Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences
What Are Ocean Tides?
If you live near (or have visited) the ocean, you might know that the water level changes throughout the day. You can be swimming at the beach and watch the water slowly creep farther up the sand. A few hours later, the water level will go down again. This repeats on a regular cycle, every single day, and we refer to this water movement as ocean tides.
High tide is when the water level is highest, and low tide is when the water level is lowest. There are two periods of high tide and two periods of low tide every day. All together, this is called the tidal cycle.
What Causes Ocean Tides?
Ocean tides are mainly caused by the gravitational pull of the moon. Really big objects like planets and moons have their own gravitational pull, and the force of gravity gets stronger when two objects are closer to each other.
The moon is the closest planet-like object to Earth. In fact, Earth's gravitational pull keeps the moon in orbit around the Earth. But, because the Earth and moon are so close to each other, the moon also exerts a gravitational force on the Earth, and we see this in the ocean tides.
This picture shows you how the moon affects the oceans on Earth.
Earth spins (or rotates) one full turn every day. At the same time, the moon is orbiting around Earth. Because both objects are moving, the moon changes its position relative to Earth throughout the day and night.
There are two high tides occurring at the same time. One high tide occurs where the moon is closest to Earth, and the water gets pulled toward the moon until land stops it. This high tide is caused by gravity and causes water to accumulate near the shoreline.
Then, on the exact opposite side of the Earth, the farthest point from the moon, there is also a high tide. This second high tide is caused by another force called inertia. Inertia causes the water of the ocean to want to keep moving in a straight line, so it causes a second area where water is pulled in one direction and builds up near shore.
If we take the points in the middle of these two water bulges, we get areas of low tide. The water levels drop in areas of low tide because the extra water has been pulled to the areas with high tides. As Earth spins and the moon orbits, the areas of high and low tide shift around the planet.
In total, two periods of high tide and two periods of low tide occur in most areas along the coast every day. Tides are also affected by the sun, wind, and weather patterns.
In most places, there are two high tides and two low tides every day, which is called the tidal cycle. One high tide occurs where Earth is closest to the moon, and this is caused by the force of gravity from the moon. The second high tide occurs on the opposite end of the Earth, the farthest point from the moon, and it happens because of a second force called inertia.
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