From: John A Brown
Date: 2018 Jan 14, 00:01 -0800
Hello to all,
When I started experimenting with shadows my idea was to adopt a minimalist approach using a ruler no longer than about 8 cms and a multi height gnomon made from no more than 7 blocks of Lego with a height of 66.9 mm By increasing the height of the gnomon the problem of umbra and penumbra is introduced and although it still occurs to a lesser extent with a lower height gnomon it can be overcome with experience.
If I had not known about the semi diameter of the sun I would still have got the same result when calibrating my piece of ruler and I guess if there are other things that I don't know about they have been taken care of too. I wonder if when Thales posted on the Pyramid Lovers Forum that he had found the height of their pyramid with just a stick and its shadow they would have found a reason for why it would not work! Talking of Greeks, they had a beautiful word for shadow catcher - Skiotheron. If you put this word into Google and look at the reference Dodwell chapter 2 you will find some really interesting information on the use of the gnomon in the ancient world and experiments that were carried out many years ago in my home town of Adelaide.
To address the idea that an increase in gnomon height increases accuracy I have attached photos of sights taken this morning. This is probably as minimalist as you can get - about 16 mm of ruler and a gnomon of 19 mm. I managed to get within 1 minute of longitude.
The instrument is the mini shadow catcher MSC 122.2 R/W. If only it happened every time as well as this!
Best wishes to all,
John A Brown.