The earth is the largest of the inner or terrestrial planets. It is nearly 8,000 miles in diameter and nearly spherical. Its oblateness is minimal.   It can be seen as a series of 3 concentric spheres, increasingly more dense inwards: 
                                            1. the gaseous atmosphere, 
                                            2. the liquid hydrosphere and 
                                            3. the solid earth, whose outer layer is called the lithosphere.