Tides are the constant rising and falling of sea levels across the globe. Newton’s laws of gravity say that two bodies in the universe are attracted to each other by gravitational pull. The laws express how the mass and distance between those bodies affect the attraction. The sun and moon both hold sway over movements on Earth, and the attraction between all three -- sun, moon and Earth -- affect the tides.
Gravitational Influences
The sun is much larger than the moon -- therefore having a larger mass -- but it is over 360 times farther away than the moon, so the sun has a diminished effect on Earth's tides. As the moon orbits, it pulls at objects on Earth. Although landmasses show a very slight upward movement, large bodies of water are pulled toward the moon by its gravity.
Bulges
The motion of the seas toward the moon produces a bulge on the side of the Earth facing the moon. Because of centrifugal force, a lesser bulge occurs on the Earth opposite of where the moon is positioned. High tides occur at each of these gravitationally induced bulges with low tides occurring in between. Since the moon rotates around the Earth and the Earth rotates around its own axis, the oceans are constantly in flux between high and low tide, which occur in intervals of about six hours and twelve minutes.
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Sun and Moon Alignment
Although the moon influences tides the most because of its proximity to Earth, the sun’s gravity affects tides to about 45 percent that of the moon. This is especially notable when the Earth, moon and sun line up together, which occurs twice each month -- during the full and new moons. At these times, high tides are taller and low tides lower than they are during the rest of the month. The additional pull of the sun combined with that of the moon creates a larger bulge.
Other Factors
The shape and size of structures, such as landmasses, have an effect on tides. Large continents have a tendency to amplify the height of high tides compared to small islands in the middle of vast oceans where tides may only rise a few feet. Even the shape of a bay or estuary influences tidal ranges, which are the differences in height between high and low tides. Funnel-shaped bays are especially influential in producing larger tidal ranges. Weather patterns can also play a role in determining tides. Strong winds can keep water away from land structures and the result is a lower low tide. High-pressure systems can depress sea heights and also lead to lower low tides. Conversely, low-pressure systems with rain and cloud cover tend to raise high tide levels.
