Question 429
HardOriginal questionThe rise and fall of the sea along a coast is caused by gravity: the pull of the Moon and the Sun on the Earth and its oceans. Tides are really very long waves that move through the ocean. When the of one of these waves reaches a coast, the water is at high tide; when the arrives, it is low tide. The difference in height between the two is called the tidal range.
You might expect the Sun to be the stronger of the two. It is about 27 million times more than the Moon, and its pull on the Earth is more than 177 times as strong. Yet it is the Moon that has the bigger effect on the tides.
The reason is distance. The pull of gravity weakens with the square of the distance, but the force that raises tides weakens faster still, with the cube of the distance. The Sun is about 390 times farther from us than the Moon, so its tide-raising force is cut by 390 × 390 × 390, or about 59 million times. Weighing its great mass against its great distance, the Sun's tide-raising force works out at only about half of the Moon's.
Why does the Sun raise smaller tides than the Moon, though its pull is stronger?
Show the answer and explanation
Correct answer: C
Explanation
Tide-raising force "weakens faster still, with the cube of the distance", and the Sun is about 390 times farther than the Moon.
That cuts its tide-raising force by about 59 million times, enough to outweigh its huge mass.
Worth noting
The Sun is far more massive than the Moon, so size cannot be the reason.
- Concept
- Following a scientific explanation
- Typical timeTypical
- 55 s
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