The Moon is a familiar sight in the night sky, a constant celestial companion whose phases define our calendar and whose gravity drives the ocean tides. Although humanity has looked up at its face for millennia, scientific exploration has revealed a world far more dynamic and complex than its calm, cratered surface suggests. Our nearest neighbor is a planet-sized body with a dramatic origin, an unstable orbit, and internal processes that still shape its structure today.
Violent Birth and Early History
The leading scientific explanation for the Moon’s existence is the Giant Impact Hypothesis. Approximately 4.5 billion years ago, a Mars-sized protoplanet, sometimes named Theia, slammed into the early Earth. This impact was so energetic that it vaporized and ejected massive amounts of material from the outer layers of both bodies into orbit around the proto-Earth, from which the Moon then coalesced.
Lunar rocks exhibit oxygen isotope ratios nearly identical to those on Earth, suggesting a common origin for the source material, largely derived from Earth’s mantle. The high-energy nature of the impact also explains the Moon’s relatively small iron core. The dense, metallic core of the impactor likely merged with Earth’s core, leaving the Moon to form predominantly from lighter, iron-poor silicate material.
The Moon’s Perpetual Motion and Recession
The Moon has a unique relationship with Earth that results in two distinct orbital phenomena. The first is tidal locking, meaning the Moon rotates on its axis in the exact same time it takes to complete one orbit around Earth, about 27.3 days. This synchronization is why we only ever see one side of the Moon, often called the near side.
The second phenomenon is a slow, measurable recession from Earth caused by the gravitational interaction between the two bodies. The Moon’s gravity creates tidal bulges on Earth. Because Earth rotates faster than the Moon orbits, the bulges are pulled slightly ahead of the Moon. The Moon’s gravitational pull on this leading bulge transfers angular momentum, causing the Moon to spiral outward at a rate of approximately 3.8 centimeters per year. This loss of angular momentum means Earth’s spin is gradually slowing down, causing the length of an Earth day to increase by about 2.3 milliseconds per century.
Unique Internal Structure and Seismic Activity
The Moon is not a solid, geologically inert sphere but a differentiated body with a crust, mantle, and a small core. Geophysical data confirm the existence of a partially molten, iron-rich core with an estimated radius of about 330 kilometers, which includes a solid inner core. The Moon also experiences seismic events known as moonquakes, detected by seismometers left on the surface by Apollo astronauts.
These quakes are not primarily caused by tectonic plate movement like earthquakes, but by different mechanisms. The most common are deep moonquakes, which occur hundreds of kilometers below the surface and are triggered by the immense tidal stresses exerted by Earth’s gravity as the Moon follows its eccentric orbit. Other types include shallow moonquakes, which can reach magnitudes up to 5.5 on the Richter scale and are likely caused by the Moon shrinking as it continues to cool and contract.
The Extreme Conditions of the Lunar Surface
The Moon’s surface environment is defined by a near-perfect vacuum, as it has an almost complete lack of atmosphere. This absence of an insulating atmosphere leads to dramatic temperature fluctuations between sunlight and shadow. The equatorial surface can reach scorching temperatures of over 120 degrees Celsius during the lunar day.
In stark contrast, areas on the lunar poles that never receive direct sunlight, known as permanently shadowed regions, act as cold traps. Due to the Moon’s minimal axial tilt of about 1.5 degrees, the Sun always hovers low on the horizon at the poles, allowing the floors of deep craters to remain in perpetual darkness. Temperatures in these regions can plummet to approximately -250 degrees Celsius, which is colder than the surface of Pluto, and cold enough to preserve deposits of water ice. The lack of wind or liquid water erosion means that any disturbance, such as the footprints and rover tracks left by astronauts, remains pristine indefinitely.