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Relative Sizes of the Earth and the Milky Way Galaxy

-- | September 9, 2012

Question: Hi, I just want someone to verify my math –needed to visualize the scale of the galaxy. I found that if the Earth were the size of a grain of salt (0.004 inches) the diameter of the Milky Way would be about 5.1 million miles. Is this about right? I can find lots of comparisons on the web of the solar system to the galaxy, but your average Joe is not really familiar with the scale of the solar system or the relative size of the sun to the planets or the diameter of the orbits. Comparisons to the size of the Earth hit home much better. Thanks. – Tom

Answer: Assuming a diameter for the Milky Way galaxy of about 15 kpc, which is about 4.6×10^(17) km, and a diameter for the Earth of about 12756 km, the ratio of the diameter of the Milky Way galaxy to that of the Earth is about 3.6×10^(13). Since 5.1 million miles is about 3.2×10^(11) inches, your ratio of the size of a grain of salt (0.004 inches) to the diameter of the Milky Way is about 8×10^(13). So, yes, your comparison between a grain of salt and the diameter of the Milky Way is correct.

Jeff Mangum

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