https://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_the_Earth.27s_circumference

It's an old method and quite accurate.

Eratosthenes' calculations are not proof of any particular model, but based on an assumption of which earth model is correct.

The reason I ask about the Northern and Southern lights is because in conventional physics it is believed that these phenomena are the result of the interplay between the Earths EM field and massive particles slung out from the sun. It is quite well understood and basically means that you can use solar "weather forecasts" (since light moves faster than particles with mass) to tell you if/when there will be northern/southern lights appearing. But in order for this to be true, something needs to create the EM field. A spinning liquid metal core; which doesn't fit well with your hollow earth assumption.