How the Greeks Knew the Earth was a Sphere

‘As long ago as 340 BC the Greek philosopher Aristotle, in his book On the Heavens,
was able to put forward two good arguments for believing that the earth was a round sphere rather than a Hat plate. First, he realized that eclipses of the moon were caused by the earth coming between the sun and the moon. The earth’s shadow on the moon was always round, which would be true only if the earth was spherical. If the earth had been a flat disk, the shadow would have been elongated and elliptical, unless the eclipse always occurred at a time when the sun was directly under the center of the disk. Second, the Greeks knew from their travels that the North Star appeared lower in the sky when viewed in the south than it did in more northerly regions. (Since the North Star lies over the North Pole, it appears to be directly above an observer at the North Pole, but to someone looking from the equator, it appears to lie just at the horizon. From the difference in the apparent position of the North Star in Egypt and Greece, Aristotle even quoted an estimate that the distance around the earth was 400,000 stadia. It is not known exactly what length a stadium was, but it may have been about 200 yards, which would make Aristotle’s estimate about twice the currently accepted figure. The Greeks even had a third argument that the earth must be round, for why else does one first see the sails of a ship coming over the horizon, and only later see the hull?’

– Hawking. S. (1998) A Brief History of Time New York, United States: Bantam Books p. 2

The Human Race

The thinking ape
200,000 years ago

Our species, Homo sapiens, is ridiculously young. We have only existed for a fifth of a million years. In that time we have expanded from our African birthplace to reach every continent, and even outer space. Our activities have precipitated the sixth mass extinction and unleashed the fastest episode of climate change in Earth’s history. Yet we are also the only species that has ever managed to piece together the history of Earth.

See other: History of Life

The First Hominids

The road to humanity
13-7 million years ago

The first apes appeared in Africa around 25 million years ago. Then at some point, the group split into the ancestors of modern humans and the ancestors of modern apes. It is hard to say exactly when, but thanks to modern genetics and a host of fossil discoveries, we have a rough idea. The oldest known hominid was Sahelanthropus tchadensis, which lived about 7 million years ago.

See other: History of Life

C4 Photosynthesis

Supercharged plants
32-25 million years ago

Plants have been busily harnessing sunlight to make sugar for hundreds of millions of years – a process called photosynthesis. But fairly recently, some plants have found a better way to do it. C4 photosynthesis is far more efficient than normal photosynthesis, allowing C4 plants to cope with harsh conditions. Today scientists are trying to engineer rice to use C4 photosynthesis, to help feed the growing population.

See other: History of Life

Death of the Dinosaurs

The fifth extinction
65 million years ago

Boom. Extinction. 65 million years ago, a huge chunk of rock from outer space smashed into what is now Mexico. The explosion was devastating, but the longer-term effects were worse. Dust was thrown into the upper atmosphere and blocked out sunlight, and in the ensuing cold and darkness Earth suffered its fifth and last mass extinction. The dinosaurs were the most famous casualties, but pterosaurs and giant marine reptiles were also wiped out.

See other: History of Life

The First Mammals

Hairy beasts
220 million years ago

At the same time that the dinosaurs were spreading and diversifying, the first mammals evolved. Their ancestors were reptiles called cynodonts, whose faces looked a little like those of dogs and may have had fur or whiskers. Early mammals such as Morganucodon were small and shrew-like, and probably only active at night. This may have spurred them to evolve warm-bloodedness: the ability to keep their body temperature constant.

See other: History of Life

The Great Dying

Permian extinction
252 million years ago

Just as the reptiles were flourishing, life on Earth faced perhaps its greatest challenge. The Permian extinction was the worst mass extinction in the planet’s history, obliterating up to 96% of marine species and similar numbers of land animals. We don’t know for sure what caused it, but massive volcanic eruptions – creating what is now the Siberian Traps – may have been to blame. In the aftermath, the first dinosaurs evolved.

See other: History of Life


300 million years ago

For the last time, all Earth’s continents came together to form one giant supercontinent. Known as Pangaea, it was surrounded by a world-spanning ocean called Panthalassa. It lasted until 175 million years ago, when it began to tear itself apart over tens of millions of years. Its shattered remnants became the familiar modern continents.

See other: History of Life