Tides, the periodic rise and fall in the levels of large bodies of water, are the product of gravitational forces. They result from the interaction of the Earth with both the sun and the moon, though the moon is the most significant influence year round.
The first person to link tides with the forces of gravity and attraction to planetary bodies was Isaac Newton in 1687. He then surmised that the phenomenon could be at least partially explained through his theory of universal gravitation: that the gravitational attraction between two bodies is directly proportional to their masses, and inversely proportional to the square of the distance between them. The ocean's attraction to the moon causes it to bulge out in the direction of the moon itself. Simultaneously, another bulge occurs on the opposite side of the earth, with the side being pulled toward the moon and away from the water on the far side.
Due to the nature of Earth's rotation, oceans experience two tides each day. Several times a year, oceans worldwide experience particularly strong tides called spring tides. Despite the name, these tides have nothing to do with springtime, but instead pertain to the appearance of new and full moons. During spring tides, the moon, sun and Earth become aligned, thus amplifying the effect of gravitational attraction.