# Why Earth Is a Sphere By The unique one Topic: Earth & Geography [Open the interactive Jig](https://ask.jig.com/j/sGddNvYYMzXS) ## Guide ### 1. Why Earth Is a Sphere Every photo of Earth from space shows a ball\. Why not a cube, or a lumpy potato? The answer is a force you feel every moment: gravity\. Drag the scene to turn it and look around\. Earth is about 12,742 kilometres across\. ### 2. Gravity pulls to the middle Gravity is the pull that every bit of stuff has on every other bit\. Add up the pull of all Earth's rock and it points to one place: the centre\. It pulls the same way from every side, so no direction is special\. That is why a dropped apple falls straight down, wherever you stand on Earth\. ### 3. Rubble becomes a world About 4\.5 billion years ago, Earth grew from rocks and dust circling the young Sun\. Every new rock made the pile heavier, so its pull grew stronger and drew in still more\. Press the button to gather the rubble\. The crashes released so much heat that much of the young Earth melted\. ### 4. High places sink, low places fill On a world this big, gravity is stronger than rock\. Tall heaps are squeezed down and low places fill in, until every part of the surface sits about the same distance from the centre\. The only shape like that is a sphere\. Scientists call this settled shape hydrostatic equilibrium: gravity pulling in is balanced by the rock pushing back\. ### 5. Too small to be round Small worlds have weak gravity, so their rock keeps whatever lumpy shape it had, like a potato\. Slide the size up and watch gravity win\. Rocky worlds become round once they are roughly 600 kilometres across\. Icy worlds are softer, so they round up at a smaller size, about 400 kilometres\. ### 6. Spinning makes a bulge Earth also spins, once a day\. Spinning flings the middle outward a little, so Earth bulges at the equator, its middle, and is slightly flat at the poles, its top and bottom\. Spin it faster to see the effect, greatly exaggerated beside the pale perfect circle\. Saturn spins so fast that it is about 10% wider than it is tall\. ### 7. How round is Earth, really? At its true size, Earth's bulge is tiny\. Across the equator Earth is about 12,756 kilometres wide, and from pole to pole it is only about 43 kilometres less\. That is just 0\.3%, far too small to see from space\. Shrink Earth to a 1\-metre ball and it would be flattened by only about 3 millimetres\. ### 8. Mountains are tiny wrinkles Mount Everest rises almost 9 kilometres above the sea\. That sounds huge, but Earth's radius, the distance to its centre, is 6,371 kilometres\. On a 1\-metre ball, Everest would stand less than a millimetre tall\. Everest's official height is 8,848\.86 metres\. ### 9. Every big world is round The Moon, Mars, the Sun and every other large world are round for the same reason\. Gravity pulls everything toward the middle until a ball is all that is left\. Next time you see a full Moon, you will know why it is a circle\.