The first definition of earth in the dictionary is the third planet from the sun, the only planet on which life is known to exist. It is not quite spherical, being flattened at the poles, and consists of three geological zones, the core, mantle, and thin outer crust. The surface, covered with large areas of water, is enveloped by an atmosphere principally of nitrogen, oxygen, and some water vapour. The age is estimated at over four thousand million years. Distance from sun: 149.6 million km; equatorial diameter: 12 756 km; mass: 5.976 × 1024 kg; sidereal period of axial rotation: 23 hours 56 minutes 4 seconds; sidereal period of revolution about sun: 365.256 days related adjectives terrestrial tellurian telluric terrene. Other definition of earth is the inhabitants of this planet. Earth is also the dry surface of this planet as distinguished from sea or sky; land; ground.