The Moon's Gravity: How Tides Work | Earth Science

🌊 The Moon’s Silent Pull: The Full Story of Earth’s Tides

How a celestial dance 384,400 km away shapes our shorelines twice a day

The gravitational pull of the Moon affects Earth’s oceans, causing the phenomenon known as tides. This simple statement describes one of the most elegant and complex interactions in the Earth–Moon system. But what really happens? Why do most coastlines experience two high tides and two low tides every day? And why does the Sun, despite being enormously larger, play only a supporting role? Let’s expand this cosmic ballet.

The Moon’s gravity does not pull uniformly on Earth. Because gravitational force weakens with distance, the side of Earth facing the Moon feels a stronger pull than the planet’s centre, and the far side feels a weaker pull. This difference—called the tidal force—stretches the entire planet, but because water flows easily, the effect is most visible in the oceans. The water on the near side is pulled toward the Moon, creating a bulge. Meanwhile, on the far side, a second bulge forms because the solid Earth is pulled away from the water, leaving relatively more water behind. These two bulges are the high tides. As Earth rotates on its axis, any given point on the coast passes through both bulges each day, explaining the familiar two high tides.

But the Moon is not stationary—it orbits Earth every 27.3 days. This movement means that a fixed point on Earth needs, on average, an extra 50 minutes to realign with the Moon, shifting tide times daily. This delay creates the tidal cycle we observe. And the Sun? Though 27 million times more massive than the Moon, it is 390 times farther away, making its tidal force only about 46% that of the Moon. When the Sun, Moon, and Earth align (new moon or full moon), their forces combine to produce spring tides—higher high tides and lower low tides. When they are at right angles (quarter moons), the Sun partially cancels the Moon’s pull, creating milder neap tides.

The story grows richer with coastal geography. In narrow bays or river mouths, the tidal bulge can funnel into a towering tidal bore. In places like the Bay of Fundy (Canada), the shape of the basin resonates with the tidal period, producing the world’s largest tidal range—over 16 metres. Conversely, some enclosed seas like the Mediterranean experience barely perceptible tides because their connection to the open ocean is narrow and the basin’s natural period does not match the tidal forces.

Tides are not just ocean phenomena; they also create Earth tides (the solid ground rises and falls by about 30 cm) and even atmospheric tides. Moreover, tidal friction is slowly slowing Earth’s rotation and pushing the Moon farther away—about 3.8 cm per year. This means that millions of years ago, days were shorter and the Moon loomed larger in the sky, producing far more extreme tides.

🌙 Beyond the textbook: Tides also influence marine ecosystems, coastal navigation, and even the timing of animal behaviour (like grunion spawning or horseshoe crab mating). Ancient fishermen and sailors understood tidal patterns long before Newton explained gravity. Today, we use sophisticated models to predict tides decades in advance, but the primal engine remains the same: the patient, persistent pull of our nearest cosmic neighbour.

So next time you stand at the shoreline and watch the water rise or retreat, remember: you are witnessing the direct touch of the Moon—a gravitational conversation between Earth and its partner, spoken in the language of moving oceans. The tides are proof that even 384,400 kilometres away, the Moon never stops reaching for us.