Tides are the regular rise and fall of ocean water caused mainly by gravity from the Moon and, to a lesser extent, the Sun. They matter because they shape coastlines, affect navigation, influence marine ecosystems, and provide a predictable source of renewable energy. The key idea is not just that the Moon pulls on Earth, but that it pulls with slightly different strength on different parts of Earth.
This difference in gravitational pull creates tidal bulges in the oceans.
Understanding Physics: Tides and Gravity
A useful way to picture tides is to compare the Moon's pull at different places on Earth. Earth and the Moon travel around a shared balance point called the barycenter. The whole Earth is pulled toward the Moon, but the near side is pulled a little more strongly than Earth's center.
Water there shifts outward relative to the solid ground. On the far side, the Moon pulls less strongly than it pulls Earth's center. As Earth moves toward the Moon, water on that far side is left slightly behind relative to the ground.
This creates a second broad bulge. The two bulges are not separate piles of water racing around the planet. They are large patterns in the ocean that Earth rotates through.
As a coastline turns into a bulge, the sea level rises. As it turns away, the level falls. The Moon moves eastward in its orbit while Earth spins, so Earth needs extra time to face the Moon again.
That is why the tidal cycle follows a lunar day rather than an ordinary clock day. A typical place with two daily tides gets high water roughly twelve hours and twenty five minutes apart. Not every coast follows this pattern neatly.
Some places have one high tide each day. Others have two highs of unequal height.
These are called diurnal and mixed tides. They happen because the Moon's orbit is tilted and because local ocean basins respond in different ways.
The shape of the ocean floor can matter as much as the pull from space. Water entering a narrow bay or estuary can be squeezed into a smaller area, making the rise much larger. In a wide open ocean, the same tidal forcing may produce a much smaller change in height.
Some seas have their own natural sloshing period. When tidal forcing is close to that period, the water motion can build up through resonance.
The Bay of Fundy in Canada is a famous example of an unusually large tidal range. Friction against the seabed and coast slows the water, so the highest tide often arrives after the Moon has passed its highest point in the sky.
During the lunar month, the relative directions of the Sun and Moon change. Their effects can reinforce each other or partly cancel. The resulting change is seen most clearly in tidal range, which is the height difference between high and low water.
Large ranges can expose mudflats, refill salt marshes, and create strong currents in channels. Fishers, ferry crews, surfers, and people walking near beaches need tide tables because water level and current speed can change quickly.
When learning tides, pay attention to the difference between gravity itself and the difference in gravity across Earth. That difference explains the two-bulge pattern far better than the simple idea that the Moon pulls ocean water upward.
Key Facts
- Newton's law of gravitation: F = Gm1m2/r^2
- Tidal force depends on the change in gravity across Earth, so it scales approximately as 1/r^3.
- The Moon produces stronger tides than the Sun because it is much closer to Earth, even though the Sun is more massive.
- Most coastal locations have about two high tides and two low tides each lunar day, which is about 24 h 50 min.
- Spring tides occur when the Sun, Earth, and Moon are nearly aligned, giving the largest tidal range.
- Neap tides occur when the Sun and Moon pull at right angles, giving the smallest tidal range.
Vocabulary
- Tidal bulge
- A tidal bulge is a raised region of ocean water produced by differential gravity and the motion of the Earth Moon system.
- Tidal force
- A tidal force is the stretching effect caused by gravity being stronger on the near side of an object than on the far side.
- Spring tide
- A spring tide is a tide with an unusually large tidal range that occurs when the Sun, Earth, and Moon are nearly in a line.
- Neap tide
- A neap tide is a tide with an unusually small tidal range that occurs when the Sun and Moon pull on Earth's oceans at about right angles.
- Lunar day
- A lunar day is the time it takes a point on Earth to face the Moon again, about 24 hours and 50 minutes.
Common Mistakes to Avoid
- Thinking tides happen because the Moon only pulls water upward on the near side, which is wrong because tides come from differences in gravity across Earth and the Earth Moon system's motion.
- Assuming the Sun has a larger tidal effect than the Moon because it has more mass, which is wrong because tidal effects depend strongly on distance and the Moon is much closer.
- Saying spring tides happen only in spring, which is wrong because the word spring means the water springs higher and these tides occur near new moon and full moon.
- Expecting high tides exactly every 12 hours, which is wrong because the Moon moves in its orbit while Earth rotates, making the lunar day about 24 hours and 50 minutes.
Practice Questions
- 1 A coast has a high tide at 6:10 a.m. Assuming two nearly equal high tides per lunar day of 24 h 50 min, estimate the time of the next high tide.
- 2 The average distance from Earth to the Moon is 3.84 x 10^8 m, and the Moon's mass is 7.35 x 10^22 kg. Using F = Gm1m2/r^2, explain how the gravitational force on 1 kg of ocean water depends on distance, then calculate the Moon's gravitational pull on that 1 kg mass using G = 6.67 x 10^-11 N m^2/kg^2.
- 3 During a first quarter moon, the Sun and Moon pull on Earth's oceans at roughly right angles. Explain whether spring tides or neap tides are expected, and describe how the tidal range compares with normal tides.