Earth-Moon interactions

Interactions between Earth and its moon have led to many effects on both bodies. Some of these effects are ocean tides, rates of rotation and revolution, and other basic forces, especially gravity.

Gravitational Interplay between the Moon and Earth

The moon, Earth's closest celestial neighbor, exerts a variety of forces on the earth. These forces affect planetary motions and activities on the bodies' surfaces. Even though the moon is far smaller than the sun, it applies regular and measurable forces that affect the movement of the earth and activities on the earth's surface because it is so close to Earth.

The moon orbits Earth approximately once every 27.3 Earth days. At an average distance from Earth of about 384,400 kilometers (km), or 238,900 miles (mi), the moon's mass has a gravitational force that pulls Earth toward it as Earth, likewise, tugs at the moon. The physics of rotation and orbital motion prevents the two objects from crashing into each other; rather, they are “falling together” toward and around the sun. These interacting forces have a number of effects.

One of the most noticeable effects of the moon on Earth is tides, or the daily rising and subsiding of sea levels worldwide. The side of the earth that is closest to the moon at a given time will feel a stronger pull from the moon's gravity than the sides not facing the moon. This uneven pull, called a tidal force, causes Earth to stretch very slightly, like a squashed globe. The parts of the globe that stretch out on the side facing and opposite the moon are called tidal bulges, and are where ocean tides rise. As the earth rotates on its axis once every twenty-four hours, the tidal bulges try to follow the pull of the moon, causing ocean levels to rise and fall twice a day in most places on Earth. Ocean levels can change as much as 40 feet in some places because of high tides.

The sun also affects Earth's tides. Even though it is far larger than the moon, its greater distance makes the tidal force from the sun weaker than that of the moon. Generally, how close an object is rather than how large an object is will increase its tidal force. However, when the moon and sun are roughly aligned on the same side of the planet (during the full or new moon), their forces combine to cause slightly bigger tides than normal; these tides are dubbed spring tides. When the sun and moon are 90 degrees apart and when their forces are counteracting each other, Earth experiences smaller tides than normal, which are called neap tides.

The gravitational pull between Earth and the moon also cause the moon to become tidally locked, synching the moon's rotation around its axis and its revolution around the earth. This synchronous rotation causes the same side of the moon to always face Earth, and the other half (dubbed the “dark” side of the moon because it is never seen from Earth) to always face away.

Motions of the Earth and Moon

The tidal effect has additional consequences besides rising and lowering water levels on Earth. For one, the moon is accelerating away from Earth and appears to be farther away. This occurs because Earth's tidal bulges have a gravitational force that tries to pull the moon along with them. The gravitational pull from the tidal bulges attempts to speed up the moon in its orbit around Earth, adding more torque to the moon's orbit and increasing its angular momentum. The overall effect results in the moon accelerating forward in a larger orbit, moving it away from Earth at about 3 to 4 centimeters per year.

This tidal tug-of-war also has an effect on Earth's motion, gradually slowing its rotation and lengthening its days. According to Newton's third law, objects will exert equal and opposite forces on each other, so energy is conserved within the Earth-moon system. The angular momentum transferred to the moon's orbital movement, and energy added to its orbit from Earth's rotation, causes Earth to slow down. Just as Earth tries to speed up the moon, the moon's own gravitational pull is trying to slow down the rotation of the tidal bulges.

Because the moon is a relatively substantial mass (larger than Pluto and about 1/81 of the mass of Earth), its pull causes Earth's rotation to slow very slightly. This effect is great enough that Earth's days are slowly getting longer at about 1 second every 50,000 years. So, in one hundred years, Earth days will be 1.6 milliseconds longer. This slowing-down effect was first indirectly suggested in 1695 by British astronomer Edmond Halley, who said the moon was moving faster than before.

The Wobble Effect

In addition to affecting Earth's rotation, the moon helps the earth maintain a fairly stable rotation on its axis. The earth does not rotate in a straight-up or straight-down fashion, like a spinning top. Rather, it tilts on its axis at 23.5 degrees. This tilt means that over time, its axis, pointing to the sky, will swing in a small circle. Like a spinning top that is tapped from the side, Earth wobbles a bit around its tilted axis as it rotates. It stays at its 23.5-degree tilt, but it points in different directions.

Earth's axis moves in a circular motion called precession, a phenomenon first noted by ancient Greek astronomers. Earth completes this axis wobble, or a precession, once every 2,600 years. This means that Earth's axis will point in a different direction over time, so the star that appears over the North Pole will change. Earth's north now points to Polaris in the constellation Ursa Minor, but two precessions ago, Thuban in the constellation Draco was the North Star, and in 13,000 years, Vega in the constellation Lyra will be the new North Star. This motion is also called the precession of the equinoxes.

The wobble occurs because Earth is not a perfect sphere, but rather is flattened slightly with a bulge around its equator that acts like a thick belt. The moon and the sun pull on the equatorial bulge, which causes the wobble. These gravitational forces cause nutations, or slight periodic vibrations, within the larger precession movement. The most notable nutation that Earth experiences is an 18.6-year cycle (identical to the precession of the moon's orbit, which lies at an angle of 5 degrees to that of Earth).

In addition to contributing to Earth's precession, the moon helps to stabilize Earth's tilt by acting as a constant force. The planet Mars, for example, wobbles dramatically on its axis because it is pulled in different directions by large celestial bodies. If Earth's wobble was more extreme, climate cycles on Earth would be much more drastic and life as it is presently known might not exist in its current form. By stabilizing Earth's wobble, the moon has helped contribute to Earth's climate and, consequently, its biodiversity.

Broader Lunar Effects

The moon also influences Earth's magnetism, though how it does so is not yet completely understood. Earth's magnetic field, or magnetosphere, surrounds Earth, arising as a result of the motion of its molten core. The magnetosphere is not a circular bubble, and it is pushed away from Earth's sun-facing side because of the solar wind, which is a constant flow of high-speed, charged electrons, protons, and other particles emitted from the sun. The solar wind pushes back on Earth's magnetic field, creating a long, magnetic “tail” facing away from the sun, which the moon crosses into during its full-moon period.

Earth's magnetic tail likely charges the moon's surface and can cause a host of activity on its surface, such as dust storms and potentially floating lunar rocks. Before the solar wind reaches Earth, however, it will interact with the moon's weak, varying magnetic field, perhaps affecting how the solar wind reaches Earth's magnetosphere. It is not entirely known how the solar wind affects Earth or how it electrifies Earth's atmosphere. Space missions such as the National Aeronautics and Space Administration's Artemis (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) and LRO (Lunar Reconnaissance Orbiter) are working to provide more insight. Artemis, for example, aims to glean understandings about the effects of the solar winds on the two bodies by remaining outside Earth's magnetosphere and monitoring solar wind streams.

Tidal interactions between Earth and the moon may also be the cause of the moon's mysterious magnetic source. Normally, a planetary body needs the churning of a dynamically moving liquid core, called a dynamo, to generate a magnetic field. The moon's core dynamo likely ceased around 4.2 billion years ago, but magnetic lunar rocks much younger than that have been found, suggesting another, external source of local lunar magnetism. Some researchers theorize that Earth's pushing of the moon into a farther orbit created the churning needed to generate magnetism in these rocks. It may also be that a large impact on the moon created its faint, uneven magnetism.

The moon has also played a large role in shaping human life on Earth culturally, mathematically, and scientifically. Since the beginning of human history, humans have been inspired by the moon and have sometimes even feared it. Countless mythological beings, such as the goddess figures Hecate and Artemis (ancient Greece), Isis (ancient Egypt), and Shing-Moo (ancient China), are associated with the moon. Stories of werewolves and wild behavior have long been associated with the moon. (The word “lunatic” is derived from the moon's Latin name, luna.) Lunar (and solar) eclipses have frightened and excited people for millennia and have inspired a great deal of thought and discussion.

The moon has also inspired scientific insights. By observing its motions and phases relative to other celestial bodies, humans could begin to calculate astronomical distances, such as how far Earth is from the moon and sun, and to calculate the approximate size of Earth. The moon provided a way to keep time on a scale larger than that of days by providing a longer rhythmic cycle, that of months.

The moon also prompted technological advancements as nations raced to be the first to reach the moon. The Soviet Union sent the first satellites to the moon in 1959, culminating in the first human landing on the moon by the United States in 1969. Rocks retrieved from the moon and the many orbiting satellites and landing spacecraft that have studied its surface have helped humans to understand astrogeology, the history of the solar system, and Earth itself.

Most astronomers believe the moon formed when a large celestial body crashed into Earth about 4.5 billion years ago, spitting up debris that circled Earth and eventually became the moon. This theory may also explain how other planets came to have moons. In the unprotected surface of the moon's asteroid-pocketed landscape, scientists can see a history of the earlier universe and of cosmic events such as asteroid showers that might have enveloped both Earth and its moon.

Principal Terms

axis: the point around which a planet rotates

dynamo: the actions of a moving, liquid core of a planetary body

neap tides: smaller than normal tides that occur when the sun and moon are 90 degrees apart

nutation: a periodic wobble or oscillation in an axis of rotation

precession: the axial movement of a spin axis of a planet, moon, or other astronomical body in a circular motion around a cone shape

precession of the equinoxes: the slow precession of the earth over 2,600 years

spring tides: larger than normal tides that occur when the sun and moon are approximately aligned

synchronous rotation: when a planetary body revolves at the same rate as it rotates; synchronous rotation of the moon around Earth is why only one side of the moon is visible from Earth

tidal bulge: the areas of a planet's surface and bodies of water that become “stretched out” when acted upon by gravitational pull

tidal force: the gravitational pull of an astronomical body that causes a squashed, stretched-out effect of a planet's surface (and, in Earth's case, bodies of water)

tidally locked: the phenomenon in which the gravitational pull between Earth and the moon results in the moon revolving around Earth at the same rate as it revolves on its axis so that the same side of the moon always faces Earth

tides: the rising and falling of water levels, usually twice a day, caused by the moon's gravitational pull

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