Do you really know how the earth is spinning around the sun? There is a lot going on there to create climate changes and seasons of different lengths and intensities on the different places on earth. I was not aware how many factors count into the equation if you want to calculate the exact length of a year… A nice scientific explanation in under 10mins:

httpv://www.youtube.com/watch?v=9Ml4_Jv_HkE

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