Tides are largely due to the gravitational forces exerted by the Moon and Sun on the Earth, combined with their relative movements.
According to Newton’s Law of Universal Gravitation, every particle in the Universe attracts every other particle with a force proportional to the product of the two masses and inversely proportional to the square of the distance between them. This relationship means that, though the Sun is much more massive than the Moon, the Moon has a far greater effect on the Earth’s tides because it is very much closer.
At any time, the gravitational pull on the oceans is greatest on the side of the Earth that is facing the Moon. This causes a bulging of water towards the Moon, which is experienced as a high tide on Earth’s coastlines. Meanwhile, on the far side of the planet, there is a second bulge. Here, the gravitational pull of the Moon is less because it is further away; instead, forces due to the Earth’s rotation are dominant, and these cause a movement of the ocean away from the Moon.
As it rotates on its axis, the Earth passes under two bulges during every 24-hour period, which explains why there are two high and low tides each day. While the sides of the Earth facing towards and away from the Moon experience high tides, the two perpendicular sides are having low tides. Between these extremes, sea levels are rising or falling as the next tide approaches.
Similar bulges are generated by the Sun’s gravitational pull, but their effect is usually small compared with those produced by the Moon. The exception is when the Earth, Moon and Sun are aligned – as occurs twice every month during the full and new moons. At these times, the Sun’s pull is acting in the same direction as that of the Moon; the result is a larger bulge in the oceans, producing a spring tide. The opposite effect occurs during neap tides. At the time of the quarter moons, the Earth, Moon and Sun form a right angle, and so the Moon and Sun’s combined pull on the oceans is at a minimum.
