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If the Milky Way galaxy is as big as planet earth, how big is Earth?

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Well, they are different shapes, but we can get a rough answer by doing a ratio calculation based on their radii.

Let $g$ be the radius of the galaxy, about $5\times 10^{20}\ \mathrm m$.
Let $e$ be the radius of the Earth, about $6371\ \mathrm{km}$.
Let $s$ be the radius of the scaled-down Earth.

Then, $$\frac e g = \frac s e$$ So $$s = \frac{e^2} g = \frac {(6371\ \mathrm{km})^2} {5\times 10^{20}\ \mathrm m}$$ which is approximately 80 nanometres. So it's about 1000 times the size of an atom.

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