Earth is the third planet from the Sun and the only world known to have life. It is about 12,742 km across, mostly covered by liquid water — about 71% ocean — under an atmosphere of nitrogen and oxygen that we can breathe. Earth orbits the Sun once every 365.25 days and spins once every 24 hours, and its axis is tilted 23.5°, which gives us the seasons. A protective magnetic field, generated by its molten iron core, shields us from harmful solar radiation. Earth has one large Moon that steadies its tilt and makes the tides. It sits in the 'habitable zone,' the just-right distance where water can stay liquid — the foundation of all life.