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When the earth revolves around the sun it spins around its which is tilted at 23.5 degrees. It is actually the tilt in the earth's axis and this is what is responsible for creating the seasons in the hemispheres. So if you think about it, if a particular hemisphere is tilted toward the sun it is summer, but the other hemisphere would be winter because it is tilted away from the sun. Example: When the northern hemisphere is tilted towards the sun it would be summer and the southern hemisphere would be winter.
its got to do with the intensity of the light as it hits the earth
think of a torch pointing straight at a wall with a circular bright spot on the wall
as opposed to
the torch pointing at angle to the wall with an oval less bright spot on the wall
it is the incident angle of the light that makes all the difference. a square metre of land in summer time will receive far more energy from the sun that the same square metre in wintertime [assuming in each case that the sun is shining on it!!]