Our Constant Companion

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A familiar sight in our skies from ancient times, the Moon is threaded through humanity's history.

Bulliadus Crater by Mark Bell

Bulliadus Crater by Mark Bell

The source of our ocean tides, subtle chronobiological cycles and the only other world that humankind has so far set foot upon, the Moon seems a familiar and tangible place. A quarter of Earth's diameter and just a quarter of a million miles away, it's 100 times closer than Venus. Given its proximity, brightness and large apparent size, it's easy to see why the Moon has enchanted humankind for centuries.

Before the emergence of widespread street lighting, the Moon was the primary source of light for nocturnal activities. Its sheer size and regular cycle of phases made it an obvious timepiece to our ancient ancestors, forming the basis of some early calendars, and in various cultures the Moon either had deities associated with it or was considered to actually be one. In the following centuries, when astrology and astronomy were one and the same, it continued to bear a supernatural significance, marking when certain activities and plans would go well — and when they were doomed to fail.

Pre-telescopic observers noticed an unchanging pattern of darker patches that would later become known as maria, or 'seas', because they were assumed to be vast bodies of water. They act as a Rorschach test for different cultures — the face of the 'Man in the Moon' observed in Western tradition, the 'Rabbit' pounding rice of East Asian folklore, or the 'Lady Reading a Book' from the southern hemisphere, to give just three examples.

Until the middle ages, the Moon was believed to be a smooth sphere, neatly slotting into the Aristotelian view of the 'perfect heavens'. It wasn't until after 1609, when Galileo turned his telescope to the Moon, that this perception was undone.

Galileo was not the first to examine the Moon through a telescope — that accolade falls to Englishman Thomas Harriot, whose sketches predate Galileo's by several months — but he was the first to publish. In his Sidereus Nuncius, Galileo revealed a world pockmarked with craters and mountains. He had seen that the terminator, the line that divides lunar day and night, was often jagged, correctly inferring that this irregularity must result from shadows cast by topographical features.

About a dozen lunar landforms can be distinguished with a keen eye. A typical pair of binoculars, if suitably steadied, will transform your view of the Moon into a scarred, airless world, and most likely will give you a better view than Galileo had in the 1600s. Through even the smallest modern scope innumerable impact craters appear, often fringed by long rays of ejecta. Alongside them sit grand basins of solidified lava, soaring mountain peaks, curious fissures and escarpments — it's a whole new world to explore.

Locked on Earth
You don't need a telescope to reveal that night after night we always see the same lunar features staring back at us. This is because the Moon has a synchronous rotation with respect to Earth, meaning that spins once on its axis in the same 27.3 days (the sidereal month) it takes to complete an orbit of our planet.

This is no coincidence. Earth's gravitational pull on the Moon has caused a bulge in the body of the Moon itself, similar to the tides in Earth's oceans. This bulge unbalanced the Moon's gravitational force, slowing its rotation until the bulge aligned with the Earth. Despite its appearance in the sky, our Moon is nowhere near round; it is closer to a lemon shape.

A consequence of this 'tidal locking' is that for much of human history the Moon held a closely guarded secret: no one knew what the far side was like. This didn't change until 1959, when the Soviet Luna 3 probe became the first to pass an image of the hitherto unseen side.

In a memorable episode of The Sky at Night broadcast on 26 October 1959, Patrick Moore announced the success of the Soviet mission, revealing the first shadowy photographs of the Moon's far side live on air. Luna 3's imagery was crude by today's standards, but it revealed that the 'dark side' was strikingly different in a number of ways.

While 35 per cent of the Moon's Earth-facing hemisphere is covered with mare lava, very little molten material made it to the surface on the far side, so maria account for just one per cent. It's thought this is because the far side's crust is thicker — it may be up to twice as thick as that of the near side — possibly due to the slow accretion of a companion satellite after an impact. This theory seems to be supported by the discovery of the far side's 3.9 billion-year-old South Pole — Aitken Basin, over 2,400km wide and around 13km deep. To date, our best views of the Moon come from NASA's Lunar Reconnaissance Orbiter, now in its sixth year of operations and, at the time of its launch, the first US mission to the Moon in 10 years.

The Sun Always Shines
It's equally obvious that the illumination of the Moon's Earth-facing hemisphere changes over the course of the month — a word, incidentally, that we get from 'Moon'. Although the Sun is always shining on a full half of the Moon, the proportion of the lit side we are able to see depends on where the Moon is in its orbit around Earth, giving rise to the phases we see.

Imagine you are looking down on the Earth, Moon and Sun from above. When the three line up with the Moon in the middle, the Moon's lit half points away from us on Earth, producing a new Moon. Slowly emerging from its new phase into the evening sky, the lunar crescent thickens from one day to the next. The term 'waxing' is used to indicate this thickening phase. The waxing crescent leads to the Moon appearing as an illuminated semicircle roughly a week after new.

This is somewhat confusingly called 'first quarter', referring to the Moon's position in its 29.5-day orbit rather than proportion of its disc is illuminated from our vantage point on Earth. The bulging phases after first quarter are known as waxing gibbous. These increase in size until roughly two weeks after new, the Moon is on the opposite side of its orbit from the Sun and appears fully lit as a full Moon. The point of new and full Moon, when our planet, satellite and star are aligned, is technically known as a 'syzygy'.

After full Moon the phases reverse, and the illuminated part of the Moon begins to shrink or wane. After passing through the waning gibbous phases, the Moon reaches the three-quarter point of its orbit, giving rise to the 'last quarter' phase. The Moon takes the appearance of a semicircle once again, although it's the opposite half that is illuminated than that at first quarter. After this, it takes approximately a week for the Moon to go through its waning crescent phases, visible in the early morning sky, before it once again becomes new again. It takes 29.5 days for the Moon to return to complete this cycle of phases or 'lunation', slightly longer than it does to complete an Earth orbit. This is known as a synodic month.

Ellipse and Eclipse
The Moon's elliptical orbit is inclined to Earth's by an average of 5. This means that on most of the occasions that a full Moon occurs, it actually passes above or below the shadow Earth casts into space. But in the instances that the full Moon passes into Earth's shadow we see a different phenomena: a lunar eclipse.

Because the Sun is much bigger than Earth, it splits our planet's shadow into two parts: the darkest, called the umbra, and a lighter outer ring, called the penumbra. The intensity of a lunar eclipse depends on how much of the Moon passes into Earth's shadow, and which part of the shadow it passes through.

In a total lunar eclipse, the entire Moon passes through the penumbra and into the umbra, gradually darkening until it is completely covered, a point known as totality. During totality no sunlight shines directly on the Moon, but some is refracted onto it via Earth's atmosphere. As our atmosphere filters out blue light, the Moon often gains a strange orange-brown colour.

As the Moon goes into eclipse and dims, the sky gets darker too. You may not have realised how bright a full Moon can be. It lights up the sky around it with a blue haze, out of which only the brighter stars are visible. During a total lunar eclipse, the darker Moon means that the fainter stars can come out and we end up with the eerie sight of a deep-red Moon surrounded by twinkling stars.

There are two other types of lunar eclipse: partial, where only a portion of the Moon passes through Earth's dark umbral shadow, and penumbral, where part of the Moon only passes through the lighter, outer shadow. Partial eclipses can be quite noticeable, but penumbral eclipses often only cause a slight dimming.

When the same thing happens at new Moon the opposite occurs, and we may see a partial or total solar eclipse. By staggering coincidence, right now the Moon is both 400 times smaller than the Sun and 400 times closer, meaning that they appear to be the same size in the sky. The fact the Moon only just covers the Sun during a total solar eclipse allows us to glimpse our star's ghostly outer atmosphere, the corona.

A Changing Relationship
Life on Earth owes a lot to our rocky companion. Without it, our planet's axis would tilt wildly between 0 and 85, albeit over a period of a million years, sending our hemispheres veering between chaotic ice ages and searing hellscapes. It would have been a death sentence for evolving life.

But our relationship with the Moon is becoming increasingly distant. When it formed, the Moon was only 22,500km from our planet. Today, it's nearly 10 times farther away and getting more distant by 3.8cm a year — around the same rate as your fingernails grow. As a result, Earth's spin speed is slowing down and our days are getting longer.

Eventually, there will come a point when the length of the day and the month will be the same, and the Moon will cease to cross our skies. There will be no new or full Moon, only a small static disc in the night sky visible from one side of the planet, a situation we see today in the Pluto-Charon system. By the time that happens, humans will hopefully be looking out at other moons from distant planets.

The Major Classes of Lunar Features

There are 14 official valleys on the Moon, the longest around 600km. Most are named after nearby craters. One of the most familiar is the 180km-long Vallis Alpes (pictured), which cuts across the northern Montes Alpes and almost connects the Mare Imbrium and the Mare Frigoris.

These vast dark plains of solidified magma are notable for both their dark appearance and the fact that they are largely absent from the Moon's far side. One of the most distinct is the 560km-wide Mare Crisium (pictured) which is just visible to the naked eye.

The ubiquitous lunar feature, varying in size from microscopic pits to sprawling depressions up to 350km in diameter — anything larger is a basin. Some were formed through volcanism but the majority, like Tycho (pictured) are the result of ancient impacts.

The oldest and largest impact craters on the Moon, exceeding 350km in diameter. All lunar maria are found within them. The South Pole-Aitken Basin on the Moon's far side holds the record for being the largest, at around 2,400km; the biggest on the near side is the Imbrium Basin, shown here, which stretches across 1,160km of the lunar surface.

The Moon's peaks are named in two ways: 'Montes' for mountain ranges and 'Mons' for singular peaks and massifs. The most spectacular of the 18 named lunar ranges is the gently curved, 600km-long Montes Apenninus (pictured), which form the southeastern edge of the Imbrium Basin. Mons Huygens, the Moon's tallest mountain at 5.4km, soars skyward here.

The Big Myth — The Dark Side of the Moon
The phrase 'dark side of the Moon' may evoke fond memories of Pink Floyd's 40-year-old prog-rock album to the baby boomer generation, but in an astronomical context it's often used to refer (erroneously) to the Moon's far side. The phrase is something of a misnomer, since the lunar far side goes through the same cycle of illumination as the phases of the Moon seen on the Earth-facing hemisphere. Technically, the far side is the 'dark side' at the instant of full Moon. The only places on the Moon's surface permanently bathed in shadow are a few deep craters at the north and south poles.

Where Did the Moon Come From?
Most scientists now believe that the Moon was formed around 4.5 billion years ago when an object the size of Mars (and since named Theia) collided with the early Earth, giving it a glancing blow. The impact spewed debris into Earth's orbit, which coalesced to form the Moon at just the right distance to be an independent body; any closer and Earth's gravity would have pulled the material back.

This theory was born from the chemical analysis of lunar samples returned by the Apollo missions, which showed a remarkable similarity between Earth's composition — hinting at a common heritage. But there is a problem: the compositions look too similar. If this collision occurred, the Moon should have more of Theia's material and should therefore be more different from Earth.

The Apollo samples were obtained from a very small area - could this explain the similarities? It would seem not, because we do have other lunar material. The Russian Luna programme returned 0.33kg of Moon samples and we also have a number of lunar meteorites. Analysis of this material brings up a similar problem, it is just too similar to the composition of Earth.

So where does this leave the collision theory? It still has a lot of support, but what would be a great help is having more lunar samples from known but more varied locations.

What's Our Moon Made Of?
Our natural satellite has a small core composed predominantly of iron, a distinct mantle, and a crust of varying thickness comprised of anorthosites and basalt.

Moon Facts

  • Age: 4.5 billion years
  • Diameter: 3,475km
  • Mass: 0.0123 Earths
  • Average distance: 384,400km
  • Average orbital velocity: 3,679km/h
  • Orbital period: 27.3 Earth days
  • Lunar cycle: 29.5 Earth days
  • Surface gravity: One-sixth that of Earth

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