The well-known rapper B.o.B has been on a mission to convince his Twitter followers that the Earth is flat. 

How can a 16-mile distance between two points in a picture look flat, he questioned: "Where is the curve? please explain this." A torrent of similar tweets followed.

Many people have tried to reason with him, including the astrophysicist and broadcaster Neil deGrasse Tyson. B.o.B has not been swayed

We at BBC Earth wouldn't usually weigh in on conspiracy theories, but we couldn't let this one go. For one thing "Earth" is in our name, and anyway we think it's fun to know how these big facts about the universe got discovered.

Let's start with some pictures. Here is the Earth from one million miles away. This image was composed from a series of images taken by a NASA camera on the Deep Space Climate Observatory satellite.

And here is a picture of the Earth from the Moon. The Moon, as we can all see when we look up, is also round. 

None of this concerns B.o.B, because he also claims that all such NASA imagery is faked.

You will have your own view on the plausibility of that, but regardless, consider this: we knew the Earth was round long before we mastered space travel.

Over 2,000 years ago, the Greek philosopher Aristotle had it all figured out.

The fact that Earth is round has been common knowledge ever since

In his book On the Heavens, he wrote: "Again, our observations of the stars make it evident, not only that the Earth is circular, but also that it is a circle of no great size. For quite a small change of position to south or north causes a manifest alteration of the horizon."

In other words, you see different sets of stars in the night sky depending on where you are. The sky over the northern hemisphere is not the same as the sky over the southern hemisphere. If the Earth was flat, then at any given time we would all see the same stars, and we don't.

Another Greek thinker and mathematician, Eratosthenes, went further and managed to measure the Earth's circumference.

He discovered that at noon in one Egyptian city, the Sun was directly overhead, whereas in a different city the Sun did not rise quite so high. Eratosthenes knew the distance between the two cities, measured how high in the sky the Sun rose to in each at the same time, then did some trigonometry. His method was crude, but his answer was in the right ballpark.

The fact that Earth is round has been common knowledge, at least among the educated and powerful, ever since.

More recently, people have gone all the way round the Earth. The Portuguese explorer Ferdinand Magellan famously circumnavigated the Earth from 1519 to 1522, which would have been even more difficult if it had had an edge.

But long before Magellan, it was obvious to observant sailors that the Earth is round. If you sail towards something tall, like a mountain, you will see the top of it appearing over the horizon before the rest of it.