NavList:

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

HOME

Re: Shadow sights for longitude
From: David Fleming
Date: 2018 Jan 12, 19:40 -0800

Mark, I meant to present an idea that would open a shadow instrument up to in principle sextant level accuracy.

A backstaff or an astrolabe had edges which produce shadows as John Brown is doing and suffers from umbra/penumbra uncertainty ie. light from all points of the sun showing at the same points in the measurement plane.

If instead of an edge we use a pinhole, then we will get an image of the sun which gets bigger the further we get from the pin hole. I then meant to suggest we could use say a hat pin or a small bead on a thin rod as an inicator to mark the center of the image.

I've attached a sketch of what this might look like. You would level the device on a tripod and change position (L) of pinhole to produce an image of the sun on the paper with the bead centered.

Cheers,

Dave

On Fri, Jan 12, 2018 at 5:33 PM, Mark Knudsen <NoReply_Knudsen@navlist.net> wrote:

I am familiar with pin holed cameras have taken many photos with them. I could use a little more detail for the use of a ball .it sounds interesting I am very interested in simplified practical navagation methods on land thanks mark

On Jan 12, 2018 4:31 PM, "David Fleming" <NoReply_Fleming@navlist.net> wrote:
Increaseing shadow length will not improve the accuracy. As has been noted the accuracy is limited by the sun not being a point source of light. A sextant or the observers eye in the case of a sextan sight taken without a telescope produces an image of the sun atop the horizon, so that the angle to a specific point on the sun is measured.
If you wish to achieve such accuracy using shadows, then build a pin hole camera. And position a small ball in the center of the image. Then the angle of the ray from the pinhole to the ball relative to the horzontal will more accurately duplicate a sextant height measurement.
The reference to longitude is only due to John's taking prime vertical sights. Apply to meridian transit sights and you have latitude.
DaveF

File:

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.

Subject:
Author:
Start date:	(yyyymm dd)
End date:	(yyyymm dd)

NavList:

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

Compose Your Message

NavList

What is NavList?

Join / Get NavList ID Code

Retrieve a NavList ID Code

Email Settings

Custom Index

Add Images & Files
Name or NavList Code:	Email:

Name:
	(please, no nicknames or handles)
Email:

NavList ID Code: