Tides

The rise and fall of tides

The Earth appears blue from the darkness of Space. This is because more than 70 percent of its surface is covered with oceans. The seas make up more than one million million million tons of seawater.

Every 12 hours or so, the seawater rises, then falls back again. These rises and falls are called the tides. When the water is rising, we say the tide is flowing. When it is falling, we say the tide is ebbing. The movement of the ocean waters is caused by the Moon and by the Earth spinning. Gravity pulls the Moon and Earth together. As the Earth turns, the Moon pulls at the ocean water directly beneath it, causing the water to rise. A similar rise in sea level occurs on the opposite side of the Earth, where the water bulges out as a result of the Earth spinning. At these places, there is a high tide. Some six hours later, The Earth has turned 90 degrees. The sea then falls to its lowest point and there is a low tide.

The two experiments opposite explain how the oceans rise and fall without any change in the amount of seawater, and how the tidal bulges of water stay in the same place below the Moon, as the Earth spins beneath it.

How tides occur

YOU WILL NEED
Soccer or beach ball, glue, glue brush, glass, tinfoil, scissors, reusable adhesive, flashlight.

YOU WILL NEED
High and low tide: plastic bowl, water, plastic ball to represent the world.

The tidal bulge: strong glue, one 8in length and two 16in lengths of thin string, plastic ball to represent the world, plastic bowl, hand drill, water.

High and low tide

The tidal bulge

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