Shadows and gnomons
In the third century BC Eratosthenes using the shadow of a grounded
vertical rod (gnomon) calculated with a margin of error of 5% the
Earth’s radius.
His experiment led him to travel from the city of Siene to Alexandria
For those times it was an extraordinary result, but if it had a valid
measuring instrument that measured the distance between the two cities
and if the second measure had taken it 4 years after the first, the
margin of error would have been almost nil.
It is understood that using shadows over time is a practically perfect
tool because it is based on a constant that is the revolution of earth
around the sun and around its axis.