Natural satellite
The Moon
The Moon is familiar enough to seem simple. Its changing face is actually a lesson in lighting and orbital motion, while its surface keeps a record of impacts and volcanism that Earth has largely erased.
Updated · About 6 minutes to read
- Orbits
- Earth
- Mean radius
- About 1,740 km
- Average distance from Earth
- About 384,400 km
- Orbit against stars
- About 27.3 Earth days
- Phase cycle
- About 29.5 Earth days
- Rotation
- Synchronous with its orbit
- Atmosphere
- Extremely thin exosphere
- Polar water
- Ice in some cold traps
Rounded reference figures: primary source. Distances and periods describe stated averages or reference frames, not live positions.
The far side is not permanently dark
The Moon rotates once in approximately the time it takes to orbit Earth, so the same hemisphere generally faces us. That is tidal locking, not a failure to rotate. The far side receives sunlight too; it simply does not face Earth from our usual viewpoint. Small changes in viewing angle let us glimpse a little more than half the lunar surface over time. Calling the unseen hemisphere the 'dark side' can be confusing because it suggests perpetual night. What changes across the Moon is which part is illuminated at a given moment.
Why the shape of moonlight changes
Lunar phases come from our changing view of the Moon's sunlit half as it travels around Earth. Earth's shadow causes a lunar eclipse, not the ordinary monthly phases. The Moon completes an orbit relative to distant stars in about 27.3 days, while the familiar new-moon-to-new-moon cycle lasts about 29.5 days. The difference appears because Earth and Moon move around the Sun during that time. A month in a calendar is a human convention; a synodic lunar month is an astronomical interval. Naming the reference frame keeps these numbers from seeming contradictory.
A surface that keeps old scars
Without rain, rivers, or active plate tectonics like Earth's, the lunar surface preserves impact craters over immense stretches of time. The bright highlands are heavily cratered, while darker maria are broad plains filled by ancient lava. 'Sea' is a historic name for those dark areas, not a claim that they contain liquid water today. Ejecta from impacts can spread bright rays across the terrain. Scientists can count craters and study returned samples to place events in relative and absolute time, though a crater count alone is not a perfectly precise clock.
How the Moon may have formed
The leading formation model begins with a large impact on the young Earth, followed by debris gathering into the Moon. It accounts for important features of the Earth-Moon system, but the exact impact conditions and later mixing are still studied. Lunar rocks, orbital dynamics, and measurements of the interior all test versions of this idea. A widely repeated animation can make the sequence look settled frame by frame, even though nobody observed it directly. It is better read as a model built to explain evidence than as a recording of a single known event.
Water in places that never see the Sun
The Moon has no global ocean or ordinary water cycle. Some permanently shadowed polar regions are cold enough to retain water ice, and instruments have found evidence for water in different forms and settings. Amount, distribution, depth, and accessibility are separate questions. Detecting a water-related signal from orbit does not mean a bright block of clean ice lies ready at every landing spot. Sunlight can vary dramatically across nearby ridges and crater floors, so maps of temperature, illumination, and terrain are needed alongside chemical observations.
Almost no atmosphere, but plenty of dust
The Moon has a very thin exosphere rather than a dense atmosphere that can smooth temperature swings or stop most small impacts. Its surface is covered by regolith, broken rock and dust made by repeated impacts and space weathering. Dust can be abrasive and cling to equipment, a practical issue shown by lunar exploration. A footprint can last a long time in this environment, but that does not mean the landscape never changes. Micrometeoroids, solar particles, and thermal cycling continue to work on the surface, just much more slowly than familiar Earth weather.
The Moon helps us understand Earth
The Moon raises tides through gravity, and its presence helps stabilize Earth's rotational behavior over long periods. It also gives scientists a nearby place to test ideas about rocky worlds and early Solar System impacts. NASA's Lunar Reconnaissance Orbiter has mapped terrain and lighting in far greater detail than a casual telescopic view. Those maps are measurements with particular resolutions and coverage, not a perfectly scaled physical globe. The Moon's value is both local and comparative: it is an Earth companion and a separate world with its own history.
Eclipses depend on orbital alignment
The Moon's orbit is tilted relative to Earth's path around the Sun. That is why every full moon does not produce a lunar eclipse and every new moon does not produce a solar eclipse. An eclipse occurs when the geometry lines up closely enough for a shadow to fall where it can be seen. The changing phase tells you how much of the sunlit lunar half is visible from Earth, while an eclipse is a separate shadow event. Keeping those ideas apart makes ordinary phases, eclipses, and the Moon's synchronized rotation easier to picture together.
For the wider context, read Tidal locking, tidal heating, and changing orbits.
A surface record that Earth has mostly erased
The dark lunar maria are largely basaltic plains formed by ancient volcanic activity. The brighter highlands generally preserve older crust and a denser impact record. The contrast visible from Earth therefore reflects geology as well as lighting. Individual impact basins, volcanic units, and ejecta deposits supply overlapping parts of a long history.
Crater counting estimates relative surface ages by comparing impact densities. Turning those counts into numerical ages requires a calibration, helped enormously by radiometric ages from returned samples. A plain with fewer craters may be younger because lava covered the older record, not because it somehow avoided every later impact. Small craters and secondary impacts complicate the counting.
The loose surface layer, or regolith, formed through repeated impacts and space weathering. It behaves differently from ordinary soil shaped by Earth’s water and life. Lunar samples also carry evidence of the solar wind and impact processing. Those laboratory records connect the Moon to the wider Solar System and provide checks on interpretations based only on orbital photographs.