INSpiREzine Making Waves | Page 33

TIDES

Tides are the biggest waves on the planet, causing the oceans to rise and fall along the shores around the world. Tides exist due to the gravitational pull of the Moon and the Sun, but vary depending on their location relative to the ocean as the Earth rotates on its axis. The Moon, being so much closer to Earth, has more power to pull the tides than the Sun and therefore is the primary force creating the tides. The Moon’s pull is about twice that of the Sun’s.

As the Moon revolves around the Earth, its gravity pulls Earth toward it. The Earth’s outer shell (lithosphere) is unable to move much but the water above it is pulled by the gravity and a “bulge” is created. This bulge is the high tide beneath the Moon. On the opposite side of the planet the centrifugal force created by the Moon and Earth orbiting around one another pulls the ocean water out creating a second high tide bulge on the opposite side of Earth from the Moon. These two water bulges on opposite sides of the Earth aligned with the Moon are the high tides. Since so much water is pulled into the two high tides, low tides form in the "valleys" between the two high tides. During one planetary rotation (or one day) most locations will pass through both bulges and both valleys, thus, why we usually have two

high tides and two low tides every day.

Is a relationship between the phases of the Moon and the tides?

The Sun also has a part to play in causing the tides, and its location relative to the Moon alters the strength of the pull of the Moon on the ocean.

When the Sun and Moon are in line with one another, as in the case of full and new moons, they reinforce each other’s gravitational pulls and create larger than usual tides called

spring tides. This happens when the Moon is either on the same side of Earth as the Sun or directly on the opposite side of Earth.