9.6: Tides

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Did you ever build a sandcastle?

A sandcastle lasts only until the next tide comes in. To build a sand castle the sand must be moist enough for the grains to stick together. So it has to be near the water. But that puts the castle in position to be buried by the next high tide. Oh well. You can build another one when the tide goes back out!

Tides

Tides are daily changes in the level of ocean water. They occur all around the globe. High tides occur when the water reaches its highest level in a day. Low tides occur when the water reaches its lowest level in a day. Tides keep cycling from high to low and back again. In most places the water level rises and falls twice a day. So there are two high tides and two low tides approximately every 24 hours.

Below, you can see the difference between high and low tides (Figure below). This is called the tidal range.

High tide and low tide in the Bay of Fundy

Where is the intertidal zone in this picture?

Why Tides Occur

The figure below shows why tides occur (Figure below). The main cause of tides is the pull of the Moon’s gravity on Earth. The pull is greatest on whatever is closest to the Moon. Although the gravity pulls the land, only the water can move. As a result:

Water on the side of Earth facing the Moon is pulled hardest by the Moon’s gravity. This causes a bulge of water on that side of Earth. That bulge is a high tide.
Earth itself is pulled harder by the Moon’s gravity than is the ocean on the side of Earth opposite the Moon. As a result, there is bulge of water on the opposite side of Earth. This creates another high tide.
With water bulging on two sides of Earth, there’s less water left in between. This creates low tides on the other two sides of the planet.

Diagram of how the moon causes tides

High and low tides are due mainly to the pull of the Moon’s gravity.

Spring Tides and Neap Tides

The Sun’s gravity also pulls on Earth and its oceans. The Sun is much larger than the Moon, so is its gravitational pull on Earth greater than the Moon’s? The pull of the Sun’s gravity is much less because the Sun is much farther away. In fact, the Sun’s gravitational pull on Earth is a little less than half of the Moon's pull.

The figure below shows where the Moon is relative to the Sun at different times during the month (Figure below). The positions of the Moon and Sun relative to each other affects the tides. This creates spring tides or neap tides.

Spring tides occur during the new moon and full moon. The Sun and Moon must either be in a straight line on the same side of Earth, or they must be on opposite sides of Earth. Their gravitational pull combines to cause very high and very low tides. Spring tides have the greatest tidal range.
Neap tides occur during the first and third quarters of the Moon. The Moon and Sun are at right angles to each other. Their gravity pulls on the oceans in different directions so the highs and lows are not as great. Neap tides have the smallest tidal range.

The Sun and Moon both affect Earth’s tides

The Sun and Moon both affect Earth’s tides.

Studying ocean tides' rhythmic movements helps scientists understand the ocean and the Sun/Moon/Earth system. This QUEST video explains how tides work, and visits the oldest continually operating tidal gauge in the Western Hemisphere.

Summary

The primary cause of tides is the gravitational attraction of the Moon. This causes two high and two low tides a day.
When the Sun's and Moon's tides match, there are spring tides. When the two tides are opposed, there are neap tides.
The difference between the daily high and the daily low is the tidal range.

Review

How does the Moon make two high and two low tides a day?
How does the Sun make two high and two low tides a day?
What causes spring tides? What causes neap tides?