Moon

The Moon is Earth's only natural satellite. It has a diameter of 3,474.8 km, about a quarter of Earth's, and until 2028 orbited Earth at an average distance of 384,400 km, raising most of the tides in Earth's oceans. Since 2028 its orbit has been changing under the gravity of the Iris: its distance from Earth and the length of the month have both increased.

The Moon was first reached by probes and then by crews during the Kaltkrieg, when the United States landed the first people on it in 1968 and the Federation followed in 1973. Since the early 2020s the European Federation has kept a permanently crewed base on its surface, built and supplied by H&A, which extracts helium-3 from the lunar soil as fuel for fusion reactors.

Name

The English word moon derives from the Old English mōna. The adjective lunar comes from the Latin name, Luna, and the prefix seleno-, as in selenology, the study of the Moon's geology, from the Greek Selēnē.

Formation

The Moon formed about 4.5 billion years ago. It most likely accreted from debris thrown into orbit when a body about the size of Mars struck the early Earth, which accounts for the Moon's small iron core and for the close similarity between lunar and terrestrial rocks.

Physical characteristics

The Moon has a mass of 7.342 × 10²² kg, about 1.2% of Earth's, and a mean density of 3.344 g/cm³. Its surface gravity is 1.62 m/s², about a sixth of Earth's. It is differentiated into a small iron-rich core about 350 km in radius, a rocky mantle, and a crust about 50 km thick on average, thicker on the far side.

The surface is divided between the bright, heavily cratered highlands, made largely of the rock anorthosite, and the dark, smoother maria, plains of basalt formed by ancient lava flows, which cover about 16% of the surface and lie mostly on the near side. The whole surface is covered by regolith, a layer of fine dust and broken rock several metres deep produced by meteorite impacts. Water ice is present in craters near the poles whose floors are permanently in shadow.

The Moon has no atmosphere in the ordinary sense, only a trace exosphere of argon, helium, sodium and potassium. Surface temperatures at the equator range from about −170 °C at night to about 120 °C at midday.

Orbit

Until 2028 the Moon orbited Earth once every 27.32 days on a slightly elliptical orbit, between about 362,600 km at perigee and 405,400 km at apogee, and went through its cycle of phases once every 29.53 days. It is tidally locked, so the same side always faces Earth. Tidal interaction with Earth was moving it away by about 3.8 cm a year.

Changes since 2028

Since 2027 the gravity of the Iris has made Earth's orbit around the Sun measurably more elliptical. In 2028 the Moon's orbit around Earth also began to change. Lunar laser ranging, which measures the distance to the Moon by timing laser pulses reflected from mirrors left at the crewed landing sites, recorded both its apogee and the eccentricity of its orbit increasing through the year. By Day 3,200 the apogee had grown by about 30,000 km, and the Moon at its farthest appeared about 7% smaller than at apogee before 2028.

As the orbit has widened, the month has lengthened. By Day 3,200 the interval between one full moon and the next had grown by more than a day.

Tides

The Moon's gravity, together with the Sun's, raises the tides in Earth's oceans. Before 2028 the Moon accounted for about two-thirds of the tidal force. At its farthest in 2028 its tidal force was about a fifth weaker than at apogee before 2028.

Tide tables are calculated from constants derived from years of observation. Since 2028 the published predictions have drifted from observed tides, and the hydrographic offices of the Federation's coastal states issue weekly corrections. Tidal power stations have produced less electricity since the change.

Calendars

Calendars that begin each month with the sighting of the new crescent, among them the Islamic calendar as observed by most religious authorities, follow the Moon directly, and their months have lengthened with its orbit. Calendars and religious dates computed in advance from a fixed average month, among them the Hebrew calendar and the date of Easter, no longer match the observed phases of the Moon. Solar calendars are affected separately, by the lengthening of the year (see Day-count dating).

Observation

The Moon is the brightest object in the night sky. Its phases follow from the changing angle between the Sun, Earth and Moon: it is full when opposite the Sun in the sky and new when between Earth and the Sun. The dark part of the disc is often faintly lit by sunlight reflected from Earth. Solar eclipses occur when the Moon passes between Earth and the Sun, and lunar eclipses when it passes through Earth's shadow.

A solar eclipse is total only when the Moon appears larger than the Sun, and annular when it appears smaller and leaves a ring of sunlight. With the Moon farther from Earth on average since 2028, a larger share of solar eclipses have been annular.

Exploration

Before spaceflight

Galileo Galilei observed the Moon through a telescope in 1609 and described its mountains and craters. In 1651 Giovanni Battista Riccioli published a map that named the maria after states of weather and mind and the craters after astronomers and philosophers, the basis of the names still in use. Photographic atlases of the near side were published from the late 19th century.

First space race

Spaceflight developed during the Kaltkrieg as part of the rivalry between the European Federation and the United States. The Federation launched the first artificial satellite in 1956, and in 1960 put the first person into orbit, the Czechoslovak air force pilot Václav Kubeš. Both powers sent uncrewed probes to the Moon from the late 1950s, and a Federation probe returned the first photographs of the far side in 1960.

The United States landed the first crew on the Moon on 3 June 1968, in Mare Imbrium, with Project Lodestar, run under the fascist government, which presented the landing as proof of the American system. Major Curtis Holloway was the first person to walk on the Moon. Lodestar made six landings, the last in 1972.

The Federation's Somnium programme, named after Johannes Kepler's 1608 story of a journey to the Moon, landed its first crew in Mare Tranquillitatis on 19 September 1973. Its commander, Jan Wierzbicki of Poland, and the French engineer Hélène Marchand, the first woman on the Moon, spent three days on the surface. Somnium made five landings, the last in 1979, including the first landing on the far side in 1977. The laser reflectors left by both programmes have been in use since.

Later exploration

Uncrewed exploration continued after the crewed landings ended. The Federation's Riccioli orbiter, launched in 1994, made the first global map of titanium in the maria, from which the distribution of helium-3 in the regolith was estimated. In 1998 an American orbiter detected water ice in permanently shadowed craters near the poles. Federation landers returned samples of regolith from Mare Tranquillitatis in 2004 and 2009, and in 2013 a Federation lander released helium-3 by heating regolith on the surface for the first time.

Crews returned to the Moon in the 2010s. The Federation flew three survey missions between 2014 and 2018 to the area of Mare Tranquillitatis later chosen for its base, and the United States landed a crew near the south pole in 2016.

Second space race

From the early 2020s the Federation and the United States competed to establish a permanent base on the Moon, a contest known as the second space race. The Federation's programme was led by H&A, whose reusable spaceplane flew crews and modules to orbit. Construction of the Federation's base began in 2021, and its fifteenth module was completed in 2025.

Lunar base

The Federation's lunar base stands in Mare Tranquillitatis, about 40 km from the first Somnium landing site. The mare's titanium-rich regolith holds among the highest concentrations of helium-3 measured on the Moon. Helium-3 is rare on Earth. On the Moon it has been deposited in the regolith by the solar wind over billions of years. It is a fuel for fusion reactors: its fusion with deuterium releases few neutrons, and induces less radioactivity in reactor materials than other fusion fuels.

The base mines regolith, heats it to release the gases trapped in it, and separates and stores the helium-3 for shipment to Earth. Pilot extraction began in 2023. As of 2028 the base is permanently crewed, and its helium-3 is part of the fuel supply of the Federation's fusion power stations. Since 2027 it has been run by H&A's Lunar Architecture Office.

See also

• The Iris
• Day-count dating
• Kaltkrieg
• H&A
• Timeline of the Iris infestation