Learn & Understand

The Moon's Pull: What Actually Makes the Tides

The tide time calculator projects tides using the regular rhythm most coastlines follow, roughly two highs and two lows each day. Behind that rhythm lies one of nature's grand spectacles: the daily rise and fall of the seas, driven chiefly by the gravitational pull of the moon. Understanding what actually causes the tides, and why there are typically two of each per day arriving a little later each time, transforms the calculator's regular pattern from an arbitrary schedule into a reflection of the heavens.

Gravity Reaches Across Space

The moon, though far away, exerts a gravitational pull on the Earth, and this pull is slightly stronger on the side of the Earth nearer the moon than on the far side. The oceans, being fluid, respond to this uneven pull by bulging. The water on the side facing the moon is drawn toward it, heaping up into a bulge, and a corresponding bulge forms on the opposite side of the Earth as well. These two bulges of water are the essence of the tides.

Two Bulges, Two Tides

The existence of two bulges, one facing the moon and one on the far side, explains why most coasts experience two high tides each day. As the Earth rotates beneath these bulges, any given coastline passes through both of them in the course of a day, encountering a high tide at each bulge and a low tide in between. So the daily rotation of the Earth, sweeping the land through the two standing bulges of water, produces the familiar sequence of roughly two highs and two lows.

Why the Tide Is Late Each Day

The tides do not arrive at the same clock time each day; they slip a little later, and this too has a celestial cause. While the Earth rotates, the moon is also moving along its orbit, so the Earth must turn a bit further to catch up to the moon's position again. This makes the tidal day slightly longer than the ordinary day, which is why high tide comes roughly fifty minutes later each day. The calculator uses this known tidal period to project the next tide from the last.

A Rhythm Written by the Sky

Because the tides follow the steady, predictable motion of the moon, the pattern is regular enough to project forward, which is exactly what the calculator does, advancing from a known tide by the tidal period, and estimating the height in between. The sun contributes too, its own smaller pull combining with the moon's to make tides stronger or weaker at different times. The calculator's regular schedule is thus a reflection of orbital clockwork. In predicting a tide, it is really reading the position of the moon, the great author of the ocean's daily breathing.

For the shape of the tidal curve between highs and lows, see the Rule of Twelfths Tide Calculator; to plan anchoring around tide swings, the Anchor Chain Length Calculator.