Unpromising as a barren mountainside seems at first, it contains rich information about an astonishing array of subjects. Pick up some fossil mollusks or corals. Clams from the intertidal zone add a thin calcareous layer to their shells when the tide is high, then stop as the water falls. Each layer represents one tidal cycle, two tides per day. High spring tides produce thicker layers that mark off the lunar month. Where the winter is cold, growth slows or even stops, allowing us to identify the seasons and count the days of the year. In warmer waters, corals respond to the sun: a growth layer for each day, but little at night. The length of the year can be inferred for the remote past, for as long as corals and mollusks inhabited the earth (Figure 1.1).

Why should one wish to know the number of days or of lunar months for years so long ago? As we go farther into the past, we find more days in the year and fewer hours in the day. This is because the rotation of the earth has gradually slowed as a result of tidal friction. Also, there is reason to believe that the moon has not maintained a constant distance from the earth; if that is so, it ought to be reflected in the length of the lunar month.