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 tides, and they each correspond to low tides. 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 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 changes the Earth’s shape slightly. This is not very noticeable on land, which does not change shape easily. On the 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 Affects 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, 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 cycle’s highest high tides and the lowest low tides. Neap Tides occur with a half-moon, when the sun and moon are at right angles to each other, resulting 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 across Earth would be similar. However, the earth is not completely round, and landmasses 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 affect the earth’s tides most, other forces are also at work. 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.
Resources
- NOAA National Ocean Service Education – “Tides and Water Levels.“
- NASA Science – Earth’s Moon – “Tides.“
- National Oceanic and Atmospheric Administration (NOAA) – Scijinks – “What Causes Tides?“
