The Earth is the largest and most massive of the inner group of planets. Data are given in Table 6.1. In the Solar System, only the Earth is suited for advanced life of our kind; it lies in the middle of the ‘ecosphere’, the region round the Sun where temperatures are neither too high nor too low. Venus lies at the extreme inner edge of the ecosphere, and Mars at the extreme outer edge.

The Earth–Moon system is often regarded as a double planet rather than as a planet and a satellite. The effect of tidal friction increases the Earth's axial rotation period by an average of 1.7 m s–1 per century.

STRUCTURE OF THE EARTH

The rigid outer crust and the upper mantle of the Earth's globe make up what is termed the lithosphere; below this comes the æsthenosphere, where rock is partially melted. Details of the Earth's structure are given in Table 6.2. The crust has an average depth of 10 km below the oceans, but down to around 50 km below the continents. The base of the crust is marked by the Mohorovičić discontinuity (the Moho) named after the Jugoslav scientist Andrija Mohorovičić, who discovered that the velocity of seismic waves changes abruptly at this depth, indicating a sudden change in density. Between 50 and 100 km below the surface the lithospheric rocks become hot and structurally weak.