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### Science Dad on the Size of the Solar System

#### by Vince Harriman

Click to enlarge images

After our post on the ISS and space objects, Beckett and I have had many conversations about outer space. And with all the exciting space news lately (alien life news at NASA, the Akatsuki probe arriving at Venus, the tripling of the number of stars), we decided to revisit one of our favorite topics.

One of the hardest things to talk about and understand is the size of the universe -- even the size of the solar system. When a scientist at Yale announced this week that there were three times as many stars in the universe as previously thought, Beckett and I sat down to do the math. Writing down a one followed by dozens of zeros gives a glimpse at size, but still doesn't really get at how big something is. So we decided to start locally and build a scale model of the solar system to get a sense of the size of our corner of the universe.

We found a great website that scales the solar system by entering in any value for the sun. We started by finding a beach ball that measured exactly twelve inches in diameter. We filled it with air and cracked out the Play-doh. Beckett and Rowan sat down and started rolling out little balls to be the planets.

The website indicated that if the sun is 12 inches in diameter, Mercury would be 0.04 inches in diameter, or just under 1 mm. This was just about as small as we could make with Play-Doh. Next, Venus and Earth were both about 0.1 inches in diameter, or 2.6 and 2.7 millimeters -- roughly around the size of a ball bearing. Mars was a little smaller, but when we got to the outer planets we finally had something we could hold on to. Jupiter was almost an inch and quarter in diameter and Saturn was an inch. Neptune and Uranus were both under half and inch, and Pluto was half the size of Mercury and too small to make.

We bagged them all up and headed outside where we hung the beach ball from a tree over the street. This is where the big surprise came. After looking at a typical 'scale drawing' (see above), we checked the math and started walking. We started counting off the feet -- Mercury was 41 feet away, Venus was 77 feet away, and Earth was 107 feet away!

In the photo you can just see the beach ball hanging in the tree -- it sure looks tiny! From over a hundred feet away, Earth looked incredibly tiny and distant. We started counting paces again, stopping at 163 feet for Mars, then hiking to 559 feet for Jupiter. By now we could barely see our 'sun' hanging from the tree. By the time we made our way to Saturn (1025 feet or .2 miles!) we really had to search for our scale 'sun'. We made it all the way to the end of the street (2062 feet) and paused to consider how cold Uranus must be that far from the sun. If we kept walking, Neptune would have over half a mile away at 3232 feet, and Pluto (still a planet in book) at 4248 feet! We decided that we were getting a pretty good idea of how big our solar system is and stopped in a coffee shop to warm up and talk about our scale model.

We talked a bit about the universe outside our solar system and did a little math. Our solar system is approximately 27 light days across, if we use Pluto's average orbit as the boundary. Our neighborhood is only light days across, while the current estimate is that the Universe itself is 16 billion light years across! The universe is so big it doesn't even make sense to make a scale model. Here is the math:

300,000 kilometers a second X 60 seconds = 18,000,000 kilometers per minute
18,000,000 kilometers a minute X 60 minutes = 1,080,000,000 kilometers per hour
1,080,000,000 kilometers an hour X 24 hours = 25,920,000,000 kilometers per day
25,920,000,000 kilometers per day X 365 days = 9,460,800,000,000 kilometers per year
Current estimate 16,000,000,000 light years X 9,460,800,000,000 = 151,372,800,000,000,000,000,000 kilometers across!

That is one big Universe!

_____
Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-5 1/2 and Rowan-2 1/2 ask him 'why' approximately 6,411 times a day.

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.