Newton writes the Principia; reverts to agnostic action-at-a-distance theory.

Pytheas discovers correlation between spring tides and the phases of the Moon.

Seleucus of Seleucia theorized that tides were caused by the Moon.

Kepler guesses that gravitation of the Moon causes the tides.

Huygens derives formula for the centripetal force (exerted by an object describing a circular motion).

Hooke, Newton, and others infer an inverse proportion between gravity and the square of distance.

Newton was the first person to explain tides as the product of the gravitational attraction of astronomical masses.