[DB 1507 (39); OAB 60] It is claimed that tidal interactions between the Earth and the Moon are causing the Moon to move away from the Earth, and the Earth to rotate more slowly. This much is true, and in fact paleontological studies of ancient corals and stromatolites has confirmed that the Earth did rotate faster in the past, resulting in more than 365 days in a year. It is also true that such a faster rotation would have caused a much greater equatorial bulge in the past than currently exists. The fallacy is the assumption that such a bulge would have remained for us to observe today. The Earth's mantle, made up of rock subjected to high temperatures and pressures, acts like a fluid over long time periods — it does not hold its shape over billions of years. The current equatorial bulge is very close to what you would expect to be produced by the current rotation rate, although it is slightly larger because the Earth has not completely relaxed from previous times when it rotated faster.

 

A related question concerns the rate at which the Moon is receding from the Earth. If you simply extrapolate the Moon's orbit backwards in time, assuming that the rate at which it is currently receding has not changed, you find that the Moon would have been close enough for the Earth's gravity to pull it apart only 2 billion years ago. However, K.S. Hansen described a very plausible answer to this question (Reviews of Geophysics and Space Physics, v.20, no.3, pp.457-480 (1982)). He pointed out that the current Earth-Moon configuration contains a resonance which increases the efficiency of the tidal interactions that are causing the Moon to recede, and that therefore the Moon is currently receding faster than usual. In his computer models, by carefully keeping track of the changing tidal parameters as the Moon spirals away from the Earth, Hansen determined that the Moon would have been at an acceptable distance from the Earth 4.5 billion years ago (for a more detailed discussion, including more recent research based on Hansen's breakthrough, see Thompson (1999), http://www.talkorigins.org/faqs/moonrec.html).

 

Incidentally, a misunderstanding of how "leap seconds" work has led some people to grossly overestimate the rate of change of Earth's rotation. The U.S. Naval Observatory, along with other international agencies, adds a "leap second" to the calendar whenever they determine that Earth's rotation is out-of-sync with their atomic clocks. Properly understood, the rate of about one "leap second" every two years does not mean that Earth's rotation is slowing by a half-second every year. Rather, it means that Earth's rotation is consistently a tiny fraction slower than it was when the length of the second was rigorously defined, a discrepency that builds up over a year to a difference of half a second. If Earth's rotation were really declining measurably, we would expect to see "leap seconds" become more and more frequent, since every year the discrepency in year-length would be greater than it was the previous year. In fact, we do not see this. "Leap seconds" are due, not to a consistent decline, but to fluctuations in Earth's rotation rate about a mean value, which are caused by entirely different processes and have little long-term effect. On the other hand, the consistent deceleration of Earth by the Moon is so slow that it cannot be directly measured (physical calculations put it at about one second every 70,000 years), although it is corroborated by fossil corals that show more days per year in the past.