8. The length of a day was ever approximately constant.
It is not easy to change a spinning rotation of a cosmic body. Practically, only a central collision with another cosmic body can do it. Proto-Mars has been destroyed 3.5 milliard years ago by Saturn (the rest of the cosmic “projectile” that has collided with Andrea-star, a stellar companion of our Proto-Sun). Later, the largest part of that Proto-Mars has collided with Proto-Earth (by the way, giving birth to our Moon). It is the reason, why Saturn, Mars, and the Earth have all the same inclination of their axes of rotation, and Mars and Earth have even (almost) identical rotation velocity (day length). Traditional geophysics has assumed that the reason for more days in a single year was the quicker rotation of the Earth millions of years ago. It was impossible (as discussed above). The reason is even simpler. The Proto-Mars—Proto-Earth collision has occurred at the distance of 1.33 AU (3.5 milliard years ago). Since then the Earth-Moon duo is approaching the center of mass of the Solar System (in Venus) with a small but constant speed of about 15 m/year. It causes the shortening of the Earth’s orbit and thus also the observed lowering of the number of days during a single year.
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