When teachers explain ocean tides, they frequently describe how the Moon's gravity pulls on Earth and all of its water. This, they often say, leads to a gravitational imbalance, which stretches the ocean into two opposing bulges: one that's closest to the Moon (where the Moon's gravity is strongest); and one on the opposite side from the Moon (where its gravity is weakest). But this differential in gravity doesn't mean the Moon is 'lifting' or 'stretching' the oceans. If that were how tides worked, we'd also see lakes, ponds, or even backyard pools bulge in sync with the Moon. "Every YouTube video I've ever seen about tides, including ones made by smart people, explains tides incorrectly," Gabe Perez-Giz, an astronomer and astrophysicist at NYU, said in a video by PBS Space Time. "Don't get me wrong, the facts are correct. There really is a gravity differential. … What's wrong is the explanation for the bulges." At any given point on Earth, the Moon's gravity is about 10 million times weaker than our planet's own gravitational force, Perez-Giz says. So the difference between the side of Earth that's facing the Moon and the opposite side is minimal. Our tidal bulges are actually the product of a complex dance of gravity between the Moon, Earth, and Sun. And the total effect is more of a 'push' than a 'pull' on Earth's water. Although each drop of water on Earth is indeed pulled by the Moon's gravity, the effect isn't noticeable on a molecular level since Earth's inward pull is overpowering. The key, however, is that ocean water covers about 71 percent of Earth's surface and is connected as one liquid body. This allows the small force on each water molecule to collectively add up to "a pretty decent increase in water pressure," Perez-Giz says. Molecules of water near Earth's poles are pulled mostly straight down toward the planet's center of gravity (near its core), and the molecules closest to the Moon (at Earth's equator) experience the strongest pull toward the Moon. Water molecules that are farthest from the Moon, meanwhile, feel the weakest gravitational acceleration. Since water molecules can easily move and bump into one another, these countless tiny nudges add up and 'squeeze' seawater away from the poles. This global water pressure works against Earth's gravity to form two bulges: the high tides. "The ocean isn't being lifted or stretched," Perez-Giz says. "The ocean is bulging along the Earth-Moon line in the same way that a blister or pimple will bulge up if you start to squeeze it from the side[s]." These high-tide bulges stay put as Earth rotates underneath them every 24 hours, leading to a tide change every 6 hours. Low tides occur where the disruption caused by tidal force is weakest (and water pressure caused by Earth's gravity is strongest). Dramatic tides can result where land and seafloor terrain funnel more seawater into one spot. Smaller bodies of water, like lakes and pools, don't have noticeable tidal bulges because they lack enough liquid to create pressure that can visibly overcome the pull of Earth's gravity. The Sun's gravity also affects the tides, accounting for roughly one-third of the phenomenon. When the Sun's gravity counteracts the Moon's, it leads to lower-than-average "neap tides." When the Sun lines up with the Moon, it triggers larger 'spring tides'. Watch Perez-Giz's full explanation below: This article was originally published by Business Insider. More from Business Insider:
Why are tides higher not just during a New Moon, but also during a Full Moon? I understand this has to do with the alignment of the Sun, the Earth and the Moon, but I would expect that the gravitational effects of the Moon would be weaker during a Full Moon as the Sun is "pulling" from the opposite direction? That's a great question! Tides are caused by tidal forces, and the answer to your question lies in the definition of a tidal force. A tidal force is related to gravity, but it isn't the same thing. It's really the difference between the the strength of gravity at two locations. The gravitational attraction between two objects (say the Earth and the Moon) decreases with distance. This means that the Moon's gravity pulls most strongly on the side of the Earth closest to the Moon and least strongly on the side of the Earth farthest from the Moon. Tidal forces on the side of Earth closest to the Moon pull material (mostly water) toward the Moon. Tidal forces on the other side of Earth actually pull material away from the Moon. The resulting deformation of Earth looks the same when the moon is at opposite sides of its orbit, like full moon and new moon or first quarter and third quarter, as shown in the diagram on this page. That's why tides around the equator are higher during both a new moon and a full moon (spring tide). The Sun also affects the Earth's tides. However, tidal forces due to the Sun are about half as strong as those due to the Moon. This seems strange, because the Sun's gravity at Earth is much stronger than the Moon's. But remember that tides concern the difference between gravity's pull at opposite sides of the Earth. The radius of the Earth is a very small fraction of the distance between the Sun and the Earth, about 0.005%. As a result, the difference between the Sun's gravitational pull on either end of the Earth is small. In contrast, the radius of the Earth is about 1.7% of the distance between the Earth and the Moon. So even though the Moon's gravity isn't as strong as the Sun's, lunar tidal forces are stronger than solar tidal forces, so lunar tides are stronger than solar tides. This page was last updated on February 11, 2016.
Michelle is a second year astronomy graduate student at Cornell. She works with Professor Dong Lai to study tidal interactions between white dwarfs and black holes.
- Anonymous, Rochester, NY Greetings! Many people believe that the Moon does not have any gravity. In fact, the Moon, like every other massive object in the Universe, attracts every other massive object gravitationally. Even subatomic particles such as protons and neutrons exert a gravitational pull on proximate objects, although it is so slight as to be negligible. We use the term "surface gravity" in reference to the downward "pull" that objects experience when resting or moving on a larger body. Earth's average surface gravity is about 9.8 meters per second per second. When an object is tossed off a building top or a cliff apex, for instance, it accelerates toward the ground at 9.8 meters per second per second. The Moon's surface gravity is about 1/6th as powerful or about 1.6 meters per second per second. The Moon's surface gravity is weaker because it is far less massive than Earth. A body's surface gravity is proportional to its mass, but inversely proportional to the square of its radius. (To see how one can calculate the Moon's surface gravity, consult the Math Zone 6: http://usm.maine.edu/planet/mz-6-calculating-planets-surface-gravity) The Apollo astronauts were able to walk on the lunar surface because the Moon exerted a gravitational pull on them. Of course, the astronauts were able to leap higher on the Moon than on Earth because the Moon's surface gravity is so comparatively weak. When on Earth, a fully suited Apollo astronaut weighed about 500 pounds, equipment included. His weight was only about 80 pounds on the Moon.* We also draw your attention to the now-famous hammer-feather drop demonstration that Apollo 15 astronaut David Scott performed on the lunar surface. He demonstrated that in a vacuum, the falcon feather and a hammer, when dropped simultaneously from the same height, reach the ground at the same time. www.youtube.com/watch?v=5C5_dOEyAfk You will notice that the objects fall slowly, because their acceleration toward the surface is only 1/6th what it would be on Earth. Commander Scott's demonstration proved that objects of unequal mass fall at the same rate and, of course, proved that the Moon does, indeed, have gravity. *Some people use the terms "mass" and "weight" interchangeably. In fact, these values are quite different. "Mass" measures body's resistance to inertia. Provided you don't add or lose body matter, your mass is the same here as it would be on the Moon, Pluto or any place in this or any other galaxy. "Weight" measures the gravitational attraction the planet exerts on your body. You do not have the same weight on Earth as you would on the Moon, Pluto, or even the Sun or a neutron star. |