The earth revolves around the sun in the speed of 30 kilometers per second, around the sun takes 365 days to five forty-eight 46 seconds, there is just one year, called the tropical year on astronomy. The earth revolves around the sun's orbit is an ellipse, the long diameter and short diameter difference, can be approximated as a perfect circle. The sun was a focus of the ellipse, and the focus is not on the ellipse center, so the distance between earth and the sun is sometimes a bit closer, sometimes a little farther. At the beginning of the January, the nearest to the earth, 147100000 kilometers, it is called perihelion. Seven early Earth farthest from the sun, is 152100000 kilometers, it is called aphelion. In fact, when the earth is at perihelion, the northern hemisphere winter, the southern hemisphere summer at aphelion; when, the northern hemisphere summer, the southern hemisphere winter. This shows, the changes of the seasons and the perihelion and aphelion independent
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