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August 20th: Walking With Planets


365daysDate: August 20, 2009

Title: Walking With Planets


Podcaster: Andy Briggs

Organization: Science File (

Description: Astronomers blithely speak of the millions or billions of miles between the planets in the solar system, but our brains cannot comprehend what those distances actually look like. Andy Briggs suggests a new, and much more meaningful, way of visualising those enormous spaces between the worlds. Forget miles in their millions or billions – what’s needed is feet.

Bio: Andy Briggs is an English software developer living in Spain. He has been passionate about astronomy since he was three years old, when his father took him outside to see Echo 1A, one of America’s earliest communication satellites, sailing through the night sky. Andy is publisher of Science File (, which aims to assemble the most comprehensive set of science reference materials available on the web.

NOTE: As of December, 2016, this website is no longer active.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Andrew and dedicated to Cass, his best friend and awesome teacher at UTSA.


Walking With Planets

By Andy Briggs

Hello and welcome to the 365 Days of Astronomy podcast. I’m Andy Briggs, publisher of If you are interested in science, there´s something there for you.

Today I am going to talk about a problem shared by every living human. The problem is this: we live in a solar system where distances between the planets are measured in millions or billions of miles (or kilometres, if you prefer). Compared to the distances between stars or galaxies, they are nothing. They don’t even register on the scale of impossibly huge gulfs. But to our poor simian brains, they are enormous – and more to the point, we don’t have a chance of visualising what those distances actually mean. It’s all very well to be told, for example, that Venus is roughly 25 million miles away or Neptune is a tad over 2 billion miles distant, but do you know how far those spaces between the worlds actually are? Can you actually see them in your mind’s eye?

I thought not.

It’s not our fault, of course. Evolution has not equipped us with a brain that can process such impossibly huge numbers. Only a hundred years ago, most people never came into contact with anything measured in millions or billions. But since then, what we know about the Universe has increased exponentially, and as a result we have had to mentally adjust to an impossibly vast Universe with completely meaningless scales of distance, be they miles or light-years. We blithely accept the figures, but our neurons just give up when we try to visualise what those figures mean.

The trouble is that whatever units of measurement we use, sooner or later the numbers spiral out of control and we reach a point beyond which our brains cannot pass. And we are not necessarily talking about interplanetary or interstellar distances here – I can visualise what a mile is, but not what ten thousand miles would look like. What chance, then, of seeing the distance to Neptune?

So I would like to propose a system of measuring interplanetary distances which makes them more meaningful. It’s very simple, and, although after a while it too creates numbers which get a little too big to handle, it works better than using millions and billions when you are trying to grasp what the distances look like.

Let’s measure distances to the planets by the time it would take to walk there.

We all walk, after all. We all know how to measure a journey in walking-time, whether it’s to the pub, to work, or to the nearest supermarket. Walking-time means something to us: it’s not a functionally-abstract concept like “two billion miles.” So let us don our walking boots and take a tour of the Solar System, and see how long it takes us. We are going to assume a walking-speed of three miles per hour, with no rest or toilet breaks. We are going to walk continuously, without stopping, but obviously if you really do need breaks it is very easy to build those into the calculations. We will also assume that we are walking the minimum-possible distance to the planets, not the mean or maximum.

You will need to wrap up warm. Oh, and please leave a message for callers on your answerphone: you will be gone quite some time.

Ready? Let’s go!!

The first stop on our tour is our nearest celestial neighbour, the Moon. Walking steadily, we reach it after twenty-nine and a half years. Now, to me, that figure means more than saying that the Moon is 238,000 miles distant: if I had set out on my walk to the Moon at the end of my teens, I would just now be arriving. Or, to put it another way, I started walking the year that Louise Brown, the first test-tube baby, was born. In a nice coincidence, bearing in mind what we are discussing, it was also the year that Pluto’s moon Charon was discovered. And in the same year – 1978 in case you hadn’t worked it out – the Space Invaders game, which begat a whole new era of video games, was developed by the Taito Corporation.

Suddenly that distance to the Moon becomes a whole lot more meaningful. No?

So where shall we go next on our walking tour of the Solar System? Inwards to Venus and Mercury, or out to Mars? You choose. OK, Venus it is then.

But there is one thing I neglected to mention. You might not have come if I had told you. We want to measure the time it takes to walk from the Earth to the planets. Therefore – and I am really sorry about this – we need to walk back home again first. Don’t cry. Please.

So after soaking our aching feet, we set off again, and we arrive home from the Moon in 2037.

And when you are rested and ready, we shall set off for Venus.

It only took us 29.5 years to walk to the Moon, so how long to Venus?

Ready for this? Our walk to Venus is going to take us 951 years. If we were arriving now, we would have set off just nine years before the Battle of Hastings, in 1057. In that year Macbeth of Moray, who had united Scotland for the previous 17 years and who is often referred to as the “Last of the Celtic Kings,” was killed in battle.

Back to the Earth, then, arriving in 2959.

On, then, to Mercury, nearest planet to the Sun. Our walk takes us 1,826 years so, arriving in 2009, we would have left home in 181 AD, the year that the Roman Emperor Commodus exiles his sister Lucilla to Capreae (now known as Capri.) We return to the Earth, arriving in the year 3,834.

No slacking now. We are off to Mars, fourth planet from the Sun. The journey will take us 1,332 years. If we were arriving now, we’d have set off in the year 675 AD. In that year in Britain, Wessex had a new king, Centwine, although Ethelred of Mercia still holds parts of the region. Late in the year, Ethelred captures Rochester in Kent and ravages its surrounding countryside.

We leave the rusty deserts and almighty canyons of the Red Planet and walk back to the Earth.

Come, then, and let us walk from the Earth to Mars and then cross the vast gulf between Mars and Jupiter. Our walk will take us through the Asteroid Belt, where hundreds of thousands of boulders ranging from tiny stones to house-sized blocks careen through space. We will take comfort from the fact that no spacecraft has ever suffered damage as it crossed the Asteroid Belt, and hope that we, too, will be safe. Luckily we survive, and we arrive at the cloud tops of Jupiter after a walk from the Earth of 13,889 years. If we were arriving now, we would have set off in 11,881 BC. This was the Upper Palaeolithic era, a fascinating period of human history known for its technological advances, artistic creativity, and advances in social groups which produced the first symbols and rituals.

We return to the Earth, arriving in the year 15,897.

And on to Saturn. The ringed planet, for me, is the highlight of our tour, and our walk from the Earth takes us 28,387 years. If we were arriving in 2008, we would have left in 26,379 BC. This was an era of human history characterised by nomad hunter-gatherer societies. It was a period in which the Neanderthals decline in number to nothing, and when the first artifacts of weaving and cloth-making date from. The Gravettian people roam between Southern Russia and Spain, and stone tools are being produced in large numbers to hunt big animals like mammoth, bison, and horses.

After pausing to take in magnificent views of the ring system and to explore Saturn’s 60 moons, we return to the Earth, arriving in the year 30,395

We conclude our tour by walking to Uranus and Neptune, the two gas giants towards the edge of the Solar System. We arrive at Uranus after a walk of 60,883 years, so if we were arriving now we would have left home in the year 58875 BC. The first Australian aborigines date from this era, greatly altering their environment by discovering fire, and it was a period of migration out of the plains of Africa to Australia and Asia. The first ceremonial burials date from this period. We left a world covered with ice: the last great Ice Age was in full swing.

We return to the Earth, arriving in the year 62,891.

And so we come to our final destination of our walking tour of the Solar System. Since Pluto lost its status as a planet, Neptune is now the most distant in the Solar System. If you are not completely tired out by all this walking, one day we could indeed visit Pluto, and beyond to the Kuiper Belt where thousands of rocky bodies, some bigger than Pluto, inhabit a zone out to some 6 billion miles from the Sun.

Our walk to Neptune takes us 101,979 years. Arriving in 2008, we would have left in 99,971 BC. Around this time, humans developed the first spoken languages and domesticated horses for the first time. A giant tsunami, possible a quarter of a mile high, strikes the coast of Australia and New Zealand.

And so, one last time, we return to the Earth, arriving in the year 103, 987

Well, we have done it. We have walked to all the major planets in the Solar System. I hope you have enjoyed the tour, and that now, when astronomers tell you the distances to the planets measured in miles or kilometres, you will have a better sense of the distances they are talking about. Of course, measuring those distances in the time it takes to walk them is only meaningful within the Solar System: if we were to start walking between the stars, once again our brains would flounder to understand those unfathomable spaces and impossibly long periods of time.

At the end of all our walking, we now know that the furthest major planet in the Solar System can be walked to in the time which has elapsed since the last ice age. Yet, astronomically speaking, that’s not even round the corner.

Thank you for listening….and Happy Walking!!!

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

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