It is always cool to repeat classic experiments. How many of us know the radius of the Earth because we determined it on our own? I haven't done this, at least not yet. For all my life I have just ...

I don't know where he got this idea (surely not from the internet), but Eratosthenes estimated the radius of the Earth by looking at two shadows at two different locations on the Earth. This diagram ...

Barry Lienert, a geophysicist at the University of Hawaii, provides the following explanation. We start by determining the mass of the Earth. Issac Newton's Law of Universal Gravitation tells us that ...

If you step outside anywhere on the surface of the Earth, there's no obvious signs that tell you the Earth isn't flat. It looks flat pretty much anywhere you go, including -- as Shaq recently noted-- ...