Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Sun Moon Lunars to 155 degrees
    From: Frank Reed
    Date: 2010 Apr 5, 17:38 -0700

    George H, you wrote:
    "I don't understand. Except at very low Sun altitudes, the Moon correction
    for (parallax and refraction) is almost always far bigger that that for the
    Sun, which is mostly just refraction.
    So I would change Frank's suggested order to Sun-Moon-lunars-Moon-Sun."

    No. That would actually be worse --not a lot, but it's definitely better to put the Moon altitudes at the ends of the sequence. You don't have to measure them or interpolate them as carefully.

    Why?? Well, remember that "lunars miracle" that I've discussed with you several times in the past few years?? The altitude of the Moon matters LESS than the altitude of the other body, and when the lunar distance is close to 90 degrees, the altitude of the Moon can be wrong even by several degrees with no significant effect on the lunars clearing process. You're still thinking of the altitude as an index to get into the altitude correction table. That's only part of it. The altitudes also affect the geometric factors determining what portion of the altitude corrections act along the lunar arc. Those factors or "corner cosines" change when the altitudes change. By a lucky accident, the change in the corner cosine cancels out the change in the Moon's altitude correction as the distance approaches 90 degrees. Also notice that if you move the Sun up or down, you can significantly change the geometry at the other side of the standard lunar triangle. So even though the Sun's altitude correction may be very small, it will still impact the lunar clearing process. In other words, even if there were no refraction AT ALL, you would still need to get the Sun's altitude correct to better than 6' (actually 6'*sin(LD)/cos(h)) if you want to clear a lunar correctly. Try it yourself with a few simple cases.


    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

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

    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