[dotancohen]

I thought that the ancient Greeks considered the sky to be a 2 dimensional object: the inside of a sphere.

[antognini]

That conception was much older, probably in the early Archaic Age. By Plato's day it was believed that the planets were different distances from the Earth with the Moon being closest, Saturn being the farthest, and the sphere of the fixed stars being beyond that.

[yellsatclouds]

it sure does look like such a thing

then again, everything we see is majoritarily 2 dimensional (with a whisper or soft hint of depth)