How big is the Moon? Let me compare …



The size of the Moon can be deceptive when viewed from Earth.
Flickr/Ovi Gherman, CC BY-NC-ND

Jonti Horner, University of Southern Queensland

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?

The Moon passing in front of Earth, captured by the Earth Polychromatic Imaging Camera (EPIC), more than a million kilometres away from our planet.

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.




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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.

The Moon vs Australia.
NASA/Google Earth

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.

The land area of Australia is some 7.69 million square kilometers. By contrast, the surface area of the Moon is 37.94 million square kilometres, close to five times the area of Australia.

The Moon rising above Uluru: You’d need five Australias to cover the land mass of the Moon.
Flickr/jurek d Jerzy Durczak, CC BY-NC

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.

Notice the difference in size? The Moon viewed from Earth at perigee (closest approach at 356,700km on October 26 2007) and apogee (farthest approach at 406,300km on April 3 2007).
Wikimedia/Tomruen, CC BY-SA

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.

The full Moon rising over the Perth Hills, in Western Australia, in 2016.
Paean Ng/Flickr, CC BY-NC-ND

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.




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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.

The total solar eclipse seen from north Queensland in November 2012.

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.

The moonwalkers

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.

Astronaut Buzz Aldrin was the second man to walk on the Moon and one of the few moonwalkers still alive today.
NASA

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.




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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.The Conversation

The Moon over the Sydney Opera House.
Flickr/Paul Carmon, CC BY-NC-ND

Jonti Horner, Professor (Astrophysics), University of Southern Queensland

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Adani has set a dangerous precedent in requesting scientists’ names



The Galilee waterhole is part of the area potentially affected by Adani’s Carmichael mine.
Stop Adani, CC BY-SA

Samantha Hepburn, Deakin University

A freedom of information request has revealed Adani sought the names of CSIRO and Geoscience Australia scientists involved in reviewing groundwater management plans related to its proposed Carmichael mine.

Adani argued it required a list of people involved in the review so as to have “peace of mind” that it was being treated fairly and impartially on a scientific rather than a political basis.

Ten days before Adani’s request, Geoscience Australia’s acting director of groundwater advice and data reportedly raised concerns that Adani had “actively searched/viewed” his LinkedIn profile and that of a colleague.




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Significantly, Adani’s request to the government was made before CSIRO and Geoscience Australia had reported their review findings back to the Queensland government.

While the federal Department of the Environment and Energy reportedly declined to hand over the names, the fact the letter was sent in the first place is concerning. It fundamentally interferes with the capacity of individual scientists to provide clear and informed evaluation.

The letter obtained under freedom of information by environmental group Lock The Gate. Click to enlarge.
Lock the Gate

Was Adani denied procedural fairness?

In the absence of clear legislation to the contrary, government decision-makers have a general duty to accord “procedural fairness” to those affected by their decisions. While procedural fairness is protected by common law, Commonwealth legislation also provides some protection, and a breach of procedural fairness is a ground for judicial review.

What exactly constitutes procedural fairness varies from case to case. Fundamentally, the principles of procedural fairness acknowledge the power imbalance that can arise between an administrative decision-maker and an individual citizen. Traditionally, procedural fairness has two elements: the fair hearing rule and the rule against bias.

The fair hearing rule requires a person – or company, in this case – to have an opportunity to be heard before a decision is made affecting their interest.

The rule against bias ensures the decision-maker can be objectively considered to be impartial and not to have prejudged a decision. This rule is flexible, and must be determined by reference to a hypothetical observer who is fair minded and informed of the circumstances.

There is no indication of any breach of procedural fairness in the environmental assessment process. The review of the groundwater management plan was conducted rigorously, according to the public interest.

The letter sent by Adani requesting the names of scientists was allegedly grounded in concerns about the possibility of anti-Adani activism by expert reviewers. Despite this, Adani made it clear that it was not explicitly alleging bias. Its objective, the letter said, was a desire to be “treated fairly and in a manner consistent with other industry participants”.

The real purpose of the letter

If Adani was seriously concerned about a breach of procedural fairness in the review of their groundwater management plan, it would have sought a judicial review. It did not – because there was no breach.

The scientists working at CSIRO and Geoscience Australia are all experts in their disciplines. They were engaged in the important process of determining whether Adani’s plan for managing groundwater around their mine would meet the environmental conditions of their mining licence. In other words, the scientists were doing their job.

Deputy Prime Minister Michael McCormack has said he “understands” Adani’s actions because of the delays associated with the review, but this is not how the system works.
The delays occurred because the original plan submitted by Adani had to be revised following expert review, and the updated plan required detailed evaluation. The mine could potentially have a serious impact on groundwater, the communities and ecosystems dependent on the water, and the nationally significant Doongmabulla Springs; this deserves careful scrutiny.




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As Adani has not brought an action for judicial review, the substantive purpose of the letter appears to be, as suggested by CSIRO representatives, to pressure scientists and potentially seek to discredit their work. The potentially chilling effect is clear.

Concern about climate change is not bias

The profound concerns raised by climate change and fossil fuel emissions are shared by many scientists around the world. The reports prepared for the International Panel on Climate Change make it clear that coal fired electricity must drop to nearly zero by 2050 to keep warming within 1.5℃.

This shared concern does not make scientists political activists. Nor does it prevent scientists from acting fairly and impartially when reviewing a groundwater management plan.




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An acceptance of climate science and even a belief that coal-fired energy should be decommissioned does not constitute bias. A reasonable bystander would expect most environmental scientists to be concerned about climate change.

It is crucial the environmental assessment process for large coal mines remains rigorously independent and absolutely free from any direct or indirect pressure from the coal industry. This is even more important when dealing with groundwater assessments, given their economic, social and ecological significance.

The letter, sent before the review was handed down, sets a dangerous precedent. Not because it suggests the scientists were impartial or there was any procedural unfairness involved in the process. But rather, because it jeopardises the independence of our scientists who, in seeking to ensure the longevity of our water, food and energy resources, carry a heavy responsibility to the public interest.The Conversation

Samantha Hepburn, Director of the Centre for Energy and Natural Resources Law, Deakin Law School, Deakin University

This article is republished from The Conversation under a Creative Commons license. Read the original article.