Plate 10 · Space
How the Moon Moves the Sea
Why are there two high tides a day? Because the Moon pulls the near ocean toward it — and leaves the far ocean behind.
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Pulled Harder Up Close
The Moon’s gravity weakens with distance. It pulls hardest on the ocean facing it, less on the Earth’s center, and least on the ocean on the far side.
Those differences stretch the oceans. The near side is drawn toward the Moon; the far side is left behind. The sea rises in two bulges, one facing the Moon and one facing away.
Bulges greatly exaggerated · Earth–Moon distance not to scale
Twice a Day
The Earth turns once a day beneath those two bulges, so most coasts get two high tides and two low tides each day.
Meanwhile the Moon moves along its orbit, and the Earth must turn about 50 minutes more to face it again: a lunar day lasts 24 hours 50 minutes. That is why high tide comes about 50 minutes later each day.
The Sun Pulls Too
The Sun is far more massive, but so much farther away that its tide-raising force is only about half the Moon’s. Tidal force comes from the difference in pull across the Earth, and that difference fades fast with distance.
At new and full moon, the Sun, Moon and Earth line up and the two pulls add together: high tides are extra high and low tides extra low. These are spring tides. A week later, with the Sun and Moon at right angles, they partly cancel, and the tides are smaller: neap tides. Spring tides have nothing to do with the season; they come twice a month, all year round.
Three Real Coasts
Below are NOAA’s tide predictions for three stations in October 2026. At Eastport, Maine, there are two high tides a day, and the water rises and falls by nearly 7 meters in a single day. San Francisco also has two a day, but one high is higher than the other. At Pensacola, on the Gulf of Mexico, most days bring just one rise and fall, never more than about 60 centimeters.
Eastport’s range peaks on October 28, two days after the full moon of the 26th, and bottoms out on the 19th, the day after first quarter.
Water levels: NOAA Tides & Currents predictions · local standard time
The Shape of the Coast
The Moon and Sun act on these three coasts in almost exactly the same way, yet their tides differ enormously. The water has to slosh around ocean basins, and the shape of a bay, the depth of the seafloor and the line of the coast can amplify a tide, shrink it or change its rhythm.
So “two high tides a day” is only the beginning. To know the tide on a particular shore, you have to measure the sea there for a long time.
Your Turn
Run time and watch a coast turn into and out of the two bulges. Turn the Sun’s tide down to zero and watch the spring and neap tides disappear.
Try thisNew moon or full moon: which one brings the higher tides?
What the model leaves out
The 3D scene shows the “equilibrium tide”: an Earth covered evenly by ocean whose surface instantly follows the Moon and Sun. Real oceans are broken up by continents, and the tide travels around ocean basins as a wave, so high tide often does not come when the Moon is overhead. The bulges are greatly exaggerated, and the sizes and distances of Earth, Moon and Sun are not to scale. The curves for real coasts are NOAA’s predictions.
A common misreading
The bulge on the side away from the Moon is not water flung outward by the Earth’s spin; it is there because the Moon pulls that water more weakly than it pulls the Earth’s center. And tides are not the Moon’s work alone: the Sun supplies roughly a third of the tide-raising force, and spring and neap tides come from the two working together or against each other.
Sources
- NOAA National Ocean Service: What causes tides
- NOAA National Ocean Service: The lunar day
- NOAA National Ocean Service: Spring and neap tides
- NOAA National Ocean Service: Types and causes of tidal cycles
- NOAA National Ocean Service: What is a spring tide?
- NOAA Tides & Currents (station predictions)
Data
Tide predictions: NOAA CO-OPS, Tides & Currents. Earth: NASA Earth Observatory, Blue Marble Next Generation, October 2004 (Reto Stöckli); city lights, NASA Earth Observatory Black Marble 2016 (Joshua Stevens, Miguel Román, Suomi NPP VIIRS); clouds, NASA Blue Marble 2002 (Reto Stöckli, NASA GSFC). Elevation and seafloor (globe relief): Terrain Tiles on AWS Open Data; ETOPO1, NOAA; GMTED2010 and SRTM, USGS. Moon: NASA Scientific Visualization Studio, CGI Moon Kit (LRO LROC WAC colour mosaic; LOLA elevation). Stars: Yale Bright Star Catalogue, 5th revised edition (Hoffleit & Warren 1991), via CDS VizieR. Milky Way: NASA/Goddard Space Flight Center Scientific Visualization Studio, Deep Star Maps 2020; Gaia DR2: ESA/Gaia/DPAC.
Follow the question
Plate 12Why the Moon Changes ShapeThe Moon never changes shape. What changes is how much of its sunlit half we can see.
Plate 03The Shadow of the MoonThe Sun is about 400 times wider than the Moon, and about 400 times farther away. That coincidence lets day turn to night for a few minutes.
