Where are you getting 8000 from? The Erathostenes experiment interpreted on a Flat Earth tells us that the earth has a diameter of about 25,000 miles.
Ah, you're mistaken. No it doesn't.
Eratosthenes (and you REALLY need to learn to spell his name properly - it's wrong in the Wiki too) did an experiment which can have two possible conclusions.
EITHER:
1) The Earth is round and has a radius of around 3,500 to 4,000 miles - and the sun is very much further away than that - at least millions of miles.
...OR...
2) The Earth is flat, and the sun is 3,500 to 4,000 miles away.
But you don't get to use BOTH interpretations - pick one or the other.
Eratosthenes could
only knew the radius of the earth because he assumed it to be round. If you take away his round-earth assumption - and replace it with a flat-earth assumption - then he would have had no idea how big the Earth was. How could he possibly have known? Heck, even you Flat Earthers don't know.
In fact, there is a third possible interpretation that NOBODY likes:
3) That the Earth is round - but it's radius is larger than 4,000 miles - and the sun is not all that far away, but it's beyond 4,000 miles.
...this one sucks because you can't measure either the Earth's radius or the Sun's distance unless you assume that either one or the other is essentially infinite.