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.
A total lunar eclipse will occur on Wednesday, January 31, and Australia is in the perfect position to see it. But it’s also being called many other lunar things, from a Blood Moon to a Blue Moon and a Super Moon.
So what is really going to happen on the night?
This is the first time in three years that we have the chance to see a total lunar eclipse from Australia, and the Moon will spend just over three hours passing through Earth’s shadow.
Explainer: what is a lunar eclipse?
The great thing about lunar eclipses is that they are lovely to watch and no special equipment is needed to see the events unfold.
From light to dark
At first we’ll see the Full Moon begin to darken. For Wednesday’s lunar eclipse the shadow will approach from the bottom-right, leaving the top part of the Moon in sunlight.
It takes an hour before the Earth’s shadow crosses the Moon entirely and once the Moon is completely engulfed the period known as totality begins.
Totality brings its own surprise. The Earth’s shadow is not completely black, but has a reddish hue. This has led many cultures, including some Indigenous Australian communities, to describe a lunar eclipse as a Blood Moon.
Sunlight still manages to reach the Moon but it must first pass through Earth’s atmosphere. This both reddens the light (by scattering away the shorter wavelengths or blue light) and also bends the path of the light, directing it into the shadow.
This week’s lunar eclipse is a fairly deep one and totality will last just over an hour. Thereafter, the Moon will begin to emerge from the shadows, and it will be another hour before we see the brilliance of the Full Moon once more.
How I can see it?
The eclipse can be seen by the entire night side of the globe and everyone will experience the event at precisely the same moment. What affects the eclipse timings are local time zones.
For Western Australia, the eclipse occurs in the early evening, within an hour after sunset. The Moon will be low to the eastern horizon at the start of the eclipse but will move higher in the sky and towards the northeast as the eclipse progresses.
For the rest of Australia, the eclipse occurs two to three hours after sunset. The eclipse will begin with the Moon in the northeast and climbing towards the north.
Super Blood Blue Moon
It seems these days that it’s not enough to be treated to a beautiful natural phenomenon like a total lunar eclipse. Instead, I’ve been hearing a lot of hype surrounding this eclipse and the numerous names applied.
It’s true that lunar eclipses can only occur around the time of Full Moon. That’s when the Sun is on one side of the Earth, while the Moon is located on Earth’s opposite side.
Most of the time the Full Moon sits above or below Earth’s shadow and the Moon remains flooded with sunlight. But twice a year, the three bodies fall into line so that Earth casts its shadow on the Moon.
As well as being a Full Moon, eclipses can also be described as a Blood Moon because of the Moon’s reddish appearance, as mentioned previously.
But the descriptions of Super Moon and Blue Moon may not be quite what they seem.
Look to the sky … it’s a Super Moon!
I’ve written before about the Super Moon sensation and it’s a term that has only taken off in the past seven years.
Back in March 2011, NASA published an article describing a “super full moon”. The precise time of Full Moon that month occurred 59 minutes before perigee, that is, the Moon’s closest approach to Earth as it travels along its elliptical orbit.
As quoted in the article:
The full Moon of March 19th  occurs less than one hour from perigee – a near-perfect coincidence that happens only every 18 years or so.
It must have seemed a worthwhile curiosity to report on at the time.
Seven years later and the Super Moon craze is now a bit out of hand, with some claiming three Super Moons a year depending on the chosen definition.
As a Super Moon this lunar eclipse is definitely on the outer limits, with the Full Moon occurring 27 hours after perigee and at a distance of more than 360,000km (calculated in the usual way from the centre of Earth to the centre of the Moon).
Considering that it’s also quite difficult to tell the difference in both size and brightness between a regular Full Moon and a Super Moon, this one is really pushing the limits of credibility.
Once in a Blue Moon
According to Philip Hiscock, a folklorist at the Memorial University, USA (now retired), the classic saying “once in a blue moon” is more than 400 years old. It originated as something so absurd it could never actually happen, similar to saying “when pigs fly”.
But it is possible on rare occasions for the Moon to turn blue.
Intense volcanic activity or smoky forest fires can fill Earth’s atmosphere with dust particles that are slightly larger than usual. As a result, red light is scattered away, giving everything a blue tinge, including the Moon (normally the atmosphere scatters blue light, hence why the sky is blue).
But when it comes to this lunar eclipse, it’s not the colour of the Moon but a quirk of our timekeeping that is in play.
What a difference a day makes
A Full Moon occurs every 29.5 days, but our months are longer (excluding February). This mismatch of timing means that every couple of years there comes a month with two Full Moons.
In recent times, a Blue Moon has referred to the second full moon of a calendar month. For most of the world, this lunar eclipse is occurring during a Blue Moon, except for Australia’s eastern states of New South Wales, Victoria, Tasmania and the Australian Capital Territory.
Those states follow daylight saving, which pushes the Full Moon into the following day and out of the month of January (the actual time of Full Moon is 12:26am AEDT, February 1). This leaves January with only one Full Moon for those states and territory.
But there’s more. This modern definition of Blue Moon arose only 30 years ago.
The original definition is as follows: if four Full Moons occur between an equinox and a solstice (for example, in the three months between a spring equinox and a summer solstice) then the third Full Moon should be called a Blue Moon.
This ensured that the proper names of the Full Moons (common in North America, such as the Harvest Moon) were correct relative to the equinoxes and solstices.
But regardless of the exact flavour of this lunar eclipse, what’s certainly true is that we are part of a grand universe, and Wednesday night is the perfect reminder of that.