Fossil stromatolite from Morocco (courtesy of Hans-Ulrich Seitz, Tübingen). Stromatolites are
microbial reefs created by cyanobacteria (blue-green algae). The annular deposits represent
lithified growth patterns with lunar periodicity. Stromatolites and mussel shells are useful
paleontological tools (trace-fossils) for determining variations in the length of the lunar cycle
going back to the precambrium (modified from Panella et al. 1968, Science 162, 796-792).
The length of the lunar month is shortening because the moon slows-down the earth’s
rotation mainly to the loss of energy due to tidal torque in shallow seas. The deceleration rate
has not been constant through time, probably because the distribution of oceans and
continents changed in the past (from the Pennsylvanian to the upper cretaceous the slowing
down has been negligible; at that time the Atlantic was not yet formed, and shallow seas
were regressive).