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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@fer3.com> 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@fer3.com> 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

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