Even though we can see the Moon shining brightly in the night sky – and sometimes in daylight – it’s hard to put into perspective just how large, and just how distant, our nearest neighbour actually is.
So just how big is the Moon?
That answer isn’t quite as straightforward as you might think. Like Earth, the Moon isn’t a perfect sphere. Instead, it’s slightly squashed (what we call an oblate sphereoid). This means the Moon’s diameter from pole to pole is less than the diameter measured at the equator.
Why the Moon is such a cratered place
But the difference is small, just four kilometres. The equatorial diameter of the Moon is about 3,476km, while the polar diameter is 3,472km.
To see how big that is we need to compare it to something of a similar size, such as Australia.
From coast to coast
The distance from Perth to Brisbane, as the crow flies, is 3,606km. If you put Australia and the Moon side by side, they look to be roughly the same size.
But that’s just one way of looking at things. Although the Moon is about as wide as Australia, it is actually much bigger when you think in terms of surface area. It turns out the surface of the Moon is much larger than that of Australia.
How far is the Moon?
Asking how far away is the Moon is another of those questions whose answer is more complicated than you might expect.
The Moon moves in an elliptical orbit around the Earth, which means its distance from our planet is constantly changing. That distance can vary by up to 50,000km during a single orbit, which is why the size of the Moon in our sky varies slightly from week to week.
The Moon’s orbit is also influenced by every other object in the Solar System. Even when all of that is taken into account, the distance answer is still always changing, because the Moon is gradually receding from the Earth as a result of the tidal interaction between the two.
That last point is something we’ve been able to better study as a result of the Apollo missions. The astronauts who visited the Moon placed an array of mirror reflectors on its surface. Those reflectors are the continual target of lasers from the Earth.
By timing how long it takes for that laser light to travel to the Moon and back, scientists are able to measure the distance to the Moon with incredible precision, and to track the Moon’s recession from Earth. The result? The Moon is receding at a speed of 38mm per year – or just under 4 metres per century.
Drive me to the Moon
Having said all that, the average distance between the Moon and Earth is 384,402km. So let’s put that into context.
If I were to drive from Brisbane to Perth, following the fastest route suggested by Google, I would cover 4,310km on my road trip. That journey, driving across the breadth of our country, would take around 46 hours.
If I wanted to clock up enough kilometres to say that I’d covered the distance between the Earth and the Moon, I’d have to make that trip more than 89 times. It would take five-and-a-half months of driving, non-stop, assuming I didn’t run into any traffic jams on the way.
Fortunately, the Apollo 11 astronauts weren’t restricted to Australian speed limits. The command module Columbia took just three days and four hours to reach lunar orbit following its launch on July 16 1969.
An eclipse coincidence
The equatorial diameter of the Sun is almost 1.4 million kilometres, which is almost exactly 400 times the diameter of the Moon.
That ratio leads to one of astronomy’s most spectacular quirks – because the distance between the Earth and the Sun (149.6 million kilometres) is almost (but not quite) 400 times the distance between the Earth and the Moon.
Explainer: what is a solar eclipse?
The result? The Moon and the Sun appear almost exactly the same size in Earth’s sky. As a result, when the Moon and the Sun line up perfectly, as seen from Earth, something wonderful happens – a total eclipse of the Sun.
Sadly, such spectacular eclipses will eventually come to an end on Earth. Thanks to the Moon’s recession, it will one day be too distant to perfectly obscure the Sun. But that day will be a long time coming, with most estimates suggesting it will occur in something like 600 million years’ time.
While we’ve dispatched out robot envoys to the icy depths of the Solar System, the Moon remains the only other world on which humanity has walked.
Fifty years after that first adventure, the number of people to have walked on the Moon who are still alive is in sharp decline. Twelve people have had that experience but, as of today, just four remain.
Vast as the Moon is, those 12 moonwalkers barely scratched the surface. Hopefully, in the coming years, we will return, to inspire a whole new generation and to continue humanity’s in-person exploration of our nearest celestial neighbour.
Look up on a clear night and you can see some circular formations on the face of our lunar neighbour. These are impact craters, circular depressions found on planetary surfaces.
About a century ago, they were suspected to exist on Earth but the cosmic origin was often met with suspicion and most geologists believed that craters were of volcanic origin.
Around 1960, the American astrogeologist Gene Shoemaker, one of the founders of planetary science, studied the dynamics of crater formation on Earth and planetary surfaces. He investigated why they – including our Moon – are so cratered.
Images from Apollo
By 1970, there were more than 50 craters discovered on Earth but that work was still considered controversial, until pictures of the lunar surface brought by the Apollo missions confirmed that impact cratering is a common geological process outside Earth.
Unlike Earth’s surface, the lunar surface is covered with craters. This is because Earth is a dynamic planet, and tectonics, volcanism, seismicity, wind and oceans all play against the preservation of impact craters on Earth.
In contrast to Earth, our Moon has been inactive over long geological timescales and has no atmosphere, which has allowed the persistent impact cratering to remain over eons. The lunar cratering record spans its entire bombardment history – from the Moon’s very origins to today.
The big ones
The largest and oldest impact crater in the Solar system is believed to be on the Moon, and it is called the South Pole-Aitken basin, but we cannot see it from Earth because it is on the far side of the Moon. The Moon is tidally locked to Earth’s rotation and the same side always faces toward us.
But this crater, more than 2,000km across, is thought to predate any other large impact bombardment that occurred during lunar evolution. Impact simulations suggested it was formed by a 150-250km asteroid hurtling into the Moon at 15-20km per second!
From Earth, the human eye can observe areas of different shades of grey on the surface of the Moon facing us. The dark areas are called maria, and can be up to more than 1,000km across.
They are volcanic deposits that flooded depressions created by the formation of the large impact basins on the Moon. These volcanic eruptions were active for millions of years after these impacts occurred.
No other large impact event has occurred on the Moon since then. This is a good sign, because it implies there were no very large impacts occurring on Earth either after this time in evolutionary history. (The asteroid that wiped out the dinosaurs on Earth 66 million years ago was only about 10-15km in size and left a crater larger than 150km in size, which was substantial enough to cause a mass extinction.)
As seen from Earth
They are called complex craters because they are not entirely bowl-shaped, but are a bit shallower and include a peak in the centre of the crater as a consequence of the material collapsing into the hole made during impact. Tycho and Copernicus are both 80-100km across but have spectacular central peaks and prominent “ejecta rays” – areas where material was ejected across the lunar surface after an impact.
The formation of these craters excavated underlying material that was brighter than the actual surface. This is because lunar surface is subjected to space weathering, which causes surface rocks to darken.
Still a target for impacts
The Apollo 12, 14, 15, and 16 missions placed several seismic stations on the Moon between 1969 and 1972, creating the first extraterrestrial seismic network (ALSEP). During one year of operations, more than 1,000 seismic events were recorded, of which 10% were associated with meteoroids impacts.
So the Moon is still being hit by objects, albeit mostly tiny ones. But as there is no atmosphere on the Moon, there is no gas to help burn up these rocks from space and stop them smashing into the Moon.
The seismic network was functional until it was switched off in 1977, in preparation for new space missions. No one expected that the next fully operational extraterrestrial seismometer would not be placed on a planetary surface (Mars) until 40 years later.
Nowadays, from Earth, using a small telescope (and armed with a little patience), you can see so-called “impact flashes”, which are small meteorite impacts on the lunar surface that is facing us.
Thanks to the atmosphere on Earth, similar-sized rocks from space cannot make an impact here because they tend to predominantly burn up, but on the Moon they crash into the soil and release its kinetic energy of the impact via bright thermal emission.
The launch of 60 Starlink satellites by Elon Musk’s SpaceX has grabbed the attention of people around the globe. The satellites are part of a fleet that is intended to provide fast internet across the world.
Improved internet services sound great, and Musk is reported to be planning for up to 12,000 satellites in low Earth orbit. But this fleet of satellites could forever change our view of the heavens.
Starlink’s ambitious mission
Starlink is an ambitious plan to use satellites in low Earth orbit (about 500km up) to provide global internet services.
This is different from the approach previously used for most communication satellites, in which larger individual satellites were placed in high geosynchronous orbits – that stay in an apparently fixed position above the Equator (about 36,000km up).
Communications with satellites in geosynchronous orbits often require satellite dishes, which you can see on the sides of residential apartment buildings. Communication with satellites in low Earth orbit, which are much closer, won’t require such bulky equipment.
But the catch with satellites in low Earth orbit, which move quickly around the world, is they can only look down on a small fraction of the globe, so to get global coverage you need many satellites. The Iridium satellite network used this approach in the 1990s, using dozens of satellites to provide global phone and data services.
Starlink is far more ambitious, with 1,600 satellites in the first phase, increasing to 12,000 satellites during the mid-2020s. For comparison, there are roughly 18,000 objects in Earth orbit that are tracked, including about 2,000 functioning satellites.
Lights in the sky
It’s not unusual to see satellites travelling across the twilight sky. Indeed, there’s a certain thrill to seeing the International Space Station pass overhead, and to know there are people living on board that distant light. But Starlink is something else.
The first 60 satellites, launched by SpaceX last week, were seen travelling in procession across the night sky. Some people knew what they were seeing, but the silent procession of light also generated UFO reports. If you’re lucky, you may see them pass across your skies tonight.
If the full constellation of satellites is launched, hundreds of Starlink satellites will be above the horizon at any given time. If they are visible to the unaided eye, as suggested by initial reports, they could outnumber the brightest natural stars visible to the unaided eye.
Astronomers’ fears were not put to rest by Musk’s tweets:
Satellites are very definitely visible at night, particularly in the hours before dawn and after sunset, as they are high enough to be illuminated by the Sun. The Space Station’s artificial lighting is effectively irrelevant to its visibility.
In areas near the poles, including Canada and northern Europe, satellites in low Earth orbit can be illuminated throughout the night during the summer months.
Hundreds of satellites being visible to the unaided eye would be a disaster. They would completely ruin our view of the night sky. They would also contaminate astronomical images, leaving long trails across otherwise unblemished images.
The US$466 million Large Synoptic Survey Telescope, based in Chile, is an 8-metre aperture telescope with a 3,200-megapixel camera. It’s designed to rapidly survey the sky during the 2020s.
With the full constellation of Starlink satellites, many images taken with this telescope will contain a Starlink satellite. Longer exposures could contain dozens of satellite streaks.
Dark skies or darkened hopes?
Is there any cause for optimism? Yes and no.
Musk has produced some amazing feats of technology, such as the SpaceX Falcon and Tesla cars, but he’s also disappointed some on other projects, such as the Hyperloop tunnel transport plan.
While Starlink certainly blew up on Twitter, for now at least, Musk is 11,940 satellites short of his 12,000.
Also, initial reports may have overestimated the brightness of the Starlink satellites, with the multiple satellites closely clustered together being confused with one satellite.
While some reports have indicate binoculars are needed to see the individual satellites, they also report that Starlink satellites flare, momentarily becoming brighter than any natural star.
If the individual satellites usually are too faint to be seen with the unaided eye, that would at least preserve the natural wonder of the sky. But professional astronomers like myself may need to prepare for streaky skies ahead. I can’t say I’m looking forward to that.
Over the past few days a pair of spectacular fireballs have graced Australia’s skies.
The first, in the early hours of Monday, May 20, flashed across the Northern Territory, and was seen from both Tennant Creek and Alice Springs, more than 500km apart.
The second came two days later, streaking over South Australia and Victoria.
Such fireballs are not rare events, and serve as yet another reminder that Earth sits in a celestial shooting gallery. In addition to their spectacle, they hold the key to understanding the Solar system’s formation and history.
Crash, bang, boom!
On any clear night, if you gaze skyward long enough, you will see meteors. These flashes of light are the result of objects impacting on our planet’s atmosphere.
Specks of debris vaporise harmlessly in the atmosphere, 80-100km above our heads, all the time – about 100 tons of the stuff per day.
The larger the object, the more spectacular the flash. Where your typical meteor is caused by an object the size of a grain of dust (or, for a particularly bright one, a grain of rice), fireballs like those seen this week are caused by much larger bodies – the size of a grapefruit, a melon or even a car.
Such impacts are rarer than their tiny siblings because there are many more small objects in the Solar system than larger bodies.
That was probably the largest impact on Earth for 100 years, and caused plenty of damage and injuries. It was the result of the explosion of an object 10,000 tonnes in mass, around 20 metres in diameter.
On longer timescales, the largest impacts are truly enormous. Some 66 million years ago, a comet or asteroid around 10km in diameter ploughed into what is now the Yucatan Peninsula, Mexico. The result? A crater some 200km across, and a mass extinction that included the dinosaurs.
Even that is not the largest impact Earth has experienced. Back in our planet’s youth, it was victim to a truly cataclysmic event, when it collided with an object the size of Mars.
When the dust and debris cleared, our once solitary planet was accompanied by the Moon.
Impacts that could threaten life on Earth are, thankfully, very rare. While scientists are actively searching to make sure no extinction-level impacts are coming in the near future, it really isn’t something we should lose too much sleep about.
Smaller impacts, like those seen earlier this week, come far more frequently – indeed, footage of another fireball was reported earlier this month over Illinois in the United States.
In other words, it is not that unusual to have two bright fireballs in the space of a couple of days over a country as vast as Australia.
Pristine relics of planet formation
These bright fireballs can be an incredible boon to our understanding of the formation and evolution of the Solar system. When an object is large enough, it is possible for fragments (or the whole thing) to penetrate the atmosphere intact, delivering a new meteorite to our planet’s surface.
Meteorites are incredibly valuable to scientists. They are celestial time capsules – relatively pristine fragments of asteroids and comets that formed when the Solar system was young.
Most meteorites we find have lain on Earth for long periods of time before their discovery. These are termed “finds” and while still valuable, are often degraded and weathered, chemically altered by our planet’s wet, warm environment.
By contrast, “falls” (meteorites whose fall has been observed and that are recovered within hours or days of the event) are far more precious. When we study their composition, we can be confident we are studying something ancient and pristine, rather than worrying that we’re seeing the effect of Earth’s influence.
Tracking the fireballs
For this reason, the Australian Desert Fireball Network has set up an enormous network of cameras across our vast continent. These cameras are designed to scour the skies, all night, every night, watching for fireballs like those seen earlier this week.
If we can observe such a fireball from multiple directions, we can triangulate its path, calculate its motion through the atmosphere, and work out whether it is likely to have dropped a meteorite. Using that data, we can even work out where to look.
In addition to these cameras, the project can make use of any data provided by people who saw the event. For that reason, the Fireballs team developed a free app, Fireballs in the Sky.
It contains great information about fireballs and meteor showers, and has links to experiments tied into the national curriculum. More importantly, it also allows its users to submit their own fireball reports.
As for this week’s fireball over southern Australia, NASA says it was probably caused by an object the size of a small car. As for finding any remains, they are now likely lost in the waters of the Great Australian Bight.