Why is the Earth round?

When Earth formed about 4.5 billion years ago, its growing mass created strong gravity that pulled material inward from all sides. Any large bump would be dragged down over time, so the planet settled into the most compact shape possible, a sphere, a state called hydrostatic equilibrium. Objects need to be several hundred kilometres across before their gravity is strong enough to do this, which is why small asteroids stay lumpy like potatoes. Earth is not a perfect ball, however: its spin makes it bulge at the equator, where its diameter is about 12,756 kilometres, roughly 43 kilometres wider than from pole to pole. The Greek scholar Eratosthenes measured the planet circumference with remarkable accuracy around 240 BC using shadows.