Evolution of the Lunar Orbit: Revisiting Earth's Tidal Records

Measurements made using reflectors that Apollo astronauts placed on the Moon show that the Moon is receding from the Earth at the rate of 3.82 ± 0.07 cm per year. If this rate is assumed to have been constant throughout geological time, then it can be calculated that the Moon must have been unacceptably close to the Earth only 1.6 billion years ago. This catastrophe is avoided by invoking an increase in the rate of recession with geological time. The principal cause of this increase in rate is thought to be changes in the way Earth's ocean tides dissipate energy.

There is no easy way to calculate this change in dissipation rate, even over timescales as short as 20,000 years. However, the rising and falling of the ocean tides may be recorded by sedimentary layering (tidalites) or by growth increments in shells and other kinds of mineral skeletons. These different kinds of proxy records have long been used to validate numerical approximations that attempt to reconstruct the history of the Moon's orbital evolution, but these two endeavors have gradually drifted apart. For example, Bills and Ray (1999) make no mention of the proxy constraints in their synthesis of studies of lunar orbital evolution, and their plot of lunar semimajor axis versus time bypasses the best of the geological constraints. Conversely, there are aspects of the physics of the Earth-Moon system that have not been considered by those working on the proxy records. Therefore, a re-evaluation of reported proxy records has been undertaken in an attempt to meld the two approaches to understanding the history of the lunar orbit.