What Causes the Tides?
How are the Moon and the Sun linked to the ebb and flow of sea level? Why do we typically see two high tides per day?
Sabrina Stierwalt, PhD
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What Causes the Tides?
Spend a day on almost any beach in the world and you will see the water’s edge advance toward land and then creep back out to sea twice. What causes these tides? You may have noticed that the tides are linked to the phases of the Moon, with the highest tides occurring when the Moon is either full or new. But how exactly does our small satellite affect our oceans from its spot over 230,000 miles away?
It’s all about gravity.
As the Earth rotates on its axis, the side facing the Moon constantly changes. Even though the Moon is more than 100 times less massive than the Earth, our satellite still exerts a gravitational influence on our planet because it is relatively nearby, at least on astronomical distance scales. Since our oceans are not rigid and can flow over the Earth’s surface, they bulge slightly outward from the side of the Earth that faces the Moon. The same tug is felt by the land, but it is imperceptible in comparison.
The bulge toward the Moon due to the gravitational pull stays in the same place: on the side of the Earth that is directly facing the Moon. The Earth below rotates, however, so that bulge is observed moving over the surface of the Earth (i.e., changing in longitude) in the form of tides.
It’s all about gravity—except when it’s about inertia!
We know the Earth only rotates on its axis once per day—that’s how we define a day!—so how are there two low/high tide cycles per day and not just one? While the Earth’s surface will bulge outward toward the Moon on the side facing our satellite, the other side of the Earth—the side directly opposite the Moon—will also bulge outward but in this case the bulge faces away from the Moon.
This opposing bulge is due to inertia a phenomenon which we discussed in a few Quick and Dirty Tips Can We Feel the Earth Moving
episodes. Inertia can be summarized as the tendency for an object in motion to want to stay in motion, and the tendency for an object at rest to want to stay at rest. You feel inertia when you are sitting in a car that begins to accelerate. Inertia is the feeling that you are being forced back in your seat which is actually a result of your body resisting the acceleration in order to maintain its previous motion (or lack thereof).
So as one side of the Earth gets tugged ever so slightly towards the Moon, the ocean water on the opposite side wants to stay put and gets left behind in the form of a bulge. As the Earth rotates, and a particular beach passes under these two bulge locations, the tides observed there will be high. In order to form these bulges, ocean water must be pulled away from other locations where they experience low tide.
Each tide cycle actually takes 24 hours and 50 minutes to complete. This is the time it takes for the Moon to start from and then return to a position directly overhead of the same position or longitude on Earth. The extra 50 minutes is needed because while the Earth rotates, the Moon moves a bit too. So after a 24-hour period, the Moon will be 50 minutes shy of returning to the same spot on Earth.
There are also a few special places on Earth where sometimes only one high tide occurs each day. This effect is due to the fact that the Earth is tilted on its axis (by 23.5 degrees) and so the Moon does not orbit exactly around the Earth’s equator.
What else affects the tides?
The difference between high and low tide changes with location due to the fact that the Earth is not a perfect sphere. There are also obviously large landmasses in the form of continents that get in the way of oceans flowing freely.
The sun can also amplify the effects of the Moon’s gravitational pull. This is the highest tides are observed during new and full moons: the Sun, Earth, and Moon are all in a line during these phases so the Sun’s gravitational pull adds to the pull of the Moon, whether it be during the high tide on the side of the Earth facing the Moon, or for the high tide on the Earth’s opposing side.
Any other crossword puzzle addicts like me will know well the term “neap tide” which refers to very low tides. These occur when the Sun is at a right angle from the Moon (i.e., the Sun, Earth, and Moon form an “L”) so that the Sun’s gravitational pull goes toward canceling out the Moon’s effect.
Weather can also affect tides, particularly during periods of high winds. Powerful winds can actually force high tides back into the ocean or bring low tides further inland.
For more information on tides, or to get precise conditions for the timing of high and low tide at your favorite beach, check out the National Oceanic and Atmospheric Administration’s Click here for more information.
Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.comcreate new email.
Ocean tide image courtesy of Shutterstock. Tide diagram courtesy of noaa.gov
