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
    Shadow stick trivia question
    From: John Huth
    Date: 2009 Oct 15, 19:35 -0400

    Last week I had my class trace out the path taken by the shadow of a gnomon over the course of a day.   The results were surprisingly straight lines.     My initial hunch was that it would be a hyperbola, but since the main goal was to determine east-west (and N+S) with a sun compass, I wasn't so much worried about the rising and setting positions of the shadow-tip.

    So, I thought a bit about it, but hadn't done the actual math of the projection versus latitude and time of year.   We're at 42 degrees North here.   

    My logic was this - at the summer solstice, since the sun rises N of E and sets N of W, you'd get a hyperbola that "points south (meaning the ends of the shadows at morning and evening point south).    Then, during the winter solstice, the hyperbola "points north" since the sun rises S of E and sets S of W.    Good, so far? 

    OK, what about the equinoxes? the sun rises due E and sets due W, but since we're at 42 degrees north, it's still going to be a "south pointing" hyperbola since the stick will have a shadow that's north of an east-west line at meridian passage.

    It would then seem that there is some day, on the "winter side" of the equinoxes where the shadow-stick is close to a straight line for my latitude.   

    Has anyone looked into this phenomenon?   I should not be lazy and perhaps do the math, but perhaps someone in this group has done the math.

    Now, before George H. jumps over me, I realize that the solar declination will indeed change over the course of the day and it will *never* be precisely a straight line, but there's some moment, that, if your froze the solar declination for a day, you'd get a straight line. 

    Thanks in advance for any wisdom!

    John Huth

    --~--~---------~--~----~------------~-------~--~----~
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
    To unsubscribe, email NavList+unsubscribe@fer3.com
    -~----------~----~----~----~------~----~------~--~---

       
    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