If the universe were the size of the Earth, how big would the Milky Way be?

First of all, we do not know that the geometry of our universe is 'closed' like a sphere, but for the sake of argument lets say that you actually mean the diameter of our visible universe which is a sphere with a radius of about 15 billion light years. Now, a galaxy like our Milky Way has a diameter of about 90,000 light years, so the ratio of these two collections of matter is 15 billion/90,000 = 167,000 to 1.

The radius of the Earth is 6378 kilometers, so it this were the Visible Universe, the Milky Way would have a radius of 6378/37500 = 170 meters. On the same scale, our solar system is even smaller.

The radius of our solar system is about 40 Astronomical Units, and there are 64,200 of these per light year. This means the Milky Way compared to the solar system is 64,200/40 x 90,000 = 144 million times bigger. So, if in our model the Milky Way is 150 meters in radius, the solar system is 150/144,000,000 or 0.001 millimeters in radius or 1 micron. This is about as big as some small dust particles! The visible universe's radius would be 167,000x150 meters = 25,000 kilometers from your model.

Copyright 1997 Dr. Sten Odenwald