If you don't know that the Earth is a sphere, wrongly believing our planet to be flat, don't tell everyone on your Twitter feed and definitely don't make a song about it, or Neil Degrasse Tyson might come after you and respond in kind.

Earlier this week, rapper B.o.B publicly questioned a spherical Earth, throwing his support behind flat Earth theory in a series of tweets and memes and suggesting a NASA cover-up.

Discounting any photo taken by a NASA spacecraft - an untrustworthy source, according to B.o.B - and setting aside any images captured by other space agencies around the world or photos taken by commercial satellites while we're at it, how do we know for fact that the Earth is spherical?

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The ancient Greeks figured it out 2,500 years ago without having to fly into space and take a snapshot. How hard can it be?

Pythagoras in 500 B.C. provides the earliest arguments for why the Earth is round. The moon, he noted, was also a sphere, so it would follow that the Earth is round as well.

A century and a half later, Aristotle put forward what was likely the physical evidence that the Earth is round, according to the American Physical Society. He noted that ships sailing over the horizon disappear hull first. He also observed that the Earth casts a round shadow on the moon during an eclipse. The stars also change position at different latitudes, which also points to a spherical planet.

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"All of which goes to show not only that the Earth is circular in shape, but also that it is a sphere of no great size: for otherwise the effect of so slight a change of place would not be so quickly apparent," .gsfc.nasa.gov/stargaze/Saristot.htm">Aristole wrote in "On the Heavens."

One hundred years after Aristotle, the ancient Greek mathematician Eratosthenes put forward the first scientific estimate of the circumference of the planet using nothing more than shadows and working alongside a team of bematists, professional surveyors in ancient Greece. His estimate of 250,000 stadia (28,738 miles or 46,250 kilometers) is close to Earth's actual circumference of 24,902 miles (40,075 kilometers) if measured at the equator.

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The scientific basis for the Earth being a sphere is often misattributed to the era of Christopher Columbus, whose crew had feared they would sail over the edge of the planet, the myth goes. In reality, Columbus, who would certainly have studied Aristotle, would have known full well that Earth is round. His expedition was initially controversial instead because the scientific advisers to King Ferdinand believed the journey west from Spain to India was too great a distance based on estimates of Earth's size.

Flat Earth theory is a modern phenomenon, beginning in the 1800s with the work of Samuel Birley Rowbotham, founder of the Zetetic Society, which would later inspire the Flat Earth Society in the 20th century, according to LiveScience.

Earth looks and feels flat after all from our everyday, ground-level viewpoint. Rowbotham envisioned the planet as a disk with the sun, moon and stars all floating just a few hundred miles away. The Flat Earth Society believes gravity is an illusion, the space program is a hoax, and world governments are conspiring to keep the truth from surfacing for a still-undetermined reason.

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Simple observations have determined the shape of the Earth for thousands of years, and there's plenty more evidence to support that fact. Gravity, time zones, GPS, air travel, telescope imagery of other planets in our solar system all point to a round Earth. But even in the face of overwhelming evidence, is there any chance a fringe pseudo scientific movement based on a vague conspiracy kept alive by a small online community and now endorsed by a famous rapper might be right after all?

Here's one astrophysicist's answer.

Photo credit: NASA