Moon

The Moon is Earth's only natural satellite and the one world beyond our own that human beings have walked on. At 1,737 km in radius it is a little over a quarter the width of Earth and, among the Sun's planets, the largest moon relative to its host planet — large enough that some scientists describe the Earth-Moon pair as a double system rather than a planet and its satellite. (Only the dwarf planet Pluto, with its outsized moon Charon, beats that ratio.)

It hangs an average of 384,400 km away, close enough to dominate our sky, drive the ocean tides, and steady the tilt of Earth's axis over geological time. It is also close enough to study in extraordinary detail: the Moon is the only body off Earth from which humans have returned rock samples, and after half a century it is once again the focus of a global wave of exploration.

Born from a collision

The leading explanation for the Moon's origin is the giant-impact hypothesis. Roughly 4.5 billion years ago, a Mars-sized protoplanet — often named Theia — is thought to have struck the young Earth a glancing blow, throwing a disc of vaporised rock into orbit that coalesced into the Moon. The idea neatly explains why the Moon has only a small iron core, why it is depleted in volatile elements, and why lunar and terrestrial rocks share nearly identical oxygen-isotope signatures, as if cast from the same material.

That violent birth left a body of 7.342 × 10²² kg — barely 1.2% of Earth's mass — with a surface gravity about one-sixth of our own. It is that gentle gravity, familiar from the loping strides of the Apollo astronauts, that shapes almost everything about how the Moon behaves.

Two faces: maria and highlands

Even with the naked eye the Moon splits into two kinds of terrain. The bright, rugged highlands are the oldest crust, saturated with impact craters that record more than four billion years of bombardment. The dark, smoother plains are the maria — Latin for 'seas' — vast basins flooded by iron-rich basaltic lava mostly between roughly 3.9 and 1.2 billion years ago, when molten rock welled up through fractures left by giant impacts.

The craters themselves are a lasting record because the Moon has almost no atmosphere and no plate tectonics to erase them. Tycho, a young ray crater about 85 km across in the southern highlands, flings bright streaks of ejecta halfway around the near side. On the far side lies the South Pole–Aitken basin, one of the largest and deepest impact scars known anywhere in the solar system.

Locked in step with Earth

The Moon turns on its axis in 27.32 days — exactly the time it takes to orbit Earth once. This tidal locking is why we always see the same near side and never, from the ground, the far side. The synchrony is not a coincidence but the end state of billions of years of tidal braking, which slowed the Moon's spin until rotation and orbit matched.

Because the Moon's orbit is inclined about 5.1° to Earth's path around the Sun, the two bodies usually miss each other's shadows — eclipses happen only when a new or full Moon lands near the points where those orbits cross. The 27.32-day orbit is the sidereal month; the familiar cycle of phases takes longer, about 29.5 days, because Earth is meanwhile moving along its own orbit and the Moon must catch up to the Sun.

A world of extremes and hidden ice

With virtually no air to hold or spread heat, the lunar surface swings from around 127 °C in direct equatorial sunlight to near −173 °C in darkness. That same lack of atmosphere means the floors of certain polar craters, tilted away from the Sun, have not seen daylight for billions of years. These permanently shadowed regions are among the coldest places in the solar system — and they are cold enough to trap water ice.

Confirming that ice has transformed the Moon's strategic value. Frozen water could supply drinking water, breathable oxygen, and hydrogen–oxygen rocket propellant for crews on the surface, turning the poles from scientific curiosities into potential refuelling stations for missions deeper into space.

Restless and slowly retreating

The Moon is not a fixed lamp in the sky. Laser ranging off reflectors left by the Apollo crews shows it drifting outward at about 3.8 cm per year, as tidal interaction transfers Earth's rotational energy to the Moon's orbit — the same exchange that is very gradually lengthening our day. Over hundreds of millions of years the Moon will grow slightly smaller in our sky.

Its gravity is also the metronome of the oceans, raising the twin bulges of high tide that sweep around the planet as Earth rotates beneath them. On a longer timescale, the Moon's steadying pull keeps Earth's axial tilt — and therefore its climate — far more stable than it would otherwise be.

From Apollo to Artemis

The Moon is the most visited world beyond Earth. Between 1969 and 1972, six Apollo missions landed twelve astronauts near the equator on the near side, returning 382 kg of rock and soil that still anchor our understanding of the early solar system. After decades of robotic quiet, the pace has surged again: India's Chandrayaan-3 set its Vikram lander down near the south pole in August 2023, and China's Chang'e 6 became the first mission to return samples from the far side in June 2024, bringing back nearly 2 kg of material from the ancient South Pole–Aitken basin.

NASA's Artemis programme is now carrying crews back toward the Moon. Artemis II flew a crewed lunar flyby in April 2026 — the first astronauts to loop around the Moon in over half a century — and NASA is preparing follow-on missions aimed at landing humans in the resource-rich south polar region. Temposia's globe lets you rotate the Moon to trace the maria, the great crater rays, and the polar terrain these missions are targeting.

How to observe the Moon

No other object rewards a first look through a telescope so easily. The Moon is the brightest thing in the night sky after the Sun, and the best time to study it is not the full Moon — when the flattened, glaring light hides relief — but the days around first and last quarter. Along the terminator, the line dividing lunar day from night, low sunlight throws crater rims and mountain ranges into stark shadow, and even ordinary binoculars will show the maria as distinct grey plains. A small telescope reveals individual craters, the bright rays of Tycho, and the rugged wall of the highlands in sharp detail.