During my schooling, I was taught that summer and winter are caused due to earth getting either closer or far away from the sun respectively during its orbit to the sun. For years, I believed that this is true. Today, while on my way to the office, I tried to gaze towards the burning sun and suddenly a thought stuck to my mind. I recollected the discussion we had in a conference call couple of days back about the temperature at Hyderabad, India and Redmond, USA. Surprisingly, Redmond was having spring season whereas Hyderabad is having summer season. If what I was taught at school was correct, the earth should have same season irrespective of whether it’s India or USA. I tried to push my memory a little further to recollect, what are the time lines when earth is closest and farthest to sun. Earth happens to be closest to sun during January and farthest during July. For some reason, it appears that what I was taught at school doesn’t fit the equation!
A little research on the published articles revealed, that theory that says seasons are caused due to distance of earth from is sun is indeed false. The actual truth is something else. Apparently, the major factor that causes the seasons is tilted axis of Earth to the orbital plane. The earth’s axis is not perpendicular to the orbital plane and it actually 23.5° titled.
Here’s where the things get little complicated, probably one of the reasons why my teachers opted for a wrong but easy explanation instead of deep analysis. The tilt in the axis produces two important effects
1. Depending on the duration of the year, the North Pole and South Pole are either pointed towards the sun or away from the sun. Note that if the North pole is pointing towards the sun, the South pole is away from the sun. This is the cause for seasons on Northern and Southern Hemispheres of earth to be opposite.
2. Due to the tilt in the angle and the curved surface of earth, the angle of incidence of sun rays changes during the course of year causing the change in seasons. Note that, if the angle of incidence of sun rays is perpendicular to earth’s surface, the earth’s surface absorbs the maximum heat.
This figure taken from Jack William's The USA Today Weather Book gives a remarkable good representation of length of days at different seasons.