A Penny For Your Thoughts

One of the concepts that we try to make clear to guests in the Nightly Observing Program at the Kitt Peak Visitor Center is the scale of the Cosmos, so they leave knowing the difference between the solar system and the galaxy. There are a variety of ways to do this, but one of my favorites is to miniaturize things down to a more reasonable scale by making a model. The model doesn't have to be physical; describing things verbally will work just fine. Let me describe one of my favorites, which I call "The Penny Model."

Once I've gathered everyone inside the Visitor Center for a warm-up break (and explained astronomical units and light years), I take out a penny and show it to the crowd. "Let's suppose," I begin, "that the solar system was shrunken down small enough to fit onto this penny. We'll put the sun in the very center of the penny, and then lay down all the orbits of the planets in their appropriate positions, so that the orbit of Pluto runs along the penny's perimeter. That makes this penny about 80 AU across." An astronomical unit (AU) is the distance between the Earth and the Sun.

"At this scale," I continue, "the closest star to the Sun would be 226 feet away. That might be, say, out in the parking lot where you parked your car. So let's drop another penny out in the parking lot to represent that star and any planets it might have." I have the option here of describing that closest star, Alpha Centauri, as a three star system, in which case we could use one penny to represent the first two stars (which are 23 AU apart) and a second penny for the third star. There would be eleven feet between these two pennies, since Alpha Centauri C is 13,000 AU from its companions.

"Since stars in this part of the galaxy are, on average, about a half dozen light years apart from each other, we'll continue to lay down a penny every few hundred feet, each penny representing a star and its planets (if any). We would cover the entire mountain in pennies this way, and then we would find ourselves laying them down in the surrounding desert as well." One of the first objects I show in the telescope is the binary star Almach. If the crowd has seen Almach, then I mention that it would be two and a half miles away from "our penny," the solar system. I like to imagine Almach as two pennies, eight inches apart, lying on the asphalt where the Pan Tak turn-off connects with Highway 86.

"In fact," I go on, "the core of the Milky Way Galaxy would be about 300 miles from Kitt Peak. I like to imagine it in the Grand Canyon area. So we spread pennies from here to the Grand Canyon, and as we approach the Grand Canyon we find that the pennies are closer together, maybe only 50 or 100 feet apart, because stars are closer together in the core of the galaxy.

"The galaxy itself would be a thousand miles across, so imagine a field of pennies, stretching in all directions with a penny every few hundred feet, that reached from San Francisco to eastern Colorado, and from northern Mexico to the southern parts of Idaho. The Milky Way would totally cover Arizona, Utah, and Nevada, and would cover most of California, New Mexico, Colorado, Wyoming, Idaho, and Oregon!

"Since the galaxy is about 5000 light years thick, we can't simply leave all the pennies at ground level. We need to have them hover in the air and embed them deep in the Earth's crust as well, twenty miles up and twenty miles down. The nearest galaxies to the Milky Way, the Large and Small Magellanic Clouds, can both be located squarely in Canada. The closest spiral galaxy, called the Andromeda Galaxy (see picture), cannot be located on Earth at all. Since it is two million light years away, its collection of pennies must be placed about a tenth of the way to the Moon!"

Updated: 7/12/98