Solar energy Northern spring/ Northern winter/ Southern fall 21 March Southern summer Periapsis Line of Solstice 21 June 21. Decemb Apoapsis 3. July 23 September Northern summer/ Northern fall/ Southern winter Southern spring The earth, at 93 X106 miles(1.49 X 10 meters) from the sun, makes a complete revolution about the sun once per year Itself rotates on its north-south axis once every 24 hours 23.5 degrees from the perpendicular to the plane of its orbit about the sun 复旦大学环境科学与工程系 Department of Environmental Science and Engineering, Fudan UniversityThe earth, at 93×106 miles (1.49×1011 meters) from the sun, makes a complete revolution about the sun once per year. Itself rotates on its north-south axis once every 24 hours. 23.5 degrees from the perpendicular to the plane of its orbit about the sun. Solar Energy