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: 60° Pendulum Astrolabe
    From: Paul Hirose
    Date: 2016 Jul 21, 02:18 -0700

    On 2016-07-19 11:03, Phil Sadler wrote:
    > I will need a table of the time of 60° altitude passage for stars brighter 
    than mag 4 for every 1° of latitude to the 1/100 of a second
    
    Before you start work on that table, it would be a good idea to do a
    basic functional test on the astrolabe with the computational tools of
    conventional celestial navigation.
    
    I doubt your table specification can be attained unless custom generated
    for a given set of conditions. For example, annual aberration causes the
    coordinates of every star to vary by tenths of a minute of arc
    throughout the year. This is easily visible at Nautical Almanac
    precision. Stars separated by a small angle are similarly affected, but
    you'll be measuring stars tens of degrees apart.
    
    If the astrolabe works as well as you say, deflection of the vertical
    (often several seconds of arc) should be taken into account, so the
    table will be specific to the observing site.
    
    Then there's precession, which is about 50 arc seconds per year, and
    nutation, which superimposes a wobble of about 20 seconds. And Earth's
    variable rotation rate ... maybe you need to account for polar motion as
    well ... it gets ugly to crystallize all this in a table. The parameters
    keep changing.
    
    Getting the parameters is not a problem. For example, the National
    Geodetic Survey has an online calculator for the deflection of the
    vertical at any point in US territory. IERS Bulletin A at the US Naval
    Observatory site has daily values for Earth orientation (UT1-UTC and the
    polar motion angles). Existing models predict precession and nutation to
    better than a millisecond of arc, years in advance. And so on.
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    NavList is a community devoted to the preservation and practice of celestial navigation and other methods of traditional position-finding. We're a group of navigators, navigation enthusiasts and hobbyists, mathematicians and physicists, and historians interested in all aspects of navigation but primarily those techniques which are non-electronic.

    To post a message, if you are already signed up as a NavList member, start a new discussion or reply to any posted message and use your posting code (this is a simple low-security password assigned when you join). You may also join by posting. Your first on-topic messsage automatically makes you a member, and a posting code will be assigned and emailed to you for future posts.

    Uniquely, the NavList message boards also permit full interaction entirely by email. You can optionally receive individual posts or daily digests by email, and any member can post messages by email (bypassing the web site) by sending to our posting address which is "NavList@NavList.net". This functionality is similar to a traditional Internet mailing list: post by email, read by email, reply by email. Most members will prefer the web interface here for posting and replying to messages.

    NavList is more than an online community... more about that another day.

    © Copyright notice: please note that the rights to all messages and posts in this discussion group are held by their respective authors. No messages or text or images extracted from messages may be reproduced without the explicit consent of the message author. Email me, Frank Reed, if you have any questions.

    Join / Get 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