why questions · Ages 9-12

Why Does the Moon Pull Oceans More Than Lakes?

The moon's gravity affects all water, but ocean basins are huge enough for tides to build into visible rises and falls.

The whole story

1. At dawn, Mara knelt on a rocky shore and set her wooden ruler beside the wet sand. The ocean had slipped away from yesterday’s line, leaving dark ribbons around the stones. She copied the height into her notebook and watched the pale moon fade above the water.

2. Mara had heard people say that the moon pulls the ocean. If gravity reaches across space, she wondered, why did no one point to a lake and say that there goes the tide?

3. The next afternoon, Mara carried her question to a quiet lake. She placed the ruler at the water’s edge, made a careful mark, and promised herself she would return when the moon was high.

4. The next morning, Mara hurried back. The lake had changed hardly at all. Wind had wrinkled the surface, and a few leaves had shifted, but the waterline was almost where she left it.

5. For a moment, Mara crossed out her first idea: perhaps the moon pulled only salt water. Then she paused and uncrossed it. Gravity was not a hook that chose one kind of water; it reached toward every drop.

6. She drew Earth as a round stone and imagined the moon tugging at it. The moon’s pull reaches every bit of water. It is a little stronger on the side nearest the moon and a little weaker on the far side, so the ocean can gather into broad, slow rises.

7. A lake has water to move, too, but its basin is small. Its tiny tide can be real, yet wind, air pressure, and ordinary waves can hide it before anyone notices.

8. At the next low tide, Mara returned to the bay and watched the water begin to come back. The ocean was not being pulled more because it was ocean water; its enormous connected basin gave the moon’s small difference in pull room to build. The moon’s gravity affects all water, but ocean basins are huge enough for tides to build into visible rises and falls.

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