The Gravitational Pull of the Moon and Sun
Ocean tides are primarily caused by the gravitational forces exerted by the Moon and, to a lesser extent, the Sun, on Earth's oceans. These celestial bodies' gravitational pulls create bulges of water on both the side of Earth facing them and the opposite side, resulting in the predictable rise and fall of sea levels across the globe.
Understanding Differential Gravity
The key principle behind tides is differential gravity. The Moon's gravitational pull is stronger on the side of Earth closest to it and weaker on the far side. This difference in force stretches Earth, including its oceans. On the near side, water is pulled towards the Moon, creating a bulge. On the far side, Earth itself is pulled away from the water, leaving a corresponding bulge of water behind.
The Cycle of High and Low Tides
As Earth rotates, different locations pass through these two tidal bulges, experiencing high tide. In the regions between the bulges, the water level is comparatively lower, resulting in low tide. This typically leads to two high tides and two low tides approximately every 24 hours and 50 minutes, a period slightly longer than a day due to the Moon's ongoing orbital motion around Earth.
Influence of the Sun: Spring and Neap Tides
The Sun also contributes to tidal forces, though its effect is less pronounced than the Moon's due to its greater distance. When the Sun, Moon, and Earth align during new and full moons, their gravitational pulls combine to create unusually high high tides and low low tides, known as spring tides. Conversely, when the Sun and Moon are at right angles to each other (during quarter moons), their gravitational forces partially cancel out, resulting in less extreme neap tides.