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A Journey to the End of Time

The Solar System in 3D, in real time

A simulation of the Solar System driven by real ephemerides. The Sun, the planets, Pluto and the major moons, space probes, the International Space Station, nearby stars and the black hole at the centre of the Milky Way are where they really are at the chosen moment. You can run time from 1700 to 2100, fly from one object to another and look at the sky from anywhere on Earth.

Moon

Earth's moon

Facts

Diameter
3475 km
Mean distance from Earth
384,400 km
Orbital period
27.3 days
Rotation
27.3 days (synchronous)
Gravity
1.6 m/s² (16% of Earth's)
Temperatures
from about 127 °C in sunlight to −173 °C in darkness

The Moon is Earth's only natural satellite and the fifth largest in the Solar System. Compared with the planet it orbits, though, it is exceptional: no other rocky planet has a moon so large relative to itself.

According to the leading hypothesis it was born from a collision, about 4.5 billion years ago, between the young Earth and a Mars-sized body called Theia; the debris from the impact, left in orbit, clumped together to form the Moon. The rocks brought back by the Apollo missions, chemically very similar to Earth's, have strengthened this theory.

The Moon always shows us the same face: Earth's gravity slowed its rotation until it matched its orbit, a phenomenon called tidal locking. Thanks to small wobbles, from Earth we can still see about 59% of its surface.

The dark patches visible to the naked eye are the “seas”: they hold no water, but are vast plains of basaltic lava that set billions of years ago. The rest of the surface is scarred by craters, witnesses of a bombardment by meteorites that, with no atmosphere or erosion, has stayed imprinted over time.

The Moon raises the tides, steadies Earth's axial tilt and with it the seasons, and drifts away by about 3.8 cm every year, slowly braking our planet's rotation. There is also a remarkable coincidence: the Sun is about 400 times larger than the Moon, but also about 400 times farther away. That is why the two look almost the same size in the sky, making total solar eclipses possible.

In 1609 Galileo Galilei pointed his telescope at the Moon and found mountains and craters, showing that heavenly bodies were not the perfect spheres people had believed. In 1959 the Soviet probe Luna 3 photographed the far side for the first time, never before seen by human eyes.

On 20 July 1969 Neil Armstrong and Buzz Aldrin, on the Apollo 11 mission, became the first humans to walk on another world. Between 1969 and 1972 a total of twelve astronauts set foot on the Moon, bringing back hundreds of kilograms of samples and leaving laser reflectors on the surface that still let us measure its distance to the millimetre.

In recent years the Moon has returned to the centre of exploration. In 2019 the Chinese probe Chang'e 4 became the first to land on the far side, while observations in recent years have confirmed water ice in polar craters that never see sunlight: a precious resource for future lunar bases.

Sources