Physical Properties of the Moon
Composition and Structure
The Moon has a rocky composition similar to Earth's mantle, built in layers: a crust, a mantle, and a small, partially molten core. Its average density is about , making it the second densest moon in the solar system (after Jupiter's moon Io).
One of the Moon's most defining traits is what it lacks. It has no significant atmosphere, no liquid water on its surface, and no global magnetic field. Without an atmosphere, there's no weather or erosion from wind and water. That's why impact craters from billions of years ago are still perfectly preserved on its surface.
Size and Mass
- The Moon's diameter is about 3,474 km, roughly 1/4 the diameter of Earth.
- Its mass is approximately , which is about 1/81 of Earth's mass.
- Despite being much less massive than Earth, the Moon's gravity is strong enough to drive ocean tides on our planet.
Surface Features
The Moon's surface splits into two distinct types of terrain:
- Highlands: Bright, heavily cratered regions that represent the Moon's oldest crust.
- Maria (singular: mare): Darker, smoother plains made of ancient solidified basaltic lava. Early astronomers mistook them for seas, which is why they carry the Latin word for "seas."
The surface also includes large impact basins (like the Orientale Basin), mountain ranges (like Montes Apenninus), and volcanic features (like the Aristarchus Plateau).
The Moon's gravitational pull is the primary driver of Earth's tides. The Sun also contributes, but the Moon's tidal effect is roughly twice as strong as the Sun's because tidal force depends heavily on distance, and the Moon is much closer.
Phases of the Moon
Causes of Lunar Phases
The Moon doesn't produce its own light. What you see as moonlight is sunlight reflecting off the Moon's surface. As the Moon orbits Earth, the angle between the Sun, Earth, and Moon changes, so the amount of the Moon's illuminated side that you can see from Earth shifts throughout the month.
Two orbital periods matter here:
- Sidereal period (27.3 days): The time it takes the Moon to complete one full orbit relative to the distant stars.
- Synodic period (29.5 days): The time from one New Moon to the next New Moon. This is longer because while the Moon orbits Earth, Earth is also moving along its orbit around the Sun. The Moon has to travel a little extra to get back to the same Sun-Earth-Moon alignment.
The synodic period is what determines the phase cycle you actually observe.
Eight Main Lunar Phases
The lunar cycle moves through eight recognizable phases. "Waxing" means the illuminated portion is growing; "waning" means it's shrinking.
- New Moon: The Moon sits between Earth and the Sun. Its sunlit side faces away from Earth, so it's essentially invisible in the sky.
- Waxing Crescent: A thin sliver of the illuminated side becomes visible on the right (in the Northern Hemisphere).
- First Quarter: Exactly half of the Moon's visible face is illuminated. Despite the name, you're one-quarter of the way through the full cycle.
- Waxing Gibbous: More than half is illuminated but not yet full. "Gibbous" means "swollen" or "humped."
- Full Moon: Earth is between the Sun and the Moon, so the entire face you see is lit up.
- Waning Gibbous: The illuminated portion starts shrinking, but more than half is still visible.
- Third Quarter (Last Quarter): Half the visible face is illuminated again, but now it's the opposite half from the First Quarter.
- Waning Crescent: Only a thin sliver remains visible before the cycle resets to a New Moon.
A helpful way to remember: the Moon's illuminated area grows (waxes) from New Moon to Full Moon, then shrinks (wanes) from Full Moon back to New Moon.
Synchronous Rotation of the Moon
Tidal Locking and Its Effects
The Moon rotates on its axis in exactly the same time it takes to orbit Earth: about 27.3 days. This means the same hemisphere of the Moon always faces Earth. This isn't a coincidence. It's the result of tidal locking, a process where Earth's gravitational pull gradually slowed the Moon's rotation over millions of years until the rotational and orbital periods matched.
Tidal locking is common in the solar system. Many moons of Jupiter and Saturn are tidally locked to their planets as well.
Near Side and Far Side Differences
Because of tidal locking, we only ever see one face of the Moon from Earth. The hidden hemisphere is called the far side. It's often incorrectly called the "dark side," but it receives just as much sunlight as the near side. It's "hidden," not "dark." The far side was first photographed by the Soviet Luna 3 spacecraft in 1959.
The two sides look surprisingly different:
- Near side: Dominated by large, dark maria (like Mare Tranquillitatis, where Apollo 11 landed).
- Far side: Much more heavily cratered, with very few maria. Scientists think the near side's thinner crust allowed lava to reach the surface more easily after large impacts.
The far side also has a practical advantage for science: it's permanently shielded from Earth's radio signals, making it an ideal location for radio telescopes that need a quiet electromagnetic environment.
Lunar Surface Features
Impact Craters and Basins
Impact craters are the most common features on the Moon's surface, formed when asteroids, comets, and meteoroids slammed into it over billions of years. Without an atmosphere to burn up incoming objects or erode existing craters, even ancient impacts remain sharply defined.
- Prominent craters include Tycho, Copernicus, and Aristarchus, each with distinctive bright ray systems (streaks of ejected material radiating outward).
- The largest features are impact basins, created by especially massive collisions. The Orientale Basin, for example, spans about 930 km across and is surrounded by concentric mountain rings. Many large basins later filled with lava, forming the maria.
Volcanic Features
The Moon is no longer volcanically active, but ancient volcanism shaped much of its surface.
- Maria are vast basaltic plains formed when lava flooded large impact basins billions of years ago. Major examples include Mare Tranquillitatis, Mare Serenitatis, and Mare Imbrium.
- Domes are small, rounded hills found near the edges of maria, likely formed by thick, slow-moving lava pushing up from below.
- Rilles are long, narrow channels on the surface. Some, like Vallis Schröteri and Rima Hadley, are thought to be ancient lava tubes or channels where molten rock once flowed. Others may be tectonic in origin.
Other Notable Features
- Mountain ranges like Montes Apenninus and Montes Caucasus formed as a result of the massive impacts that created nearby basins. These aren't volcanic mountains like many on Earth; they're essentially the rims and debris from giant collisions.
- Lunar swirls, such as Reiner Gamma, are bright, curving patterns on the surface whose origin is still debated. Leading hypotheses connect them to localized magnetic anomalies in the crust that may deflect solar wind, keeping the surface from darkening over time.