# When was it discovered that the Earth wasn't round?

We know the Earth isn't a sphere: that is, the equatorial circumference isn't equal to the polar circumference. When (and how) was this discovered?

I can put a lower bound of around the 3rd century BC, when Greek astronomy showed that the Earth was round rather than flat, and an upper bound of 1791, when the French Academy of Sciences specified a quadrant of meridian in the definition of the meter. An uncertainty of 2000 years is rather large, though.

• This should probably be tagged "geography" as well, but I don't have the reputation to create tags. – Mark Jan 28 '17 at 0:26
• could you reference your lower bound please? The Earth is very nearly spherical and I would have thought you could detect deviations only over large distances (larger than size of Mediterranean anyways). – user5245 Jan 28 '17 at 3:35
• I'm assuming that "the Earth's shape deviates from spherical" could not have been discovered before "the Earth's shape is a sphere", and the Greeks started accumulating (or at least recording) solid evidence for a spherical Earth in the 300s BC. – Mark Jan 28 '17 at 3:40
• agreed... It must have been by direct measurement. Local variations of $g$ would be subject to first order to local changes in densities, and measuring the variation would require a lot of technology. The navigational clocks weren't that good until the 1800s so even measurements over navigational distances are unlikely to be the clincher before the French. So by elimination that leaves you with a bound closer to your upper bound. – user5245 Jan 28 '17 at 3:50
• Would most people have realised quite early on, that the earth is not round\a sphere? I mean, there are plenty of mountains and valleys on the earth, which precludes it from being a sphere. And yes, I know what you mean, but it seems quite hard to draw a line here. – Gerhard Jan 29 '17 at 14:20