If you have ever been to the coast and spent the day near the water, you may have noticed that at certain times of the day, the ocean reaches farther up the beach than at others. These times are known as high tide, and they each correspond to a low tide. Ocean tides occur twice a day and are caused by gravity. The sun and moon exert the most gravitational force on the earth, and this force stretches and pulls on the water. The sun exerts 177 times the force the moon exerts on the earth, yet the moon is the influences Earth’s tides most. So what exactly is the moon’s relationship to Earth’s oceans?
The Moon’s Effect on Tides
The moon’s gravitational pull on the earth actually changes the earth’s shape a little bit. This is not very noticeable on land, which does not change shape easily. On water, however, the effect is clear. The point nearest to the moon feels its attraction and bulges toward the moon while the point farthest from the moon resists its gravitational force, a process known as inertia, and bulges away from the moon.
Why This Effects Tides
This bulging, which tends to be biggest near the equator and less evident near the poles, follows the moon while the earth rotates. In other words, in a single 24-hour period, each location on earth will rotate into the positions of being closest to the moon and farthest from the moon. In each position, a high tide occurs as the water bulges either toward or away from the moon.
The Sun’s Effect on Tides
Despite the fact that the moon exerts the most influence on Earth’s tides, the sun also has an impact. In particular, this impact is most clear during the full moon phase and during the new moon phase. During these two phases, the sun and the moon line up in a straight line in relation to Earth. This phenomenon, known as Spring Tides, makes for the highest high tides and the lowest low tides of the cycle. Neap Tides occur with a half-moon, which is when the sun and moon are at right angles to each other, and this results in a high low tide and a low high tide.
The Variability of Tides
If the earth were perfectly round and entirely covered with water, then the tides all across Earth would be similar to each other. However, the earth is not completely round, and land masses also get in the way of the ocean’s flow. Thus, high and low tides in one place can be much more extreme than high and low tides in another.
Other Effects on Tides
While the moon and sun exert the most effect on the earth’s tides, there are other forces at work as well. The shape of bays and estuaries can affect a tide’s intensity. Weather patterns also play a major role. For example, strong winds blowing onshore can push tides higher, and low pressure systems can push water down and keep tides low.
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“What Causes Tides?” NASA. NASA, n.d. Web. 09 Jan. 2017.