Why does the Moon have a greater effect on Earth's oceans than the Sun?

• A. The Sun's gravity is twice that of the Moon.
• B. The Moon is closer to Earth than the Sun.
• C. The Moon is larger than the Sun.
• D. The Sun has no gravity.

Answer: (B)

Tidal forces come from how gravity changes across Earth, not from the total pull alone. The effect scales with M/r^3, so the strength of tides depends strongly on distance: the closer a body is, the more its gravity varies from one side of Earth to the other. The Moon is much closer to Earth than the Sun, so its gravitational field changes across Earth much more steeply, producing stronger tides. The Sun, while far more massive, is so far away that its gravitational gradient is smaller, leading to weaker tides on average. Hence the Moon has a greater effect on Earth's oceans because proximity makes the tidal gradient larger.