How accurate can the shadow of a large sundial be?

[gary350]

Can't find the original information but I found this, more accurate plus it lists solar noon as 12:50:54pm

It is interesting solar noon changes from 12:50 to 12:51 then 12:52 then 12:51 and so on. Some where I read this has something to do with Earth wobble & orgit not being perfect circle.

https://sunrise-sunset.org/us/murfreesboro-tn

 

[Baluncore]

Some where I read this has something to do with Earth wobble & orgit not being perfect circle.

Take a look at this page on wikpedia. It shows the time corrections and height of the Sun.
https://en.wikipedia.org/wiki/Equation_of_time#Graphical_representation

 

[Jonathan212]

Answering my own question, the -6 dB is at v = 0 below. Ie even in the straight line from a point source to the knife edge and to an aligned observer there is already attenuation by a factor of 4.

where v is defined as

where in the case of the sundial we plug in d1 = infinity. Source: http://www.waves.utoronto.ca/prof/svhum/ece422/notes/19-diffraction.pdf

But what happens at v = 1 where diffused light is 14 dB or 25 times weaker? The v = 1 isosurface of d2-versus-h is actually parabolic, not flat (d2 = 2/wavelength * h^2). A bit of a surprise:

So putting the observer a long way from the edge does not spread light very much, h does not increase in proportion. At d2 = 10 metres, h is still just 2 millimeters for red light. The sun's transition with the sun's diameter of 0.5 degrees would be 87 mm wide at the same distance.