The Moon is Planet Earth’s only natural orbiting satellite. It’s about a quarter of the size of Earth and is in synchronous rotation with us, meaning from Earth we always see the same one side of it. The near-side that we see is marked both by dark volcanic maria (plains of solidified ancient lava) and bright crustal impact craters. Although the Moon’s surface is naturally dark, it appears bright because the regolith (layers of surface dust) reflect the Sun’s light. The Moon’s gravitational influence on Earth’s water as it orbits produces our ocean tides.
The Moon’s distance from us is about thirty times the size of Earth, coincidentally causing it to have an apparent size in the sky almost exactly the same as that of the Sun from Earth (most obvious during an eclipse), though the Sun is in reality much larger and further away. This visual similarity is entirely coincidental.
Its prominence in the sky and regular cycle of phases have – since ancient human times – made the Moon an important cultural influence on language, calendars, art and mythology. Our 30-day month is an approximation of the lunar cycle – the moon’s continuous orbit around the Earth. In ancient and more ignorant times, the Moon was personified as a deity. It became known as Luna, and the word ‘lunar’ is often associated with insanity (i.e. lunatic), as a full moon was thought to influence human behaviour in strange ways.
The Moon is the only celestial body other than Earth on which humans have yet set foot. The Soviet Union’s Luna programme was the first to reach the Moon with an unmanned spacecraft in 1959; the United States’ NASA Apollo program achieved the only manned missions thus far, culminating in six Moon landings between 1969 and 1972. These missions brought back lunar rocks which have been used to develop geological understanding of the Moon’s origin and the formation of its internal structure.
The Moon is currently understood to have formed nearly 4.5 billion years ago, not long after the Earth. There are several hypotheses around the Moon’s origin, the most widely accepted today being that the Moon formed from debris left over after a giant impact between Earth and another planet or body. There are hundreds of thousands of impact craters on the Moon’s surface thought to have been formed from bombardment of debris onto the Moon between 4.1 and 3.8 billion years ago. Because of the lack of atmosphere and weather on the Moon these impact craters have remained perfectly preserved ever since.
Further unmanned orbital and rover missions to the Moon have since taken place – from the US, Russia, China, Japan, India and the European Space Agency. These missions have led to the discovery of lunar water ice at the Moon’s poles – a significant discovery because the presence of (and access to) water on the Moon is a vital question in the plausibility of humans ever being able to live on the Moon. Without it, transporting water from Earth to Moon would be prohibitively expensive. Humans have not physically set foot on the Moon since the NASA Apollo missions concluded in 1972, although future manned missions have been discussed, including both government as well as privately funded efforts.
The Moon remains, under the terms of the 1967 Outer Space Treaty, free to all nations on Earth to explore for peaceful purposes.
To learn more about whether humans living on the Moon will ever be a possibility, jump to Chapter 56: Colonization of the Moon – content to follow
To learn more about the ground-breaking American space agency and its accomplishments, jump to Chapter 57: NASA – content to follow
To learn more about the other planets surrounding our Earth and Moon, jump to Chapter 58: Solar System – content to follow
