Did the Earth rotate faster in the past?

Yes...There is actually geologic evidence for this in certain rocks which show a banded structure caused by water tides. About 10 years ago geologists discovered evidence for the day being shorter using some change in a signature, probably the layering widths, they found in ancient rocks dated from a few billion years ago. Apparently a 'day' back then was about 18-20 hours long, not 24-hours. In the future, billions of years from now, it will lengthen to 50 hours or longer.

The above figure shows the measured slow down of earth's rotation since 1890. You can see that it is not a regular decline. If we take a much longer baseline, the results is strikingly different. The graph below from researchers Mitrovika et al (Science Advances  11 Dec 2015: Vol. 1, no. 11, e1500679 DOI: 10.1126/sciadv.1500679 ) shows the clock error from 500 B.C.E. to 1600 C.E. inferred from untimed partial solar eclipses (magenta; arrows reflect allowable bounds) and untimed total and annular solar eclipses (blue lines). The green line represents the clock error associated with the slowing of Earth’s rotation due to tidal dissipation (TD)

A good discussion about the changing length of the rotation period of the Earth can be found at the US Naval Observatory in their essay on leap seconds. The '24-hour' day actually increases by 0.0014 seconds every day, per century. Every year or so, one second has to be added to the official civilian day on New Years Eve.

Return to Dr. Odenwald's FAQ page at the Astronomy Cafe Blog.