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.
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.