User talk:Rbarline
Knowing the time when measuring earths circumference. Don't need anything more than a sun dial .. You can tell the time from the shadows .. Midday was easy, if the direction was known. (which way is north ? I'm hoping you know how easy that is !). When he says there was no shadow, and that it occurred there only once a year, he could say that it was definitely the solstice at noon, and the place was on the tropic.
It doesn't have to be calculated using a place on the tropic, but the idea that it was first done using ONE point at the tropic SUGGESTS the though process.. the stimulus.. that lead to the result. He was considering WHY the shadow would not exist at the tropic at midday on its solstice, and why the shadow would not completely disappear,ever, further north, and why the shadows are longer the further north.. that rate shows something.. the difference has some meaning. He also realised that having measurements done at precisely the same time was not important. He realised that if they were done at midday, which is to say that there was no east or west component of the shadow, that was enough.