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!

Naked-Eye Universe—Part 1

Sitting in my outdoor tub, gazing at the dark (late) winter's sky, I can see hundreds of stars, even though my field of vision is significantly reduced by overhead trees and the bulk of the house behind me. Were I to be lying on my back while floating on the ocean or reposed on the flat desert sand, I would be able to count as many as 4,000 stars above me. So tonight, with my limited view, I still may be able to discern as many as a thousand stars. That's not a bad count.

But how deep a view am I looking at? How far “out there” can I see? I know that with my naked eye I can see but a tiny fraction of the number of stars that a telescope could capture. My eyes extend nowhere near as far out as telescopes go. So just how much of the whole universe can my naked eye sample?

Our ancestors were very familiar with the night sky—far more than we moderns. We may occasionally look up, but we are either limited by city lights or are so ignorant of the sky that we quickly lose interest and turn back to our electronic screens. Our ancestors did not possess telescopes (or cell phones), so they had no idea of how far away those stars were. In fact, most of the ancients believed that the stars were all about the same distance away... maybe just a little ways above the trees. So, although they were intimate with their sky, they had little idea of just how large the universe really is.

We moderns, however, know that those stars are not scattered across a hemispherical bowl immediately overhead, but are as far away as billions of light years. (A light year is really a measure of distance. It's the distance that light—the speediest thing in our universe—travels in one year; about 6,000,000,000,000 miles. That's a hell of a distance to try to wrap your head around! So we resort to light years.) So today we know something about the true size of our universe—because telescopes have told us.

But I wander. Back to my soaking in the outdoor tub: I know I can see but a tiny part of it all. How small a part? I look up and see hundreds, if not thousands of stars, and wonder what is the size of my naked-eye universe. It's ever so smaller than the whole enchilada. Astronomers tell me that there are billions of galaxies and trillions upon trillions of stars, but I can see only a wee portion... a much smaller portion of it all.

More naked-eye universe next time...