Has the Earth's rotation ever speeded up in the past?

Probably, but during the last 900 million years, any speed ups have been superimposed on a more or less steady slow down in spin rate. Even now, we can detect how the rotation rate of the Earth changes fast and slow by milli seconds per day depending on how the mass distribution of the Earth and its atmosphere change from earthquakes and the movement of water and air.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY Education and Public Outreach program.