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: Longitude by Sunset
    From: George Brandenburg
    Date: 2012 May 8, 14:03 -0700

    Marcel,

    Thanks for finally summarizing this so well! All I would add is that if you want to treat the difference between sunset and moonset as an effective lunar distance measure, then you certainly need to include the parallax correction for the moonset.

    As for witnessing actual full-moonsets, I'm not usually up at that hour. But I have seen plenty of bright full-moonrises, and I would conclude that if you can see one you can see the other. Of course here I mean the actual moonset and not the "geometrical" one, which can't be seen.

    Cheers,
    GeorgeB


    From: Marcel Tschudin
    Date: 8 May 2012 15:08
    OK, I think to be finally not only able to "see" the parallax "problem" for the observer of the moon but also to actually understand it:

    (1) If the moon would be bright enough an observer would see it setting or rising like the sun. There is indeed "nothing" related to the question whether the moon is sufficiently bright to be seen rising/setting.

    (2) Due to the parallax the observer sees the moon set earlier than it would be without parallax. We know that when the LL of the sun (which has a negligible parallax) touches the apparent horizon its *geometrical* position is already below it. The setting of the *geometrical* sun happened therefore at some moment *before* while one still observes the sun setting.

    (3) For the moon the combined effect of (non-negligible) parallax and refraction is different. When the parallax exceeds the refraction their difference has the combined effect of a negative refraction. If we could see the LL of the moon touching the apparent horizon its *geometrical* position would not be below it as in the case of the sun but rather above the position where the moon is seen.

    (4) Mr. van Asten's statement relates to the setting of the *geometrical* and not to the visible moon during the visible moon set. During the visible moon set the *geometrical* moon remains above the apparent horizon. The *geometrical* moon passes the apparent horizon only some times *after* the visible moon set.
    ----------------------------------------------------------------
    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
    ----------------------------------------------------------------

       
    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