* This is the ratio of the mean tropical year to the mean solar day; We use means because both the length of the day and the length of the year vary.
The period of rotation (day) of the planet varies due to tidal friction (the moon "drags" oceans against the earth's rotation) and other shifts in mass (e.g. earthquakes) that change the moment of inertia.
https://en.wikipedia.org/wiki/Tropical_year
* This is the ratio of the mean tropical year to the mean solar day; We use means because both the length of the day and the length of the year vary.
The period of rotation (day) of the planet varies due to tidal friction (the moon "drags" oceans against the earth's rotation) and other shifts in mass (e.g. earthquakes) that change the moment of inertia.
https://en.wikipedia.org/wiki/Leap_second#Slowing_rotation_o...
The period of revolution (year) varies mainly due to precession of the equinoxes.
https://en.wikipedia.org/wiki/Axial_precession