First, collect the objects you need. They are:
Sun-any ball, diameter 8.00 inches
Mercury-a pinhead, diameter 0.03 inch
Venus-a peppercorn, diameter 0.08 inch
Earth-a second peppercorn
Mars-a second pinhead
Jupiter-a chestnut or a pecan, diameter 0.90 inch
Saturn-a hazelnut or an acorn, diameter 0.70 inch
Uranus-a peanut or coffeebean, diameter 0.30 inch
Neptune-a second peanut or coffeebean
Pluto- a third pinhead (no longer having planet status)
You may suspect it is easier to search out pebbles of the right sizes. But the advantage of distinct objects such as peanuts is that their rough sizes are remembered along with them. It does not matter if the peanut is not exactly .3 inch long; nor that it is not spherical.
A standard bowling ball happens to be just 8 inches wide, and makes a nice massive Sun, so I couldn’t resist putting it in the picture. But it may not be easy to find and certainly isn’t easy to carry around. There are plenty of inflatable balls which are near enough in size.
The three pins must be stuck through pieces of card, otherwise their heads will be virtually invisible. If you like, you can fasten the other planets onto labeled cards too.
Begin by spilling the objects out on a table and setting them in a row. Here is the moment to remind everyone of the number of planets -9- and their order–MVEMJSUNP. (This mvemonic could be made slightly more pronounceable by inserting the asteroids in their place between Mars and Jupiter: MVEMAJSUNP.)
The first astonishment is the contrast between the great round looming Sun and the tiny planets. (And note a proof of the difference between reading and seeing: if it were not for the picture, the figures such as “8 inches” and “.08 inch” would create little impression.) Look at the second peppercorn–our “huge” Earth–up beside the truly huge curve of the Sun.
Having set out the objects with which the model is to be made, the next thing is to ask: “How much space do we need to make it?” Children may think that the table-top will suffice, or a fraction of it, or merely moving the objects apart a little. Adults think in terms of the room or a fraction of the room, or perhaps the corridor outside.
To arrive at the answer, we have to introduce scale.
This peppercorn is the Earth we live on.
The Earth is eight thousand miles wide! The peppercorn is eight hundredths of an inch wide. What about the Sun? It is eight hundred thousand miles wide. The ball representing it is eight inches wide. So, one inch in the model represents a hundred thousand miles in reality.
This means that one yard (36 inches) represents 3,600,000 miles. Take a pace: this distance across the floor is an enormous space-journey called “three million six hundred thousand miles.”
Now, what is the distance between the Earth and the Sun? It is 93 million miles. In the model, this will be 26 yards.
This still may not mean much till you get one of the class to start at the side of the room and take 26 paces. He comes up against the opposite wall at about 15!
Clearly, it will be necessary to go outside.
Hand the Sun and the planets to members of the class, making sure that each knows the name of the object he or she is carrying, so as to be able to produce it when called upon.
You can make some play with the assigning of the objects to the “gods” who are to be their bearers. Selecting a blond Sun, a hyperactive Mercury, a comely Venus, a redhaired or pugnacious Mars, a ponderous or regal Jupiter, a ring- wearing Saturn a blue-eyed Uranus, a swimming-champion Neptune, a far-out Pluto can enliven the proceedings and teach a few scraps of mytholgy or planetology. It is unfortunate that only Venus and Earth (the Moon) are female (most of the goddesses have given their names to asteroids instead).
You will have found in advance a spot from which you can walk a thousand yards in something like a straight line. This may not be easy. Straightness of the course is not essential; nor do you have to be able to see one end of it from the other. You may have to “fold” it back on itself. It should be a unit that will make a good story afterwards like “All the way from the flagpole to the Japanese garden!”
Put the Sun ball down, and march away as follows. (After the first few planets, you will want to appoint someone else to do the actual pacing-call this person the “Spacecraft” or “Pacecraft”-so that you are free to talk.)
10 paces. Call out “Mercury, where are you?” and have the Mercury-bearer put down his card and pinhead, weighting them with a pebble if necessary.
Another 9 paces. Venus puts down her peppercorn.
Another 7 paces. Earth
Already the thing seems beyond belief. Mercury is supposed to be so close to the Sun that it is merely a scorched rock, and we never see it except in the Sun’s glare at dawn or dusk-yet here it is, utterly lost in space! As for the Earth, who can believe that the Sun could warm us if we are that far from it?
The correctness of the scale can be proved to skeptics (of a certain maturity) on the spot. The apparent size of the Sun ball, 26 paces away, is now the same as that of the real Sun-half a degree or arc, or half the width of your little finger held at arm’s length. (If both the size of an object and its distance have been scaled down by the same factor, then the angle it subtends must remain the same.)
Another 14 paces. Mars
Now come the gasps, at the first substantially larger leap:
Another 95 paces to Jupiter
Here is the “giant planet”-but it is a chestnut, more than a city block from its nearest neighbor in space!
From now on, amazement itself cannot keep pace, as the intervals grow extravagantly:
Another 112 paces. Saturn
Another 249 paces. Uranus
Another 281 paces. Neptune
Another 242 paces. Pluto
You have marched more than half a mile! (The distance in the model adds up to 1,019 paces. A mile is 1,760 yards.)
To do this, to look back toward the Sun ball, which is no longer visible even with binoculars, and to look down at the pinhead Pluto, is to feel the terrifying wonder of space.
That is the outline of the Thousand-Yard Model. But be warned that if you do it once you may be asked to do it again. Children are fascinated by it enough to recount it to other children; they write “stories” which get printed in the school paper; teachers from other schools call you up and ask you to demonstrate it.
So the outline can bear variation and elaboration. There are different things you can remark on during the pacings from one planet to the next, and there are extra pieces of information that can easily be grafted on. These lead forward, in fact, to the wider reaches of the universe, and make the planet walk a convenient introduction to a course in astronomy. But omit them if you are dealing with children young enough to be confused, or if you yourself would prefer to avoid mental vertigo.
I recommend that you stop reading at this point, carry out the walk once, and then read the further notes.