How the Moon Affects Tides

Ocean tides are caused by the Moon's gravity pulling on Earth's oceans — but the actual mechanism has a genuinely counterintuitive part that's worth explaining properly rather than the oversimplified "the Moon pulls the water toward it" version: there are two tidal bulges on opposite sides of Earth at any given time, not just one on the Moon-facing side.

The bulge on the side of Earth facing the Moon happens for the intuitive reason — the Moon's gravity pulls that near-side water toward it slightly more strongly than it pulls Earth's solid body as a whole. The bulge on the far side of Earth (facing away from the Moon) happens for a less intuitive reason: the Moon's gravity pulls Earth's solid center toward the Moon more strongly than it pulls the far-side water, effectively leaving that water "behind" relative to the planet's center — the same physics (called differential or tidal gravitational force) that would stretch, not just pull, an object caught between two unequal gravitational tugs.

As Earth rotates through these two bulges roughly once every 24 hours, most coastal locations experience two high tides and two low tides per day (a semi-diurnal tide pattern), roughly 12 hours and 25 minutes apart — that extra 25 minutes beyond a clean 12-hour split comes from the Moon itself continuing to orbit Earth in the same direction Earth is rotating, so Earth has to rotate slightly farther each day to "catch up" to the Moon's new position.

The Sun also pulls on Earth's oceans, though with only about 46% of the Moon's tidal effect (despite the Sun's vastly greater mass, tidal force depends heavily on distance, and the Moon is far closer). When the Sun and Moon align (at New and Full Moon), their tidal pulls combine, producing higher-than-average high tides and lower-than-average low tides, called spring tides (the name refers to the tide "springing up," unrelated to the season). When the Sun and Moon are at right angles to each other (First and Last Quarter), their pulls partly cancel, producing smaller-than-average tidal range, called neap tides.

A supermoon's closer-than-average distance at perigee produces a real, if modest, additional effect on top of ordinary spring tides — called a perigean spring tide — since the Moon's tidal pull is measurably stronger at closer range, a genuine, physically real (if usually modest, typically a few extra centimeters to a couple of inches beyond an ordinary spring tide) effect distinct from any folklore claims about supermoons.

Coastal regions with unusually large tidal ranges, like Canada's Bay of Fundy (among the largest tidal ranges in the world, over 15 meters in places), owe their extreme tides to a combination of the Moon's basic gravitational pull and local coastline geometry that amplifies the effect — a case where the same universal lunar mechanism produces dramatically different real-world results depending on the specific shape of the coast it's acting on.

Tide tables, published for specific ports and coastal locations, incorporate the Moon's and Sun's positions along with local geographic factors to predict actual tide times and heights — a genuinely more complex real-world calculation than the simplified spring-tide/neap-tide model alone, which is why mariners and coastal engineers rely on location-specific tide tables rather than a general lunar-phase rule of thumb.

Tide-prediction apps and websites, drawing on the same gravitational calculations covered here plus local coastal data, are far more practical for real-world boating or fishing planning than working out the general spring/neap pattern by hand.

NOAA and equivalent national agencies elsewhere publish detailed, location-specific tide predictions freely online, incorporating the same lunar and solar mechanics covered here alongside local coastal geography for genuinely practical, real-world accuracy.

Coastal flood-warning systems in many regions specifically factor in predicted spring and neap tide timing alongside weather forecasts, since a storm surge arriving during a spring tide poses meaningfully more flood risk than the same storm during a neap tide.

A rewarding local project: track high and low tide times at a nearby coast for a full month, then compare your own data against the spring/neap pattern described here.

Boaters and coastal anglers who plan trips around tide tables are, whether they think about it this way or not, planning directly around this same lunar-gravity mechanism.

A local tide table, compared against the general spring/neap pattern described here, reveals whether your own coastline's geography amplifies or dampens the expected effect.

Geometry and gravity together, not either alone, fully explain the tidal pattern described here.

The Bay of Fundy's roughly 15-meter tidal range means an area of exposed seafloor that can stretch for over a kilometer at low tide becomes fully submerged again within about six hours — a genuinely dramatic, fast-moving example of the same mechanism producing only a gentle few-centimeter difference on a straighter, more open coastline elsewhere.