An unusually large companion
Compared with the size of its planet, Earth’s Moon is large. Its diameter is a bit more than a quarter of Earth’s, which is a bigger ratio than for most moons orbiting the other rocky planets. That oddity is one reason the Moon’s origin has been debated for a long time. Any good explanation has to account for its size, its orbit, and what it is made of.
Older ideas that fell short
Early proposals included several simple stories. In one, a fast-spinning young Earth flung off a piece of itself. In another, the Moon formed elsewhere and was captured by Earth’s gravity. A third said Earth and the Moon grew side by side from the same cloud of material.
Each idea struggled with some of the evidence. Capturing a body this large is hard to do without it simply flying past. A separate formation would likely leave the Moon with a different composition from Earth. A fission scenario had trouble with the amount of spin that Earth and the Moon have in total.
The giant impact hypothesis
The idea most scientists favor today is the giant impact hypothesis. It proposes that, early in the solar system’s history, roughly 4.5 billion years ago, a Mars-sized body struck the young Earth. The impactor is often called Theia, though the name belongs to the hypothesis rather than to any observed object.
The collision would have melted and vaporized large amounts of material. Debris would have been thrown into orbit around Earth and then gradually gathered into the Moon. The hypothesis explains the Moon’s size and orbit and why it has a relatively small iron core. That matches the Moon’s low overall density compared with Earth.
The evidence and the puzzles
The main evidence comes from rocks. Samples returned by the Apollo missions, later supplemented by lunar meteorites and by measurements from orbiting spacecraft, showed that lunar rock is similar to Earth’s mantle in many ways. Its ages cluster around the early solar system. Its surface once appears to have been covered by a magma ocean, which fits a very energetic beginning.
There is a puzzle, though. Simple versions of the model suggested the Moon should contain much material from the impactor, yet the two worlds look very similar in certain isotope ratios. Researchers have proposed variations to address this, such as faster-spinning impacts, multiple smaller impacts, or thorough mixing in a hot cloud of debris. These refinements are active research, and no single version is settled.
Tides
The Moon’s most visible effect on Earth is the tides. Its gravitational force pulls more strongly on the side of Earth nearest to it and more weakly on the far side. The result is a stretch of the oceans that creates two bulges, so most coastlines see two high tides and two low tides in a bit more than a day, as the planet turns beneath them.
The Sun also pulls on the oceans, but its effect is smaller. When the Sun, Earth and Moon line up, at new and full Moon, the effects combine into larger tides called spring tides. At the quarter phases the pulls partly cancel, producing smaller neap tides. Local coastline shape, water depth and weather change the details a lot, and these factors interact with ocean currents in complex ways.
Tides have mattered for coastal ecosystems, which depend on tidal zones that are exposed and flooded each day. Many shore animals time their behavior to the cycle.
A slowly changing orbit
Tidal friction has a long-term cost. As Earth spins, the tidal bulge is dragged slightly ahead of the Moon, which tugs the Moon forward and pushes it onto a wider orbit. At the same time Earth’s rotation slows. The Moon is currently receding by a few centimeters each year, and days on Earth were shorter in the distant past. Records in ancient fossils and rocks support this, though they require careful interpretation.
The Moon also keeps one face turned toward Earth because tidal forces long ago slowed its own rotation to match its orbit.
Stability of Earth’s tilt
Earth’s spin axis is tilted by about 23 degrees, and the tilt is the reason for the seasons, as covered by axial tilt. Calculations suggest that the Moon’s gravity helps keep that tilt fairly steady over very long periods. Without it, the tilt might wander more widely, perhaps swinging climates through larger changes. This conclusion comes from models, and researchers continue to debate how strong the effect would be, so it is best taken as a likely benefit rather than a certainty.
For comparison, Mars lacks a large moon, and its tilt is thought to vary considerably over millions of years.
Cultural and practical value
Beyond physics, the Moon has served as a calendar, a navigation aid and a source of myth. Its phases are a handy way to keep track of time, and many traditional calendars are built on lunar months. For scientists, its ancient surface keeps a record of impacts that Earth has largely lost to erosion and moving plates. Counting craters on the Moon helps estimate the ages of surfaces across the inner solar system.
The Moon also remains a target for exploration. Plans for future crewed missions and robotic visits are part of the wider story of the history of spaceflight, and every new sample can test the origin ideas described above.
What remains uncertain
The broad outline is well supported: a violent early collision, a molten young Moon, and a long, slow separation. The details are less certain, including the exact size and speed of the impactor, how much of its material ended up in the Moon, and how the debris mixed. That uncertainty is normal in science about events more than four billion years old, and it keeps the Moon an active research topic.