The speed of rotation of the Earth and hence the length of the day was determined many years ago, about 4.6 billion years in fact, by how much mass of star dust coalesced to form the Earth and the quantity of rotational energy transferred from that dust. This rotational speed has been decreasing very slowly since the moon split off from the Earth and started the tides in the oceans. These tides act as a drag and so are slowly reducing the rotational speed and hence the day is getting longer. The friction of the oceans in their basins is also slowly warming the Earth, but not by very much.
The rotational speed of a spinning object can be varied by frictional drag, as we have seen but it can also be affected by moving the object’s mass closer to or further away from the centre. This can be seen when a spinning skater speeds up by bringing her arms in closer to her chest.
The same effect can be seen on the Earth by cutting down trees, a laid down tree has its mass closer to the centre of Earth than does a growing one. As the Earth is progressively deforested then this effect will increase and so the day will get shorter.
One more argument for growing trees and not chopping them down.
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