This article was originally published at The Conversation. (opens in new tab) The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.
Silas Laycock (opens in new tab), Professor of Astronomy, UMass Lowell
Why does the moon look close some nights and far away on other nights? — Gabriel H., age 7, Providence, Rhode Island
Some nights the moon seems really close and bigger than usual.
One summer evening when I was a child, I remember being mystified and then startled at a huge round shape slowly creeping up behind my friend Nancy's house, which sat on a hill on the other side of our village.
At some point I suddenly realized it was the moon, and I ran yelling through the garden to tell my dad and get him to come and see. It was bigger than a house, deep orange in color and surely of great significance. My dad muttered something about perspective and went back to gardening or playing the piano.
Unconvinced, I kept watching the moon. Later, once the moon had risen higher in the sky, it was back to looking like its usual self.
Welcome to what astronomers like me call the moon illusion (opens in new tab).
It can be hard to believe that it is just an illusion when the moon looks huge, but it is true. You can actually test the illusion yourself and even capture it with a camera.
A trick of the mind
Astronomers have discussed the moon illusion for centuries, and there are some facts they all agree on.
People mainly notice the moon looking bigger and closer when it is full and near the horizon. This is because your mind judges how big or small an object like the moon is by comparing it with other, familiar things (opens in new tab).
Imagine you are standing outside close to your house. Your house will look big, and if the moon rises next to it, the moon will look normal. If you look at a house from far away, though, the house looks very small.
The illusion comes from the fact that the moon is so far away that no matter where you are on Earth, the moon always looks the same size. It is actually the things your mind compares the moon with — a house, a mountain or anything else — that look bigger or smaller depending on how far away from them you are. So when the moon rises next to a distant house or a faraway mountain, the moon looks enormous.
Photographers use this trick to take spectacular images of distant objects with the moon behind them. People often experience the moon illusion on vacations when they go to wide-open spaces. This may be why big moons become powerful memories of happy times.
Atmospheric zoom and changing orbits
There are several convincing-sounding but wrong explanations for the moon illusion. Most are grounded in some truth, so they persist.
First is the idea that the atmosphere acts like a lens and magnifies the moon. When the moon is near the horizon, its light has to travel though much more of the Earth's atmosphere than when the moon is directly overhead. It's true that all that air acts like a giant prism and bends the rays of light (opens in new tab), distorting the color and shape of the moon. But it does not act like a magnifying glass.
Next is the idea that on some nights the moon really is closer. The moon's orbit is not perfectly circular — it's more like an oval shape, called an ellipse — so the moon does get nearer and farther away over the course of a month.
When the close part of the orbit coincides with a full moon, it's called a supermoon (opens in new tab). But when the moon is closest to Earth, it is only about 12% to 15% closer than when it is farthest from Earth — too small a difference to explain the moon illusion. It is hard to notice a 15% difference in size by just looking at the moon alone in the sky.
Testing the illusion
It's easy to test the moon illusion, and you can do it yourself. Next time you see the moon looking huge and closer than usual, hold out your hand with a straight arm. Then close one eye and see which finger tip just barely covers the moon — for me, it's my pinkie finger. Wait a little while until the moon moves higher into the sky and try the experiment again. The moon may look smaller, but your same finger will cover it just the same.
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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.
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.
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.
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 50,000 km 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 38 mm 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 384,402 km. 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 4,310 km 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 16 July 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.
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.
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.
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.
Jonti Horner, Professor (Astrophysics), University of Southern Queensland.
This article is republished from The Conversation under a Creative Commons license. Read the original article.