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I've heard that the moon pulls at the water on Earth, creating waves. How does it do that and why doesn't it effect smaller water sources such as puddles/streams?
Question Date: 2016-12-01
Answer 1:

Good question! The moon does indeed tug at the water on Earth through gravity. The side of Earth facing the moon is tugged a little bit harder than the side of Earth facing away from the moon given the difference in distance between the moon and the near and far sides of Earth. The stronger gravitational pull on the side of Earth facing the moon creates a bulge in the world's oceans (water piling up slightly on the side facing the moon) when in turn creates tides (not waves), since Earth spins through this bulge.

Tidal forces affect smaller bodies of water as well, but they are so small they're hard to detect. The difference in the distance between one side of a puddle to the moon, and the other side of the puddle to the moon is minuscule, so the tidal bulge is almost non-existent. Tides are a cool phenomenon!

Answer 2:

The moon pulls on the water to create tides, not waves. This is because the ocean facing the moon is closer to the moon than the ocean on the opposite side of the Earth, which means that the moon's gravity affects the closer oceans more strongly. This works because the oceans encircle the entire planet, and are therefore big enough that the difference in distance between the near side and the Earth and the far side of the Earth is great enough for there to be a measurable difference in the moon's gravitational force. Lakes just aren't big enough, so tides on them are too small for us to see.

Ocean waves are mostly created by wind, and have nothing to do with the moon.

Answer 3:

The fluctuations of tides here on Earth are not only a function of forces imposed by the moon, but also by those forces imposed by the sun! Both the moon and the sun exert a gravitational pull on the earth. This gravitational pull is actually what creates our tides. More specifically, we have two main types of tides that we experience here on earth: spring tides and neap tides. Spring tides occur every new or full moon in which the moon forms a line with the earth and the sun (the moon is situated right in between earth and the sun). When this occurs, the moons gravitational pull on the earth is strengthened by the added gravitational pull of the sun, as can be seen in the picture below (from physicalgeography.net).

spring tides

During these spring tides, the change from the lowest tide to the highest tide (and vice versa) is at a maximum, meaning this is when we experience extremely low tides and extremely high tides.

Neap tides occur halfway between the new and full moon when the sun and moon are at a 90o angle to one another. When this happens, the sun’s gravitational pull competes with the moon’s gravitational pull so that the range in high and low tides is quite small (see picture below from physicalgeography.net).

neap tides

This gravitational pull that the oceans experience theoretically should affect all sources of water on Earth. In reality, the size of lakes, puddles, and streams are simply too small for these gravitational forces to have a large effect. The Great Lakes are quite large and do experience small tide changes, however, most other lakes and bodies of water are much more affected by changing inflows of water from rivers and streams as well as wind - both of which can change the height of the water much more dramatically than gravitational influences. It takes a large, interconnected body of water to significantly be affected by gravitational forces that cause tidal changes.

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