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Tidal forces: The gravitational influence of the sun and moon

Although the moon primarily influences Earth’s tides, the sun and the shape of the ocean also contribute

FILE - As high tide laps against the sea wall tourist walk down the Battery in Charleston, S.C. Friday, Nov. 13, 2020. According to a U.S. federal report released on Tuesday, Feb. 15, 2022, seas lapping against Americas coastlines are rising ever faster and will be 10 to 12 inches higher by the year 2050 with major U.S. Eastern cities regularly hit with costly sunny day flooding. (AP Photo/Mic Smith, File) (Mic Smith, Mic Smith Photography LLC)

ORLANDO, Fla. – Strolling along the beach, you find seashells and seaweed left behind by the receding tides at your feet.

When you gaze up at the moon, you’re looking at the primary cause for the ebb and flow of oceans along our shores.

Despite how far away the moon is, the moon’s gravitational pull on Earth significantly influences the movement of tides.

The gravitational pull of the moon on Earth causes a shift in Earth’s mass, gently skewing its shape — elongated at the equator and shortened at the poles.

While scientific instruments can detect this effect on the Earth, one can see the corresponding changes in Earth’s oceans simply by visiting the beach.

Tidal Force, Gravity

As Earth rotates within the water layer surrounding it, its landmasses traverse through two bulges, which represent Earth’s high tides.

The majority of coastlines undergo two cycles of high and low tides each day.

A complete cycle from one high tide to the next (or from low tide to low tide) spans slightly over 12 hours.

Tidal Forces

The moon holds the primary influence over Earth’s tides due to its proximity, but it’s not the only factor.

The sun, with a mass about 27 million times that of the Moon, also affects the tides on Earth.

Despite being significantly farther away — about 390 times the distance to the moon — it exerts slightly less than half of the moon’s tide-generating force.

Nevertheless, it still plays a role.

Twice a month, during alignments of the Earth, sun, and moon, their gravitational forces combine, leading to exceptionally high tides known as spring tides also called a “King Tide” at the bulges and very low tides in displaced areas.

Spring Tides

About a week later, when the sun and moon are at right angles, the sun’s gravitational pull opposes the moon’s tug, partially canceling it out and giving rise to moderate tides called neap tides.

Neap Tides

Identification of spring tides or neap tides doesn’t require being close to water.

Spring tides consistently occur during the full or new phase of the Moon, when the sun, moon, and Earth align.

Neap tides, on the other hand, occur around the first and last quarter phases of the moon, when the moon’s orbit positions it perpendicular to the sun.

Earth’s shape is far from a perfect, smooth sphere, and the tides observed at our shores are influenced by various factors, including the contours of Earth’s continents, wind patterns, and storms.

For an accurate assessment of the tides at your location, check out the local tides forecast.

Listen to Talk To Tom in the media player below:


About the Author
Michelle Morgan headshot

Michelle joined News 6 as a meteorologist in May 2023.