Chris is right. The rotation axis of the earth is tilted 23 degrees 
relative to the plane in which the earth is rotating around the sun. In 
the summer the pull of the sun is therefore more to the north (and 
rotating once a day in the frame of the observer) in the winter more to 
the south (north of the equator).

The argument against Chris' hypothesis is that there is also the moon 
pulling us, and this force is orders of magnitude bigger. The signal to 
noise ratio is therefore very small.

The next hypothesis might be that the magnetic field of the sun is 
involved. But this field changes polarity often (I think every 14 
month). So that is unlikely too.

I like Ari's comment. There is an observation. has anyone else seen that?

Jorg

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