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Four Variables That Affect the Tides

The first recorded attempt to penetrate the mysteries of tidal fluctuations is probably Pytheas, the Greek explorer, in 325 BC, who concluded spring tides were affected by the phases of the moon. Over two thousand years later, 21st century scientists are still struggling to understand every variable in the highly complex movement of the oceans.

The Moon
- All bodies in the universe possess gravitational force as their mass subtly bends the space around them. As the moon travels around the Earth it exerts a gravitational pull on the oceans. This attraction is not powerful enough to pull the seas directly upwards towards the moon as the earth is far larger and therefore possesses far more gravitational attraction. However, the moon is powerful enough to cause the water to move horizontally along the surface of the earth. The seas will then move in accordance with where the moon is in relation to the earth.
The Earth's Rotation
- The moon travels faster than the earth rotates. This means when one day has passed, the moon is slightly ahead of where it was positioned 24 hours earlier. It takes 51 minutes for the earth and the moon to be in the same position as the day before, thus high tide is 51 minutes later. High tide occurs twice a day, of course, with intervals of 12 hours and 25.5 minutes. The tides rise and fall because the earth spins slower than the moon orbits it.
Depth
- The movement of the oceans is dependent on the depth of the water. The movement of the tide is essentially a huge wave traveling across the planet. Waves with more water move more slowly than shallow waves. The deeper the ocean, the slower the tide. In addition, as the tide approaches land it becomes shallower, but because the shoreline is irregular there are significant variations in the depth and the speed of the water. This means the tide occurs at different times along the same stretch of coastline.
The Sun
- As the moon orbits the earth at various stages it occupies different positions in relation to both the earth and the sun. When the moon is on the same side of the earth as the sun, its effect on the tides is multiplied as its gravitational effect is added to by the sun and the tide is larger. Conversely, when the moon is on the other side of the earth to the suns gravitational pull acts in opposition to the moon's and the tide is lessened. The tide still occurs as the moon is so much closer to the earth and exerts more influence.

Nature