Of all the Moon's effects on Earth, the tides are the most dramatic and scientifically well-established. The rhythmic rise and fall of the ocean twice a day — a phenomenon that shapes coastlines, enables shipping, drives powerful tidal energy, and governs the lives of countless marine species — is largely the result of the Moon's gravitational pull on Earth's oceans.
How Lunar Gravity Creates Tides
Gravity follows an inverse square law: the closer two masses are, the stronger the gravitational attraction between them. The Moon exerts a gravitational pull on every part of Earth — but because different parts of Earth are at different distances from the Moon, the pull is slightly stronger on the side of Earth facing the Moon and slightly weaker on the far side.
This difference — called the tidal force — is what creates tides. On the side of Earth closest to the Moon, the ocean water is pulled toward the Moon more strongly than the solid Earth beneath it, causing a bulge. On the far side, the solid Earth is pulled more strongly toward the Moon than the ocean water, which effectively creates a second bulge on the opposite side of the planet.
The result: two tidal bulges, one facing the Moon and one on the opposite side. As Earth rotates beneath these bulges, any given coastal location passes through high tide (under a bulge) twice and low tide (between bulges) twice each day.
Why Two High Tides Per Day?
The counterintuitive part — the tidal bulge on the side of Earth away from the Moon — confuses many people. If the Moon pulls the ocean toward it, why is there a bulge on the opposite side?
The answer lies in the fact that Earth is in free fall around the Moon (and vice versa — they orbit a common centre of mass called the barycentre). On the far side of Earth, the solid planet is being pulled toward the Moon more strongly than the ocean, so the ocean is effectively "left behind" and bulges outward. It is similar to the way passengers in a turning car are pushed toward the outside of the turn — not because something pushes them, but because the car moves out from under them.
Spring Tides and Neap Tides
The Moon is not the only object exerting a tidal force on Earth. The Sun, despite being 150 million kilometres away, has a significant tidal effect as well — about 46% as strong as the Moon's, due to the Sun's vastly greater mass compensating for its greater distance.
This creates two distinct tidal patterns that repeat with the lunar cycle:
Spring Tides (New Moon and Full Moon)
When the Sun, Earth, and Moon are aligned — at both New Moon and Full Moon — the gravitational pulls of the Sun and Moon add together. This produces spring tides: higher-than-average high tides and lower-than-average low tides. The tidal range (difference between high and low tide) is at its greatest. Spring tides have nothing to do with the season — they happen twice a month.
Neap Tides (First and Last Quarter)
When the Moon is at First or Last Quarter — at 90° to the Sun as seen from Earth — the gravitational forces are perpendicular to each other. They partially cancel out, producing neap tides: more moderate tides with a smaller range between high and low. Neap tides also occur twice per lunar month.
Practical note for coastal visitors: The highest and most extreme tides of the year occur during spring tides that coincide with the Moon being at perigee (closest to Earth). These "perigean spring tides" can flood areas not normally reached by the sea. Check our homepage to track the current moon phase.
The Tidal Day: 24 Hours and 50 Minutes
Earth rotates once in 24 hours, but the Moon moves eastward in its orbit, so the tidal bulge moves too. By the time Earth has rotated once, the Moon has moved slightly ahead, and Earth needs an extra 50 minutes to "catch up" to the Moon's new position. This is why high tides are roughly 50 minutes later each day — the tidal day is 24 hours and 50 minutes, not exactly 24 hours.
Local Tidal Patterns
The theoretical two-tides-per-day pattern is modified by local geography in complex ways. The shape of ocean basins, the profile of coastlines, the presence of continental shelves, and resonance effects can dramatically alter the tidal pattern at any specific location.
- Semidiurnal tides: Two roughly equal high tides and two low tides per day. Common on Atlantic coasts.
- Diurnal tides: Only one high tide and one low tide per day. Occur in some parts of the Gulf of Mexico and Southeast Asia.
- Mixed tides: Two high tides per day, but of significantly different heights. Common on Pacific coasts.
The Bay of Fundy in Canada holds the record for the world's highest tidal range — up to 16 metres — due to resonance between the tidal period and the natural oscillation period of the bay.
Tides and Marine Life
Tidal cycles are fundamental to coastal ecosystems. Intertidal organisms have adapted to periods of immersion and exposure in precise synchrony with the tidal cycle. Many marine animals time their spawning to specific tidal phases — the California grunion famously spawns exactly at the highest tide of spring tides on summer nights. Oysters and other bivalves track both the tidal and lunar cycle even in laboratory conditions far from the sea.
Tidal Power
The energy embodied in tidal flows is enormous, and tidal power generation is one of the most reliable and predictable forms of renewable energy. Unlike wind and solar, which depend on weather, tidal energy follows a precise astronomical schedule calculable centuries in advance. Tidal barrages, tidal stream turbines, and tidal lagoons are all being developed globally as part of the clean energy transition.