When I hear a scientist, talking about some astronomical object, and they say something is (for example) 8 billion miles away, I cringe. No doubt, the reporter asked for the distance in miles, or kilometers, because that unit of distance is familiar to most people. Does it really tell you anything?

I say no.

Yes, it tells you it is very far away, but nothing much more than that.

To really understand these distances, you need to compare it to something! When I talk with school kids about weather and earth Science, I try to compare measurements to something they can relate to.

Here is a little project you can use to teach your kids about the solar system. We will use a unit of distance called an Astronomical Unit. (AU). An AU is simply the distance from the Sun to Earth. Earth is 1 AU away from the Sun!

Grab a tape measure, and a football (Americans: Read that as soccer ball). A couple of small marbles, and some crumbs from week old brownies. Tiny pieces of gravel will do. Small enough that 8 or 9 of them will fit on your thumbnail.

Now, Go outside and put the ball in an open area. Use the tape measure to mark 1 meter. Put a piece of tiny gravel down. A persimmon seed will work too. At 5 meters from the ball put down your large marble. (A jaw breaker works really well here) At 9.5 meters put down your second biggest marble. At 19 meters put down a small marble and then another one at 30 meters.
Almost done now.

At 39.5 meters put down a tiny grain. now measure all the way out to 84 meters away from the ball. drop your tiniest grain on a piece of paper.

Welcome to the solar system. You have just built a pretty darn good model that is correct in size and scale!

At one meter is the Earth. You can put down a seed at 1.5 meters for Mars, and another seed about earth size at 70 centimeters. That is Venus.

The big marble at 5 meters is Jupiter. The one at 9.5 meters is Saturn. Neptune is at 30 meters, and while not a planet, Pluto is at 39.5 meters. Step back to Earth and look at the ball. It should look about as big as the disk of the sun in the sky. Now walk out to Neptune. Notice how tiny the sun looks!

Lastly, walk out to your grain of dust at 84 meters. Look back at the sun. This is where Voyager One is now. It is the most distant man made object from Earth. Launched in 1977, it now so far from sun, that the stream of particles from our Sun (The Solar Wind) is nearly gone. It’s not as big as the grain you have, it’s as big as the cold virus on the grain!

The fastest anything can travel is light speed. Physics people refer to it as “c”. Now, I can tell you that this is 186,282 miles per second, but I would be guilty of doing just what this post is about!

Lets do it this way.

At light speed, you can go around the Earth 7 times in one second. In our scale model, it takes light, 8.15 minutes (489 seconds) to go from the ball, out to Earth at 1 meter. In other words, on our newly built solar system, light speed is one meter every 8 minutes. I will let you do the math to figure out how long it takes to send a message to Voyager One, by radio.

The nearest star to our Sun is 272,021 Astronomical units away. Since we used 1 meter as an AU in our solar system, we can put it on our scale. Get a rubber ball 5 centimeters wide, and drive 272 kilometers away. Set it down, and write Proxima Centauri on it! It will take a radio signal moving at 1 meter every 8 minutes, over 4 years to get there.

That should give you an idea of the size of the solar system! Cool ay?

Dan