Insolation
The earth is  revolving around  the  sun  in  an  elliptical  orbit, resulting  continuous  change  in the distance between sun and the earth on annual basis. It leads to seasonal variation in solar energy received by  the  earth.The  mean  distance  between  the  earth  and  sun  is  about  149,600,000 kilometers(92,900,000  miles).When  earth position is  farthest(152  million  km)  from sun  is known as ‘aphelion’ on  July  4.  It  is  perihelion(147  million  km)occurs on January  3  each  year which  is  the  closest  distance. During  aphelion the  northern  hemisphere  is  facing  the  sun  and therefore  receives  energy  about  7  percent  less  than  the  perihelion  (southern  hemisphere).