The rhythmic rise and fall of sea levels, known as tides, are one of Earth’s most visible astronomical phenomena. These movements are caused not by total gravitational pull, but by the subtle difference in gravitational force exerted across Earth’s diameter. This stretching force is called the tidal force, and its strength depends on a body’s mass and, more significantly, its proximity.
The Dominant Force: Earth’s Moon
The Moon is the most important factor shaping Earth’s tides, primarily due to its extreme proximity. While its mass is small compared to the Sun, its closeness means its gravitational influence changes drastically across Earth’s diameter. This difference in pull is known as the gravitational gradient or differential gravity.
The tidal force exerted by any celestial body is inversely proportional to the cube of its distance, heavily favoring the nearby Moon. This strong gradient creates two high-tide bulges on opposite sides of the planet. On the side facing the Moon, gravity pulls the water toward it, creating the first bulge.
On the far side of Earth, a second bulge forms because the Moon pulls the solid Earth more strongly than it pulls the distant water. This effect is due to inertia and results in a bulge that lags behind the planet’s center. As Earth rotates, any location passes through these two bulges daily, experiencing two high tides and two low tides. The Moon’s tidal force is approximately 2.2 times greater than the Sun’s.
The Secondary Influence: The Sun
The Sun is the secondary contributor to Earth’s tides. Although the Sun is vastly more massive than the Moon, its distance is about 400 times greater. This distance factor significantly weakens the Sun’s differential gravitational pull across Earth.
Because tidal force diminishes rapidly with the cube of the distance, the Sun’s stretching effect is less pronounced than the Moon’s. The Sun’s tidal force is about 45% to 50% of the Moon’s, which dictates the basic daily tidal cycle. The Sun’s gravitational contribution modifies the lunar tides.
Tidal Cycles and Alignment
The interaction between the Moon’s and the Sun’s forces creates the two primary tidal cycles observed monthly. When the Earth, Moon, and Sun align in a straight line, their gravitational pulls combine to create the most extreme tides. This configuration is known as syzygy, occurring twice per lunar month.
Spring Tides
These periods are called Spring tides, happening during the new moon and the full moon phases. During a Spring tide, the combined forces result in the highest high tides and the lowest low tides, creating the greatest tidal range. The term “Spring” refers not to the season, but to the water surging.
Neap Tides
Conversely, a different pattern emerges when the Moon and Sun are positioned at a right angle relative to Earth. This alignment, called quadrature, happens during the first and third quarter moon phases. In this scenario, the Sun’s gravitational pull partially counteracts the Moon’s pull.
These moderate tides are known as Neap tides, occurring approximately seven days after a Spring tide. Neap tides are characterized by lower high tides and higher low tides, resulting in the smallest overall tidal range.
Why Other Solar System Objects Are Negligible
The solar system contains numerous massive objects, yet only the Moon and the Sun meaningfully affect Earth’s tides. Every object with mass theoretically exerts a tidal force on Earth, but the immense distances render their influence entirely imperceptible.
Even the largest planet, Jupiter, and the closest planet, Venus, have a negligible impact on ocean tides. Venus, at its closest approach, exerts a tidal force more than 7,500 times weaker than the Moon’s. Jupiter’s effect is weaker still, making the tidal influence of all other planets astronomically small.
The rapid decay of the tidal force with distance ensures that the Moon and the Sun are the only two celestial bodies relevant to the ebb and flow of Earth’s oceans. Their combined forces account for virtually all observable tidal phenomena.