Earth's moon

How was our moon formed? How do we know? What is it made of? What do the craters on the Moon tell us? What is tidal locking, and why is the Moon tidally locked to the Earth?

Earth's moon, with color enhanced

Subtle differences in the Moon's color have been enhanced to provide detail about the chemistry of the Moon's surface. Titanium rich areas show up as blue in this photograph, while orange and purple areas are much more deficient in titanium. The large crater in the upper right is Tycho crater, with the Sea of Tranquility below it, where Neil Armstrong first walked on the Moon. The silhouette of the International Space Station (ISS) is also seen, although it is much closer to Earth than it seems to appear here.

Formation of the Moon

There are three main possibilities of how the Moon was formed:


  1. It formed at the same time and place as the Earth, as part of the same system
  2. It formed elsewhere and was captured by Earth's gravity
  3. It formed via a collision between Earth and another protoplanetary object

If the Moon formed along with the Earth, we would expect it to be very similar to the Earth in composition, as it would have formed out of the same material.


If the Moon was a captured object that formed elsewhere in the solar system, we would expect its composition to be different from that of the Earth.

giant impact that could have formed the moon

The third theory, called the Giant Impact Hypothesis, is the one that is most widely accepted by astrophysicists today. This theory suggests that the present-day Earth and Moon resulted from a glancing collision between the early Earth and a Mars-sized object that was named Theia. The Earth recovered from this massive collision, part of Theia's core merged with that of Earth, and some of Earth's mantle was ripped away. The material torn from Earth and the remnants of Theia formed into a debris disk around Earth, much like the rings that can be seen around Saturn today. The debris disk coalesced in time to form the Moon.


The reason this theory is more widely accepted than the first two is the degree of similarity between the Earth and Moon. Their compositions are extremely similar, differing by only a few parts per million when measuring the isotopes in their chemical makeups. Also, the Moon is considerably less dense than Earth, just as we would expect if much of Theia's core merged with Earth's core.


The amount of tungsten present on the surface of the Earth and Moon offers another clue. Tungsten binds strongly with iron, so any tungsten present in the early stages, during the collision, would have sunk to the core. Tungsten present on the surface today should have accumulated later, so the amounts seen on the Earth and Moon should be different. Measurements of surface tungsten on the Earth and Moon show that the amounts of this element agree with impact models.


In essence, the Earth and Moon are very much alike, but just enough different that we do not think they formed primordially as one system. They are the result of a collision early on in the history of Earth.

early coalesced moon

After the debris disk coalesced into a sphere, the early Moon was extremely hot. It cooled off over time, from the outside in.

early volcanism on the Moon - formation of maria

As the early Moon was cooling, the young Solar system underwent a period of heavy bombardment. Many asteroids and comets collided with the Moon, and since the young Moon was still so hot inside, the surface heated up to form volcanic seas known as Maria. These large basalt regions are darker than much of the Moon's surface because they are rich in iron. They form the large dark splotches still seen today on the Moon's surface. Earth also suffered many impacts during the heavy bombardment period, but the effects on Earth have largely worn away via erosion and other resurfacing mechanisms like tectonic plate activity.

Tidal locking

Tidal locking is the coupling of two gravitating bodies, where one of the bodies keeps the same face toward the other. Our Moon is tidally locked to Earth - that is why we always see the same side of the Moon. The other side is mistakenly called the "dark side of the Moon." It is not always dark, sometimes it is lit by the Sun. However, we never see it, because of tidal locking. It is more accurately called the "far side of the Moon."

tidal locking slide 1

The Earth and Moon both rotate in the same sense, that is, as viewed from above the North Pole, both rotate in the counterclockwise direction. The Moon is also orbiting the Earth in the same sense.


The gravitational force from the Earth tended to pull the Moon into an oval shape. The young, hot Moon was deformed under this pull. As it rotated, the bulge tended to point toward the Earth, but was also pulled around by the rotation of the Moon. The Moon cooled and solidified over time. The pull from Earth tended to draw the bulge backward, slowing the rotation of the Moon.

tidal locking slide 2

Over time, the rotation of the Moon slowed until it matched the orbital motion. The same side of the Moon now always faces Earth. This is called tidal locking.

tidal locking slide 3

The Moon's gravity also pulls on Earth, especially affecting the ocean. The tidal force causes the water to be pulled toward the Moon, elongating the Earth slightly. Friction between the Earth's crust and the ocean drags the water slightly forward, sing the Earth is rotating faster than the Moon is orbiting.


The tidal force of the Moon pulls backward on the bulge, slightly slowing down the Earth's rotation. This has the equal and opposite effect of slightly speeding up the Moon's orbital motion, sending it spiraling outward. The moon is slowly moving away from the Earth, at a radial speed of about four centimeters per year.


Given enough time, Earth would become tidally locked to the Moon, always keeping the same side pointed toward the moon. Tidal locking is a natural consequence of gravitational coupling between any two closely orbiting massive bodies.


The tidal locking of the Moon to the Earth is an example of resonance. Resonance occurs when there is an agreement in timing between objects in a system that happens because of physical reasons.


In the case of our Moon, there is a 1:1 resonance between its rotational period and its orbital period. It rotates once on its axis every time it orbits the Earth.


There can be relationships in timing that are not examples of resonance. For example, Venus orbits the Sun every 225 Earth days and Earth orbits the Sun every 365 days. We would not say that there was a 225:365 resonance between the orbital periods of the two planets because there is not a physical reason for the agreement in time. Nothing between the two planets causes the timing.


Solar and lunar eclipses

solar and lunar eclipse diagrams
  • Solar eclipses occur when the Sun is blocked by the Moon
  • Lunar eclipses occur when the Moon is shadowed by the Earth


The plane of the orbit of the Moon around the Earth is not level with the plane of Earth's orbit about the Sun. There is a small tilt (about five degrees) between the planes. If it were not for this small tilt, there would be solar and lunar eclipses every month. The tilt is high enough that most often, the shadows do not intersect.

solar eclipse

When a solar eclipse happens, the Moon is between the Earth and Sun. Sometimes the bodies do not line up perfectly, as in this partial eclipse.

This map from shows the paths of all solar eclipses from 2021 - 2030 with total eclipses shown in blue and partial eclipses shown in red. For more information on solar eclipses, please see NASA's Eclipse site or their Solar System Exploration: Eclipses site.