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We live on a sphere whose axis of rotation or spin is tilted compared to the axis of revolution around the Sun. This makes light from the Sun hit the surface of the Earth at different angles at different times of the year. Combined with the fact that we rotate a given area of Earth is exposed for different lengths of time each day as well.
During June- August the tempertures are hotter in the Northern Hemisphere and cooler in the Southern hemisphere. The Northern hemisphere is tilted towards the Sun during those months. This makes the Sun appear more dirctly above the horizon and the light hits the ground at an angle almost 90 degrees. This maximizes the light and heat being absorbed. Think about a piece of paper and a lamp. The paper is illuminated best when the paper is held vertically and worst when the paper is held horizontally. In addition due to the tilt the Northern Hemisphere is illuminated over 12 hours per day. The Southern hemisphere is more like the paper held horizontally and most of the day is spent in the shadow side of the Earth. In Dec-Feb the opposite occurs because the Northern hemisphere is now in a position where it tilts away from the Sun (horizontal position) and is illuminated less than 24 hours.
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