What are tides?
Tides are the regular rise and fall of the sea. At most coasts there are two high tides and two low tides about every 24 hours 50 minutes, and the difference between high and low water, the tidal range, grows and shrinks over about two weeks. Both patterns are controlled by the Moon and the Sun.
The tidal force
Gravity weakens with distance. The side of Earth facing the Moon is about 12,700 km closer to the Moon than the far side, so the Moon pulls it slightly harder than it pulls Earth’s centre, and pulls the far side slightly less. What matters for the oceans is this difference in pull, called the tidal force. Compared with Earth’s centre, water on the near side is pulled towards the Moon, water on the far side is left behind, and water at the sides is squeezed slightly inwards. The arrows in the simulator show this pattern.
Two bulges, two tides
The tidal force stretches the oceans into two bulges: one facing the Moon and one on the opposite side. As Earth spins, a coast is carried through both bulges and both low regions between them, giving two high and two low tides. Because the Moon moves along its orbit while Earth turns, Earth needs about 24 hours 50 minutes to face the Moon again, so high tides come roughly 12 hours 25 minutes apart and about 50 minutes later each day.
The Sun’s part
The Sun raises tides in the same way. Its overall pull on Earth is about 180 times stronger than the Moon’s, yet its tidal effect is less than half as large. Tidal force depends on how much the pull changes across Earth, which falls off with the cube of distance. The Sun is about 390 times farther away than the Moon, and 390³ is about 59 million, which outweighs its 27-million-times greater mass. The result: the Sun’s tidal effect is about 46% of the Moon’s.
Spring and neap tides
At new moon and full moon, the Sun, Earth and Moon are in line, and the two sets of bulges add together:spring tides, with higher high tides and lower low tides. At the first and third quarter moons, the Sun and Moon pull at right angles; the Sun’s bulges partly fill in the Moon’s low tides, giving neap tides with the smallest range. The cycle repeats about every 14.8 days. When a spring tide happens while the Moon is near its closest point, tides are especially large; these are sometimes called king tides.
Real tides on real coasts
The bulges in the simulator are the equilibrium tide: the shape the ocean would take if it could respond instantly on a planet completely covered by water. Real oceans are interrupted by continents and have varying depths, so the tide travels around each ocean basin as enormous slow waves, which can be amplified or cancelled. The Bay of Fundy in Canada has ranges of over 15 m, while parts of the Mediterranean have only a few tens of centimetres. Some places get one tide a day, and many get two unequal tides, because the Moon and Sun are often north or south of the equator. Accurate local predictions use decades of measurements at each harbour.
Formulas
tidal effect ∝ M ÷ d³tidal range ≈ 2 √(A_moon² + A_sun² + 2 A_moon A_sun cos 2ψ)Here M is the mass of the Moon or Sun, d its distance, A the height of each body’s equilibrium tide (about 27 cm for the Moon and 12 cm for the Sun on the equator) and ψ the angle between the Moon and the Sun as seen from Earth.
Real-world applications
- Shipping and fishing rely on tide tables to enter shallow harbours safely.
- Tidal power stations, such as La Rance in France and Sihwa Lake in South Korea, turn tidal currents into electricity.
- Coastal ecosystems: rock pools, mudflats and salt marshes are shaped by the daily rhythm of the tides.
- A slowing Earth: tidal friction lengthens the day by about 2 milliseconds per century and pushes the Moon away by about 3.8 cm per year.
Common misconceptions
- “The Moon only pulls water on the side facing it.” There is a bulge on the far side too.
- “The Sun is too far away to affect tides.” It contributes almost half as much as the Moon.
- “Spring tides happen in spring.” They happen twice every month.
- “High tide is when the Moon is overhead.” Local geography often delays high tide by hours.
What the model simplifies
The simulator uses the equilibrium tide for an Earth covered by ocean, with real masses and distances for the Moon and Sun. The town is on the equator and the Moon and Sun are assumed to lie in the plane of the equator, so its two daily tides are equal. It ignores the oceans’ response time, continents, depth, the slight lead of the bulges caused by Earth’s rotation and friction, weather and the Moon’s elliptical orbit (except through the distance setting). The drawing is not to scale, and the water heights are exaggerated millions of times. It is a model for understanding tides, not a prediction of the tides at any real place.
Key takeaways
- Tides are caused by the difference in the Moon’s and Sun’s gravitational pull across Earth.
- Two bulges, facing towards and away from the Moon, give most coasts two high tides about every 24 h 50 min.
- The Sun’s tidal effect is about 46% of the Moon’s, because tidal force falls off with distance cubed.
- Spring tides (largest range) occur at new and full moon; neap tides (smallest range) at the quarter moons.
- Real local tides also depend strongly on coastlines and ocean basins.