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
    The Moon and Parallax
    From: David Pike
    Date: 2019 Oct 14, 08:07 -0700

    I’ve called this question The Moon and Parallax rather than Lunar Parallax to avoid anyone confusing it with lunars, which are still a bit outside my pay-grade.  I’ve decided to break the habit of a lifetime and start observing the Moon.  Why not when I can kneel against a radiator and observe it most nights through a south facing open bedroom window?  However, there are one or two things which puzzle me.

    One.  How is it that air navigators are allowed to get away with a relatively simple P in A correction in the Air Almanac, while marine navigators are forced to chase parallax though HP on the daily page to two corrections from the Altitude Correction Tables?  I presume this is because a. with all the other problems we have to deal with deflecting our bubble, we’re not interested in working to the nearest 0.1 of a minute of arc; we’re happy to accept corrections to the nearest minute, and b. being simple souls, we prefer to aim our bubble at the middle of any large object in the sky.

    Two.  The top Altitude Correction Table ought to reduce from a maximum at zero altitude like P in A, but it doesn’t.  It reduces either side of about 20 degrees altitude.  I presume this must be, because designed for operating at sea level, the Altitude Correction Table can also correct for atmospheric refraction, whereas the P in A tables don’t.  The air navigator must correct for atmospheric refraction separately, because it changes with aircraft height as well as star altitude. 

    Three.  This is the bit which always baffles me.  Why do they use the name Horizontal Parallax when it leads to a vertical correction?  DaveP

       
    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