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In 1687, Sir Isaac Newton described how the combined gravitational pull of the Moon and Sun generates the rhythmic rise and fall of our oceans. That insight remains the foundation of modern tidal science.
While gravity pulls water toward the Moon, inertia— the tendency of moving bodies to keep moving—resists that motion. The interplay of these forces creates the familiar tidal bulge on the side of Earth facing the Moon and a complementary bulge on the opposite side.
Because the Moon orbits Earth, a given coastal point experiences a high tide approximately every 12 hours and 25 minutes, the half of a lunar day. A full lunar day lasts 24 hours and 50 minutes, slightly longer than a solar day, due to Earth’s rotation in the same direction as the Moon’s orbit.
The Sun also contributes to tidal ranges. When the Sun and Moon align (new or full moon), their combined gravitational forces amplify the tide, producing spring tides. When they are at right angles (first or third quarter), the forces partially cancel, resulting in neap tides. The National Oceanic and Atmospheric Administration (NOAA) notes that storm surges often coincide with these lunar alignments.
During a full moon, the Sun, Moon, and Earth are almost perfectly aligned, generating the highest possible tidal range—a spring tide. Seven days later, when the Moon and Sun are perpendicular, neap tides occur. Coastal residents will notice these moderate tides around the first and third quarter moons.
For more detailed information, visit NOAA or read Newton’s original work, Principia.
