Forget trying to comprehend planetary distances measured in millions or billions of miles. What's needed is feet.

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?

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 neurones 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.