altitude). The azimuths are unaffected by refraction and remain therefore unchanged. The distance is then calculated between the two refracted positions (ZD1ref, Azi1 and ZD2ref, Azi2).

    Note that refraction has in this case to be calculated from the true ZD (or altitude) using e.g. the formula from Saemundsson (1986, see http://en.wikipedia.org/wiki/Atmospheric_refraction there the second formula with R=1.02 cot .... ). Looking at Bill's Excel spread sheet "A Nav Ang Sep Refract Mas" shows that he did indeed use this formula. I was however not able to calculate with it the star distance of Greg's observation.

    Marcel


    On Mon, Sep 17, 2012 at 7:47 PM, Greg Rudzinski <gregrudzinski@yahoo.com> wrote:

    Linked is a diagram showing how refraction changes observed distance between celestial bodies. For the diagram the bodies have a distance about 45* with the zenith directly above the observer. The refraction correction is added to the observed distance to get a true distance. If starting with a true distance from a GC calculator then the refraction correction is subtracted from the true distance to get the observed distance.

    Greg Rudzinski
    ----------------------------------------------------------------
    NavList message boards and member settings: www.fer3.com/NavList
    Members may optionally receive posts by email.
    To cancel email delivery, send a message to NoMail[at]fer3.com
    ----------------------------------------------------------------

    Linked File: http://www.fer3.com/arc/imgx/IMG_5188.jpg


       
    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