It is because the earth is round. It's actually quite oblong but it's mostly round. The heat rays of the sun hits the Earth in different latitudes. Take the war battles for a model. The ones who get hit first are the front liners. The ones closest to the enemy battle lines. There's a big massacre there. The front liners of our planet are the countries nearest the equator. It is nearer to the sun.These are the ones called tropics. This is where desserts are found. The ones in the middle are the countries that has the 4 season period. It's season is determined with the position of the Earth during its revolution. When it is closest to the sun it is summer. When farthest, it is winter. When the northern hemisphere has the winter season, the Southern hemisphere must have the summer season, and when fall in the northern, spring in the southern and vice versa. This part, since in the middle, will have less heat to fight, depending on the tiltation. The last liners have very little to fight. And thus it resulted to land full of frozen water, ice. The queerest part though, is how the seasons are called. The two seasons are the light season and the dark season. They experience 6 full months of pure daylight and 6 months of darkness. In case you're wondering, The front liners are the ones in the middle of the Tropic of Cancer and Tropic of Capricorn. The middle liners are the ones in the middle of (northern hemisphere) Tropic of Cancer and Arctic Circle, and in the middle of (southern hemisphere) Capricorn and Antarctic Circle. The last liners are the ones in the inside circle of (northern)Arctic Circle and, (southern)Antarctic Circle.