# Thread: Comparing our occupation of the known universe to a planetary scale.

1. Hello. I thought this might do better here, as it's a maths question. If I were to scale down the size of the known universe to the size of our planet, would anyone know how much of the planet we would occupied on this scale? I'm assuming it's a very small space. Perhaps a better comparison might be to our galaxy. How much of a galaxy would we occupy, if that galaxy were shrunk to planetary size? I hope this question makes sense, and I just think it's an interesting comparison.

2.

3. Use our galaxy, not the known universe. There's a pretty reliable number for the size of our galaxy.

The Milky Way galaxy is approx 100,000 ly in diameter and I'm not sure how thick it is, but you can find these values in wiki and other places. Now, the core of the galaxy, like most spirals, is thicker than the outward arms. Also, the globular clusters extend the center "sphere"/core of the galaxy even further. Make an estimate for the average thickness (which will be the hardest part of this calculation) and then use the formula for finding the volume of a cylinder (given diameter and height) to compute the total volume of of our galaxy. Since we only occupy the planet Earth, determine the volume of the Earth, ignoring the fact that we only live on the surface. Divide the volume of the Earth by the volume of the galaxy and you have your ratio. Given this ration you can easily use it to find what how much of the planet we would occupy on this scale.

I'm not going to look up the values or do the math. That's for you to do. The math is very simple.

NOTE: The cylinder shape that I'm talking about is a "flat" cylinder -- one where the width (diameter) is several times more than its height.

4. If you do an internet search for "images of Milky Way galaxy" you'll find thousands of pictures of what scientists think that our galaxy would look like from the outside.

Post your results when you're done so the members can verify your calculations.

5. Before you start you may want to wait a while to see if forum members have any better suggestions than mine or corrections to what I posted.

6. The diameter of the Milky Way is about 100,000 light years, which is about 9.46E20 meters. The diameter of the earth is about 12.74E6 meters. So that is a ratio of 7.4E13 to 1. So if you divide 12.74E6 meters by 7.4E13 you get 1.72E-7 meters. That's about the size of viruses.

7. Thanks Harold, I had done similar math in relation to diameter. I was hoping the scale would be something familiar, like a pin head, but ended up going from a distance so vastly expansive it's difficult to imagine, to a scale so minute, it's difficult to imagine. Maybe there's a revelation in that.

8. I think the point is: