Working models of the heliocentric solar system are called orreys and they consist of a small model with the Sun in the middle and a clockwork mechanism that moves little planets on rods around the Sun. The speed is relatively correct so Mercury makes many more revolutions when compared to a planet further from the Sun like, say, Jupiter. These models are wonderful mechanisms but they suffer from two problems. The first is that the orbits of the planets are circular rather than elliptical. (Yes, it's a pedantic objection.) The second problem is one of scale. With the orrery the planets look very congenial and are grouped close together. It looks like it would be easy for Jupiter to wave to Mercury as the latter gallops around the Sun.

Real planets are much further apart and so a truly scaled orrery would involve a very large spread of planets not to mention a very large arrangement of clockwork to make them move. To get a full appreciation of the scale of our solar system the University of Maine at Presque Isle has created a correctly scaled model of the solar system. It's not like an orrery where model planets revolve around a model Sun but rather a static linear model in which the distances between the Sun and the planets are in correct proportion.

Here's how they did it. The design started by establishing a distance metric. The distance from the Earth to the Sun is 93,000,000 miles and this distance is 1 mile in the model. Another way to specify this distance is as 1 Astronomical Unit (AU) so in the model 1 AU equals 1 mile. The model Sun is located at the University of Maine Presque Isle campus. Now, using the distance metric the model Pluto (yes, the model was made before Pluto was demoted) is located about 40 miles away since the real Pluto is 40 AUs from the sun. In real distance this is 3,720,000,000 miles.

Let's pause a minute because this is a big number and this is just a distance between objects in our own little corner of the universe. Douglas Addams had something to say about distances like this. Quoting from the Hitchhiker's Guide to the Galaxy:

Now while it's hard to conceive of 3.7 billion miles it's easier to conceive of 40 miles, especially if you have to walk it.

Here is a link to the overview of model. Scroll down the page a bit and you'll see a page for the Sun and each of the planets. Note the size of the scaled down planets. Earth, for example, is 5 1/2" in diameter, Jupiter is about 5' in diameter and Saturn is a little smaller at about 52". As you drive along the model path between Pluto and the Sun it's sometimes hard to find the smaller planets. For example, we came close to where Mars should be but we never did find exactly where it was. There must be a lesson here about traveling in the universe: You can overlook interesting places if you're not paying attention.

Here are some photos of our trip. We got to see some of the planets but not all of them.

	This would be Saturn's moon Titan.
	There is one moon to the right of the planet model in this picture and there are 3 moons off to the left out of sight in this picture.
Here are three of the moons lined up.	Another moon in the background.
Earth has to be around here somewhere....	And here we are with the Moon on the pole to the right. Just to put it in perspective this distance is about 250,000 miles.
Well, when you're this close to the sun you need your sun glasses.	The scaled down Sun is still pretty big at 50' in diameter and the yellow arch sculpture represents just a portion of the circumference of the model Sun.