Thank you for the opportunity to help you with your question!
The answer is Obliquity,
the Earth's axis' tilt relative to the ecliptic, has the biggest impact on insolation, that causes the seasons. Eccentricity has little
effect, such that northern winters are a couple of degrees warmer compared to the southern
winters. Precession occurs over a 26000 year cycle, so actually contains almost
no measurable effect.
Please let me know if you need any clarification. I'm always happy to answer your questions.
Jun 14th, 2015
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