(WOE) Tides Lesson

Tides

The daily rise and fall of Earth's waters on its coastlines are called tides. During high tide the water level will reach its highest point on a beach. Some sandy beaches become nothing more than a thin line barely wide enough for even one person to walk the shore. But, gradually the water will begin to flow back toward the sea. When the water reaches its lowest point this is called low tide. We have learned that surface currents are caused by winds, and deep currents are caused by water density. Tides are different. Tides are caused by the interaction of Earth, the moon, and the sun.

In the photo you can see the difference in water level on the shore as the tide rises with the moon at Maine, USA.. Low tide occurs roughly at moonrise and high tide with a high moon. This corresponds to the simple gravity model of two tidal bulges, however, at most places moon and tides have a phase shift.

Why Tides Happen

The key to understanding how the tides work is understanding the relationship between the motion of our planet and the Moon and Sun. As the Earth spins on its own axis, ocean water is kept at equal levels around the planet by the Earth's gravity pulling inward and centrifugal force pushing outward. However, the Moon's gravitational forces are strong enough to disrupt this balance by accelerating the water towards the Moon. This causes the water to 'bulge.' As the Moon orbits our planet and as the Earth rotates, the bulge also moves. The areas of the Earth where the bulging occurs experience high tide, and the other areas are subject to a low tide.

The gravity of the moon pulls on every particle of Earth. However, the moon's gravitational pull on Earth decreases with distance from the moon. As a result, different parts of Earth are pulled more strongly toward the moon than other parts are. In addition, the pull on liquids is much more noticeable than the pull on solids, because liquids move more easily. Even the liquid in a carton of milk is slightly pulled by the moon's gravity.

Neap Tides

Neap tides are especially weak tides. Because the moon & sun are perpendicular to each other, their gravity cancels each other out. When the gravitational forces of the moon and the sun are perpendicular to one another (with respect to the Earth), Neap tides occur duing quarter moons.

An Illustration of 1st quarter moon Neap tide. Neap tides occur during 3rd quarter moon as well.

Spring Tide

Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.

The Moon & the Sun's gravitational force combine to create a particularly strong tide. See the illustration below of a full moon Spring tide.

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION