What Are Tides? - Causes & Effects

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  • 0:06 Tides
  • 0:40 High & Low Tide
  • 1:50 Semidiurnal & Diurnal Tides
  • 2:38 Spring & Neap Tides
  • 4:04 Tides Affect Coastal Regions
  • 5:24 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Tides rise and fall under the influence of the gravitational pull of the moon and sun. Learn about different tides, why they are predictable and their effects on coastal regions of the world.


There are few things in nature as predictable as the tides, which are the periodic rise and fall of ocean waters. Tides are so reliable because their movements are based on the positioning of the moon and sun in relation to the earth. These celestial bodies do not change their patterns, so it's easy for us to predict when the tides will rise and fall today, tomorrow and even next year and beyond. In this lesson, you will learn more about how the moon and sun cause the tides and the effects tides have on coastal regions.

High Tide and Low Tide

Because the moon is closer to the earth than the sun, it has the most influence on the tides. In fact, it's fair to say that tides would not occur if the earth and the moon were not attracted to each other. Of course, I'm not referring to a love/hate type of attraction but rather the gravitational attraction between the two bodies. The gravitational pull of the moon causes the oceans and other major water bodies to bulge out toward the moon.

When the gravitational pull is at its highest point, the result is high tide, which is the highest level of the tide. When the pull is at its lowest point, we see low tide, or the lowest level of the tide. The earth itself is also pulled toward the moon but with less strength. This pulls the earth away from the water on the opposite side of the earth, making the water on that side bulge as well. Therefore, high tide occurs on both sides of the planet at the same time. Meanwhile the earth is rotating. So, we experience tides throughout the day.

Semidiurnal and Diurnal Tides

Now, if the earth were perfectly round with no big land masses, all bodies of water in the world would experience two nearly equal high tides and two low tides each day. This tidal pattern is known as semidiurnal tides. However, the continents of earth disrupt water bodies, and so this can produce different tidal patterns. For example, some bodies of water, such as the Gulf of Mexico, have diurnal tides, which means only one high tide and one low tide each day.

You can recall these terms by remembering that the word 'diurnal' means 'happening daily.' Therefore, we see that diurnal tides happen once a day. And if we add the prefix 'semi,' which means 'half,' we can understand that semidiurnal tides happen every half a day.

Spring Tides and Neap Tides

The earth and moon are constantly in motion around the sun, and all have their own gravitational pull. So, when the alignment between the three bodies changes, it changes the strength of the overall gravitational pull and therefore the size of the tides.

Spring tides are tides that occur when the earth, moon and sun are aligned, and the tidal range between high and low tide is at its maximum. This happens basically twice a month, during the full and new moon phases. At these times, the three bodies are in line and their gravitational pulls reinforce each other. When the spring tide is happening, we see higher than average high tides and lower than average low tides.

It's important to point out that 'spring' does not refer to the season. Instead, you can recall the meaning of 'spring tides' by thinking of them as the tides that 'spring' out and then 'spring' back with the most intensity.

A few weeks after the spring tides, we see the neap tides. These are tides that occur when the moon and sun are at right angles to the earth's orbit, and the tidal range between high and low tide is at its minimum. The neap tides occur when the moon is in its first and last quarter phase. Because of the position of the moon and sun, their gravitational pulls on the waters of earth partially cancel each other out, resulting in smaller differences between the high and low tides.

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