Just how big space is is easy to demonstrate by building a scale model of the solar system. You'll need much more room than you'd imagine to accurately model both the relative sizes of the Sun and planets, and the distances between them. Start with a soccer ball for the Sun, Mercury is a pinhead, Venus and Earth are peppercorns. The largest of the planets, Jupiter, is a pecan just shy of an inch across. These relative sizes are comprehensible: the Sun, Jupiter and Earth are about a factor of ten different from one another in size, but the distance scale is truly amazing. Earth is 26 yards away from the Sun, Jupiter 132 yards (a football field and a third) and Pluto over 1000 yards away. Space is ridiculously large, that's why they call it space.

Things spread out from there: to Proxima Centauri, the nearest star, is 4000 miles away in this scale model, some 10,000 times the distance from Pluto to the Sun. And the Andromeda Galaxy, also known as M31 and generally considered the farthest thing visible to most people without optical aid, is roughly a million times farther away than that. It's not hard to imagine all the hundreds of billions of the rest of the galaxies spreading ever more thinly in every direction. A big and lonely Universe.

But our intuition and anthropocentric perspective are deceptive at these scales. Remarkably, it turns out that the distances between galaxies are on average only a few tens of times their diameters.  For example, the Milky Way and Andromeda galaxies each span more or less a few 100,000 light years, yet are only approximately ten to twenty times that distance, 3 million light years apart. After all, we do live in the so-called “Local Group” and many clusters of galaxies are close enough to be readily visible in amateur telescopes. Numerous examples of interacting galaxies are well known, yet when galaxies collide, their stars don't crash into one another. To put it another way, consider this: it is only about 3000 times the distance between here and M31 to the edge of the Universe, about 13 billion light years away. Think about that on the next clear night as the Andromeda Galaxy rises overhead.

Paradoxically, it's the small scale, solid matter, that is really empty. More than 99% of an atom's mass is contained in the protons and neutrons of the nucleus, all in a diameter 1/100,000 that of the the atom.  Repulsion and bonding of the outer electron shells provide what we feel as the solid structure of matter. The densest matter found on this planet is in fact mostly nothing at all. 

So in reality, as well as on 1960's television, the Universe at large isn't such a lonely place. Hollow at the subatomic dimensions, it's almost cozy at the largest scales: galaxies are as close as farm houses in the countryside. And maybe all those stars and planets do harbor life, our galactic neighbors. Perhaps if we can get the dilithium crystal drive to work, space would become a  far more familiar, perhaps even welcoming frontier.

In this essay, we've attempted to “boldly go” over nearly 41 decades of spatial scales, a range of 1:100,000,000,000,000,000,000,000,000,000,000,000,000,000 (1 followed by 41 zeros), from an atomic nucleus (femtometers, 10^-15 m) to the edges of the known Universe (some 10²⁶ m away). Our perception of time is similarly human-centered. More on this in a chapter entitled “Calibrations” in Natalie Angier's insightful book, The Canon, A Whirligig Tour of the Beautiful Basics of Science (2007). 

Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 23 September 2007. Text and images on this web page are free for non-commercial use with attribution under a Creative Commons Attribution-Non-commercial 3.0 License. Ask Doug about other uses.