Naked-Eye Universe—Part 2

An ancient Greek astronomer, Hipparchus, became extremely familiar with the heavens. Like many of his compatriots, he was not limited by city lights and thus developed an intimate understanding of the night sky. Hipparchus was the first to classify stars according to their brightness. He had no idea of the distance between Earth and the stars (he even assumed that they were all the same distance away), but he carefully classified their relative brightness into six categories, which he labeled magnitude 1 to magnitude 6. His system has endured, so we still use it today.

Now that we know stars are at various distances away from us, it raises a few questions: When I look at a specific star—shining at its particular brightness or magnitude—am I looking at a faint star that is closer to us or a bright star that is farther away? How far away is the most distant star I can see with naked eye? Again, how big is the naked-eye universe that I can see from my tub? Just how far out can my eyes go?

A little research told me that the most distant star that we can see with the naked eye is in the constellation Cassiopeia. It's an extremely bright star—as brilliant as 100,000 suns—and it's 4,000 light years away. That's a mind-boggling distance of about 24,000,000,000,000,000 miles! It's amazing that my eyes can pick up light from a very bright star that far away. That seems to indicate a pretty big naked-eye universe. But let's put that in perspective, by comparing it to the size of the whole universe, which our most powerful telescopes and detailed scientific measurements have provided us.

Suppose I could imagine myself sitting at the center of a transparent Earth, letting the diameter of the Earth represent the size of our universe. If I looked outward from that center point, therefore, the surface of the Earth would represent the edge of the universe. What then (sitting in my tub, gazing at the stars with naked eye), would be the equivalent distance I could see in this scaled-down, Earth-sized universe? I could see as far out as about a tenth of an inch... about the size of a fat grape seed! I can't even fit inside a grape seed, but if I imagine myself at its center, my naked eye could only see as far as to the edge of the seed. Everything beyond that seed would be unknown to my naked eye.

Wow! My sense of sight (not amplified by a telescope) is but a grape seed at the center of an Earth-sized ball! It makes me realize just how small I am. I get to see but the tiniest part of this grand universe. The size of my minuscule naked-eye universe is really insignificant. Nevertheless, on a starry night I peer up from my tub and take in several hundred stars. It ain't much, compared to all that's out there, but it sure is magnificent!