Earth’s moon is exceptionally large relative to the planet it orbits, measuring a little over a quarter of Earth’s own diameter. According to Space.com’s breakdown of lunar size data, this makes it the largest moon relative to its host planet anywhere in the solar system. By comparison, Mars has two small, lumpy moons, Phobos and Deimos, which look more like captured asteroids. Known as Luna to the Romans and Selene and Artemis to the Greeks, Earth’s natural satellite was created 4.6 billion years ago, as detailed in the Nine Planets profile of the Moon.
The Giant-Impact Origin of Earth’s Unusually Large Moon
For a long time, three competing hypotheses attempted to explain the moon’s origin. One theory proposed that the moon formed elsewhere and was captured by Earth’s gravity; a second suggested that a young, fast-spinning Earth flung off a piece of itself; and a third proposed that Earth and the moon formed side by side from the same cloud of dust. However, none of these ideas survived once scientists compared the chemistry of moon rocks brought back by Apollo astronauts with rock from Earth’s mantle.
The Role of Theia and Inner Solar System Origins
Scientists eventually favored the giant-impact hypothesis, which posits that a Mars-sized world nicknamed Theia struck the early Earth at an angle. This catastrophic collision vaporized part of both worlds and threw a disk of debris into orbit that eventually clumped together into the moon. Recent analysis of moon samples from Apollo missions, terrestrial rocks, and meteorites indicates that Theia and proto-Earth originated from a similar region of the inner solar system, likely forming even closer to the sun than Earth, according to Live Science’s reporting on the study by Max Planck Institute geoscientist Timo Hopp and his team.
During the turbulent first 100 million years after the sun formed, the inner solar system featured dozens to hundreds of planetary embryos that frequently collided, merged, or were shifted into new orbits by gravitational chaos and Jupiter’s immense pull. Through modeling iron, molybdenum, and zirconium isotopes alongside meteorite compositions, researchers determined that Theia was likely a rocky, metal-cored world containing roughly 5% to 10% of Earth’s mass.
Tidal Forces and Earth’s Changing Rotation
The gravitational interaction between Earth and the moon produces notable physical effects, most obviously ocean tides. Because Earth and its oceans are not perfectly rigid, the moon’s stronger gravitational pull on the near side stretches the planet along the line toward the moon. Earth’s rotation carries these bulges slightly ahead of the point directly beneath the moon, creating a torque on Earth and an accelerating force on the moon. This transfers rotational energy from Earth to the moon, slowing Earth’s rotation by about 1.5 milliseconds per century and raising the moon into a higher orbit by about 3.8 centimeters per year.
Sediment layers laid down by tides over millions of years record how fast Earth used to spin. Geologists reading tidal rhythmite bands in South Australia determined that a day was 21.9 hours long some 620 million years ago, resulting in a 400-day year. Norman Murray, a theoretical astrophysicist at the University of Toronto’s Canadian Institute for Theoretical Astrophysics, noted that when the moon first formed 4.5 billion years ago, the day was less than 10 hours long, with the moon’s gravitational pull steadily slowing planetary rotation over time.
