Moon’s Gravitational Force, Tides, and Earth’s Speed

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley Ph.D., John Hopkins University

The Moon has only 17% of our Earth’s gravity. Yet, its gravitational force has saved the Earth from turning upside down in its orbit or undergoing significant climate changes every few centuries. The Moon is not only the brightest object in our night sky; it is also a caring teammate.

Planet earth and moon in the space with lens flare.
The Moon’s gravitational force affects the Earth in much greater aspects than just the tides. (Image: Pike-28/Shutterstock)

The Moon’s gravity is mostly known for the tidal waves. However, there are many more important things that the Moon’s gravitational force does on Earth.

Learn more about Saturn’s moons: Titan to Enceladus.

Lunar Tides

The tides happen due to Moon’s gravity. Different locations on Earth have different distances from the Moon: the near side of Earth is about 59 Earth radii away from the Moon, but the far side of Earth is 61 Earth radii away. Consequently, when the Moon is directly above a location on Earth, its gravitational force is the strongest. Thus, the high tide is created.

Interestingly, the same thing happens at the far side, even though the Moon’s gravity cannot pull the water against the Earth’s center. It is actually in the same direction as Earth’s pull, leading to high tide there as well. The difference in the Moon’s gravitational force between the near and far sides of Earth is about 7% of the average force.

The tides are affected not only by Moon’s gravity but also by its elliptical orbit, which shows the Moon’s distance from the Earth changes. The Sun also affects the tides.

Learn more about Neptune: Windy with the wildest moon.

Solar Tides

High tides happen at the points closest and farthest from the Sun. Even though the Sun is much further than the Moon, its gravity is significantly stronger.

Orbit of the Sun, Earth and Moon
The position of the Moon, the Sun, and the Earth affects the height of the tides. (Image: tutsi/Shutterstock)

The alignment of the Earth, Moon, and Sun determines how high the tides would get. If the Sun and Moon are on the same line with Earth, their gravities act in the same direction, creating the biggest high tides: spring tides. Spring tides occur during a new moon or full moon. On the other hand, during a first or last quarter Moon, the Earth-Moon line is vertical to the Sun-Earth line. Thus, the lunar high tide is pushed down by the low solar ride, creating the weak tides called neap tides. There is also another factor affecting the tides.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.

The Ocean Itself

Other than the Sun and the Moon, the shape of the ocean bottom near the shore, and the currents and waves in the region, also affect the tides’ height at a specific location. The Bay of Fundy between Nova Scotia and New Brunswick in Canada hosts the highest tides on Earth, whose difference with the low tides can be over 50 feet. Solid materials also experience the same force.

Moonquakes due to Earth’s Gravity

The Moon also experiences tides due to Earth’s gravity, but the 20-inch stagnant tidal bulge there changes only a few inches at every tide. However, there are some deep earthquakes on the moon caused by Earth’s gravitational force: the moonquakes. The Apollo missions left four seismometers that measured more than 7000 seismic events triggered by tides between 1969 and 1977.

Learn more about Humans on the Moon: A Never-Ending Story.

Earth’s Speed and the Moon’s Gravitational Force

Tidal forces make days get longer on Earth, and the Moon to get further away from it. About 41/2 billion years ago, the Moon was perhaps seven times closer to the Earth, and each Earth day lasted only a few hours since it was spinning much faster.

The Earth spins faster than the Moon, so when the Moon is right above a location, the Earth takes it away almost quickly. However, it takes the high tide and its effects some time to get back to normal conditions. While the tidal bulge is slightly ahead of the Moon, the Moon’s gravity pulls back on the tidal bulge, hence, slowing down the Earth’s rotation. Currently, the rate of slowing is only two seconds per 100,000 years.

Moon's surface
There is a tidal bulge on the Moon that is affected by the Earth’s gravitational force, the same way tides are created by the Moon’s gravitational force. (Image: HelenField/Shutterstock)

Corals hold the evidence of slowing down. Fossils of corals from over 400 million years ago show 420 days in one year. Younger corals show fewer days in a year, meaning the length of the days is getting longer.

Moon’s Distance from the Earth

As the Moon slows down Earth’s rotation, the Earth pulls the Moon forward. This increases the energy in the Moon’s orbit, forcing the Moon to move farther out to a slightly more distant orbit. Thus, the Moon gets further away about 11⁄2 inches per year.

Moon’s orbit and its rotation are also affected by the tides. Moon’s rotational period is equal to its orbital period, called a lunation. In other words, one year on the Moon is equal to one day.

The Earth also slows down the Moon’s rotation. The slowdown continues until the Moon’s rotational period equals its orbital period. Then, the Earth’s location would get fixed over the tidal bulge on the Moon. A Moon at this state is ‘tidally locked’ to its planet. Hence, today only one side of the Moon faces the Earth.

The Moons either slow down or speed up until they are tidally locked, and this has happened to most moons in the Solar System.

To round it up, the Moon and our Earthwork as a team and keep life stable and possible on Earth.

Common Questions about Moon’s Gravitational Force

Q: Is there any gravitational force on the moon?

Moon’s gravitational force is far less than Earth’s, but it is extremely important to the Earth. The force keeps Earth from rotating faster or changing the rotational angle significantly, thus, keeping conditions stable on Earth.

Q: What is the gravity on the moon compared to Earth?

Moon has a surface gravity 17% of the Earth’s, but its gravitational force is strong enough to keep the Earth from changing its orbital tilt drastically.

Q: How does the gravitational force of the moon affect Earth?

The gravitational force of the moon has played the significant role of keeping Earth from changing orbit tilt. Without it, the Earth could have lost the right rotation tilt for appropriate living conditions.

Q: What would happen without the moon?

In simple terms, without the Moon and its gravitational force, life would be impossible on Earth. The water that remains on the surface, the stable climate, and the stable rotational speed of the planet are all results of the Moon’s gravitational force.

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