The sun is the primary and most important source of energy for the natural processes of the earth. The sun warms the planet, drives the hydrologic cycle and generally makes life on earth possible. The sun's energy warms the surface of the earth, helps the process of evaporation, photosynthesis and atmospheric circulation. Other energy sources of the earth are the internal heat that arises from the core of the earth itself, and the gravitational forces that attract masses on earth.