I recently watched a video that explained how the length of the day was impacted by where the Earth was at in its orbit around the Sun. Consider a "day" for a point on Earth the period from when the Sun is at a highest point to the next time the Sun is at a highest point. The length of this day varies over the course of the year, getting longer when Earth is closer to the Sun and shorter when Earth is furthest from the sun.

See Vsauce: How Earth Moves

However, I got to thinking. As the tidal effects from the Moon and Sun change the shape of the Earth (most directly through the oceans), the moment of inertia must also be changing and therefore the speed of the Earth's rotation must be varying.

PLEASE NOTE: I'm not referring to the decay of rotation due to friction caused by the tidal effects.

My question(s) is(are):

  1. Is this variance in rotational speed periodic? Chaotic?
  2. Is it detectable?
  3. Could it have an impact on our calendar? If so, over what time frame?

1 Answer 1


Ocean Tides and the Earth's Rotation

There are two ways that the ocean tides can cause such rapid variations. (1) As the tides move water around the globe, the moment of inertia of the earth changes. By conservation of angular momentum, the solid earth changes its rotation rate accordingly. (2) As the tidal currents slow down or speed up, they exchange angular momentum with the solid earth, which is manifested in the rotation rate. enter image description here
Rapid variations in Universal Time as measured by VLBI and as predicted by a numerical ocean model


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