One of the earliest careful observers of ocean tides was the Greek geographer Pytheas of Massalia. In about the third century B.C., Pytheas traveled outside the Straits of Gibraltar and observed tidal action in the Atlantic Ocean. (Tides were essentially absent from the Mediterranean Sea, which was more familiar to the Greeks.) Pytheas proposed an explanation for tidal action: the pull of the Moon on the Earth's oceans, he said, caused the tides. This explanation is now known to be basically correct, although it was not widely accepted by scientists until the eighteenth century. English physicist Sir Isaac Newton (1642–1727) first succeeded in mathematically describing the tides in what is known as the equilibrium tidal theory.