A Neat Trick to Understand the Size of the Galaxy

The nearest star, Proxima Centauri, is some 25 trillion miles away. The Orion Nebula lies about 8,000 trillion miles away. And we are some 162,000 trillion miles from the centre of our own galaxy, the Milky Way. These are mind-numbing distances, completely foreign to the everyday experience of even the most hardened stargazers, and they make it hard to grasp the size of our immense Milky Way galaxy.

But there’s a happy numerical coincidence that makes it much easier to contemplate such distances.

Here’s how it works…

A light year, the distance light travels in one year, works out to be about 5.88 trillion miles. An astronomical unit, the distance from the Earth to the Sun, is about 93 million miles, a much smaller distance. A quick calculation shows there’s about 63,200 astronomical units (AU) in a light year.

So far so good.

Now it turns out, completely by chance, there’s also about 63,200 inches in a mile (I’ll let you do the math). So an inch to a mile is the same as an AU to a light year.

Using this compressed scale, where one inch is the Earth-Sun distance, you can get a better feel for the stupendous size of our galaxy…

If the Sun was a grain of sand, and the Earth a microscopic speck one inch away, then Jupiter would lie 5.2 inches away and Pluto an average of 40 inches away. Next stop… the nearest star, about 4.3 miles away, with mostly empty space between it and the Sun. The star Vega would be 26 miles away, the Orion Nebula 1340 miles away, and the globular cluster M15 some 25,000 miles distant (about three times the diameter of the Earth).

And even on this massively compressed scale, the diameter of the Milky Way Galaxy itself would be about 100,000 miles.

This little trick of scale helps you wrap your mind around relative distances and appreciate the immense scale of a big galaxy like our Milky Way, where even a light year doesn’t count for much…