Almost entirely by electromagnetic radiation, ignoring gravity's tidal effects (which are measurable, but small by comparison.)Transferring thermal energy can be done by 3 basic physical processes:...
It's slightly less in July, when the earth is farthest from the sun, and slightly more in January, when the earth is closest to the sun. But these differences are very small.
It doesn't. In fact, due to it's elliptical orbit, Earth is closer to the sun in the northern hemisphere's winter than the northern hemisphere's summer.
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