Could someone explain the idea, method of how earth orbit was derived, in non-mathematical terms (well, some math is fine too)? I am more interested in the (ingenious) engineering/experimental approach behind it. I guess it's related to or requires the determination of earth mass, and from the thread by that name I got the picture of how earth mass was first estimated and then refined. Now, what about earth's orbit? It's distance to Sun, Sun's mass, etc.? Was Sun's mass first estimated using its size estimate (from eclipse data) and gas density? Thanks in advance.