By Jonny Lupsha, Wondrium Staff Writer
The Moon’s gravity doesn’t only make tides. It also slows Earth’s rotation and stabilizes Earth’s rotation axis in space. Additionally, it could contribute to massive flooding in the 2030s.
NASA is predicting extensive flooding in the mid-2030s in U.S. coastal regions that could stay wetter longer. This could mean big losses around the nation. Flood damage in South Carolina was so extensive over the last five years that the state recently planted 3 million seeds to mitigate additional damage. However, the 2030s flooding is partly to blame on an unusual suspect: the Moon.
Half of the Moon’s 18-year cycle causes high and low tides to be less extreme; the other half makes them more extreme. When added to rising sea levels, the more severe tides could spell annoyance—or danger—for U.S. coastlines. In her video series A Field Guide to the Planets, Dr. Sabine Stanley, Bloomberg Distinguished Professor in the Morton K. Blaustein Department of Earth and Planetary Sciences at Johns Hopkins University, explained the lunar tides.
Gravitational Pull
“Tides occur because the force of gravity depends on distance,” Dr. Stanley said. “The closer you are to a massive object, the stronger the gravitational force from that object. Think about it: The near side of Earth is roughly 59 Earth radii away from the Moon, but the far side of Earth is 61 radii away.”
Furthermore, since different locations on Earth are different distances away from the Moon, the force of gravity is different at those locations, too. Of course, the Moon’s gravitational pull is weakest at the parts of Earth that are furthest away from it. Additionally, Earth is always rotating and the Moon orbits it. These combined factors make the tides.
“The difference in the Moon’s gravitational force between the near and far sides of Earth is about 7% of the average force, but it’s enough to change the sea height by a couple of feet on average,” Dr. Stanley said. “But the tides are more variable than what you would expect just from the Moon’s gravity and motion around the Earth.”
This is because there are several complicating factors affecting the Moon’s gravitational pull.
A Moon-Shaped Pool
The first complication affecting the tides is that the Moon has an elliptical orbit around Earth, which means its distance from Earth changes. Again, the closer the Moon is to Earth, the stronger its gravitational pull.
“Second, we also experience solar tides,” Dr. Stanley said. “They work exactly the same way as lunar tides, but the high solar tides occur at the points closest and furthest from the Sun. Now, the Sun is much further from the Earth than the Moon is, but the Sun is also much more massive.
“These two factors combine such that the solar tidal force is about half as strong as the lunar tidal force.”
Since the height of the tides depends on the alignment of the Earth, Moon, and Sun, during a new moon or full moon—when all three are in alignment—the tidal forces of the Moon and the Sun pull in the same direction. These make the largest high tides, known as spring tides.
A third complication is the ocean itself.
“Even the solid portions of the Earth are affected by the gravitational forces of the Moon,” Dr. Stanley said. “The shape of the ocean bottom near the shore, and the currents and waves in the region, will also affect how high the tides can be at a particular location. The highest tides experienced on Earth occur in the Bay of Fundy between Nova Scotia and New Brunswick in Canada.
“Here, the difference between high tides and low tides can reach over 50 feet.”