What causes Seasons ?
Seasons occur on the Earth because of the tilt of the Earth's axis. Because the Earth's spin is not exactly aligned with the Earth's orbit around the Sun, the Sun sometimes shines directly over the northern hemisphere, and other times shines directly over the southern hemisphere.
The Sun reaches its most northerly position (the Tropic of Cancer) on June 21. This is called the "summer solstice." If you were standing on the tropic of cancer on June 21, the Sun would be directly overhead at noon. This is the first day of summer in the northern hemisphere, and the first day of winter in the southern hemisphere. Because the Sun is shining more directly in the north than in the south, the weather is warmer in the north in June. In December, the opposite happens.
The Sun is over the Tropic of Capricorn, and is shining directly at the southern hemisphere. So December is summer in Australia and Brazil, but winter in America, Europe, and Asia.
Some people believe that summer is when the Earth is closest to the Sun. This is not true. In fact, the Earth is farthest from the Sun in early July, and nearest the sun in January. Because these differences in distance are so small however, they hardly affect Earth's seasons at all.