His comparison was inspired by a photo of the Earth taken by the Voyager 1 space probe in 1990, just as it was leaving the solar system. That's it, in the image I've posted here. That little dot in the band of light at the top is Earth, as it looks from beyond the orbit of Neptune. The "sunbeams" are actually bands of refracted sunlight created by the camera's optics, but that's a justifiable bit of poetic license. After all, Earth really is suspended in sunlight.
As I watched the dust motes in my living room swirling in that same sunlight, I thought about Sagan's comparison, and wondered how accurate it was. Is our whole world--the home of every known living thing--really that tiny? To find out, I did some figuring. I'll spare you the math and skip to the results--I think you'll find them impressive.
Imagine the Earth really were the size of one of those dust particles floating in my living room--a big one; about 30 millionths of a meter across. That speck of dust would be about 15 inches from the sun, which would be about the size of a BB. Neptune would be orbiting that BB at a distance of about 37 feet.
And the nearest star? It would be 63 miles away.
That's about the average distance between all the 100 billion or so stars in the Milky Way galaxy. If Earth were a speck of dust, the Milky Way would still be over a million miles across. And that's just one galaxy, in a universe filled with uncountable galaxies.
It seems, then, that Carl Sagan wasn't exaggerating at all. If anything, he was being too generous. On the scale of the stars, our planet is unimaginably tiny--a little speck of nothing. And yet, it's not nothing; not to us. But don't listen to me. Listen to Dr. Sagan, because he said it better than I ever could:
Consider again that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar", every "supreme leader", every saint and sinner in the history of our species lived there — on a mote of dust suspended in a sunbeam.
The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that, in glory and triumph, they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner, how frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds.
Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.
The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.
It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we've ever known.