Andromeda Galaxy. Photo by m-gucci/Stock/Getty Images.

How far can you see?

If you’ve ever wondered how far can you see with your naked eye, the answer hovers in the night sky of autumn. High in the east, under a clear sky away from light pollution, you can spot an object whose distance can’t be comprehended; only quantified.

Let’s start with distances we can comprehend. We resolve letters on an eye chart from a distance measured in feet; words and symbols on a road sign from a distance measured in yards; the outline of a city skyline or mountain range from a distance measured in miles. We Contra Costa County folk are graced with a magnificent long-distance object: Mt. Diablo, about 10 miles west of downtown Brentwood. Let’s use the mountain as a point of reference.

A little elevation – say, the crest of Round Valley’s Hardy Canyon Trail – rewards us with a view of an object 10 times farther than Mt. Diablo: the granite majesty of the Sierra Nevada Range to our east. A greater challenge to the imagination is the view of our Moon sinking into the west behind Mt. Diablo. The Moon: 24,000 times more distant than the mountain – though not nearly as impressive as the Sun: 9 million times the distance of the mountain.

Our next step takes us into interstellar space. The nearest bright star in our autumn morning sky, found in the constellation Canis Major (just southeast of Orion), is the glinting diamond we call Sirius, a whopping nine light years from Earth. Now, if nine light years doesn’t sound impressively remote, it should.

A light year is a measure not of time but distance: the distance light travels in one year. Once we leave our tiny solar system, the space between stars, and galaxies of stars, becomes so enormous that astronomers express distance in light years instead of miles. It’s hard to wrap the mind around a number ending in 18 zeroes.

How far is a light year? Well, if you could hitch a ride on a wave of light, if you could go 186,000 miles per second – seven times around Earth in one second – it would take you 8½ minutes to reach our Sun and nine years to reach Sirius.

But in the scale of the cosmos, Sirius is our next-door neighbor. The main rectangle stars above Sirius in Orion – Betelgeuse, Bellatrix, Rigel and Saiph – range from 240 to 900 light years away. Hanging below Orion’s belt is M42, the Orion Nebula, at a distance of 1,350 light years.

But 1,350 light years is a piece of cake. You can see farther than the Orion Nebula – a lot farther. All the stars you can spot with your naked eye reside within our home galaxy, the pinwheel of between 200 and 400 billion stars we call the Milky Way. But there’s a naked-eye object out there that’s well beyond our galaxy. And that would be another galaxy.

Labeled M31, the Andromeda Galaxy floats in our autumn evening sky a staggering 2½ million light years away. Expressed in miles, that’s 12,900,000,000,000,000,000. What the heck, round it up to 13 quintillion miles. At that distance, the cumulative light of Andromeda’s trillion stars strikes your retinas with a few thousand photons per second – more than enough to flip the switch of your optical apparatus.

And more than enough to flip the switch of your imagination. When you finally resolve that gossamer oval, preferably through binoculars or a telescope, keep in mind that you’re not viewing Andromeda in the present; you’re viewing it as it was 2½ million years ago.

But why stop there?

How far you can see into the universe depends partly on the innate brightness, what astronomers call the “luminosity,” of the objects out there. From Brentwood, your naked eye might not be able to resolve a puny 15-watt light bulb atop Mt. Diablo, but it sure can resolve the 1,500-watt beacon on Diablo’s Summit. The strength of the light source, not the mere distance, matters too.

Which takes us well beyond Andromeda. The luminosity of stellar events such as supernovae or gamma ray bursts allows you to spot them with the naked eye from, as those in higher astronomical circles like to say, “a really really really long way away.” The afterglow of the gamma ray burst known as GRB 080319B, detected in the constellation Boötes by NASA’s Swift satellite on March 19, 2008, brightened to visual magnitudes between 5 and 6, bright enough to be spotted by the naked eye.

GRB 080319B’s distance? About 7.5 billion light years, halfway to the edge of the known universe – a universe unfurling at an astounding rate. By the time it takes you to finish this sentence, the universe will have expanded in volume by 100 trillion cubic light years. Period. Ready for the next 100 trillion? Here it comes.

The next time you squint at your optometrist’s Snellen chart and lament what’s become of the 20/20 vision of your youth, take heart. You might not be able to resolve that P in line 8, but there’s another object you can resolve.

“By the way, Doc. I stepped outside last night and saw something really far away.”

“Yah? How far?”

“Oh, about 13 quintillion miles,” you say with an air of scientific detachment.

“Riiight.”