In the frame of reference, the days are now created by the daily circulation of the Sun. On the other hand, the change of seasons is caused by a periodic rising and falling of the solar arc: Every day the Sun's circulation slides slightly up until it reaches its highest / lowest point, then returning from to day to day until it has reached its lowest / highest point (periodicity: half year).

In the frame of reference, the solar arc is wobbling back and forth on the imaginary EW-axis (see repeated illustration). The wobbling also causes the seasons on the southern hemisphere of the Earth to be the reverse of those on the northern hemisphere. From this you can deduce that the Sun and the Earth are involved in yet another movement - an annual one - which, of course, should also be a uniform motion for the frame of reference relative to a rotating Earth, just as in the case of the solar arc.

The problem is, however, that if the annual orbit of the Sun would be uniform relative to the uniformly rotating Earth, the position of the solar arc would be the same every day. The plane of motion of the Sun would then be perpendicular to the axis of rotation or the north-south axis of the Earth. However, because of the facts that the wobbling causes seasons to emerge, and that the seasons on the southern hemisphere of the Earth are the opposite of those on the northern hemisphere, this angle can not be perpendicular and the annual orbit of the Sun cannot be uniform in reference to the uniformly rotating Earth.

The inequality of the days - and hence the existence of seasons - suggests that the Earth rotates uniformly in reference to an accelerated moving Sun.

Note 7 On the basis of the relative motions of a large number of stars, it is now known that our Sun orbits the centre of our Milky Way at a speed of about 220-250 km / s.

Continue to: 2.4. Exercise with Reference Frame Failed?