How far can you see? The answer might surprise you.

You 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.

But we’re just warming up.

We East County folk are graced with a magnificent long-distance object on a regular basis: Mt. Diablo, about 10 miles from downtown Brentwood. Let’s use the mountain as a point of reference.

Even in relatively flat East County, 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 enchanting pageant of our moon sinking into the west behind Mt. Diablo. Our moon: 24,000 times more distant than our mountain – though not nearly as impressive as our sun: 9 million times the distance of the mountain.

Our next step takes us into interstellar space. The nearest bright star in our autumn sky, found southeast of the constellation Orion in Canis Major, is the glinting diamond we call Sirius, a whopping nine light years farther than the mountain. Now, if nine light years doesn’t sound impressively remote, we need to back up a bit.

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 emptiness between stars, and galaxies of stars, becomes so enormous that astronomers describe distance in light years instead of miles. It’s hard to wrap the mind around a number ending in 18 zeroes.

So 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 about 8½ minutes to reach the 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 1,350 light years from Mt. Diablo.

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 (for those keeping score at home), the Andromeda Galaxy hangs in our autumn evening sky a staggering 2.3 million light years beyond Mt. Diablo. That’s right: beneath a clear, dark sky and equipped with normal vision, you can spot our next-door galactic neighbor at a mind-numbing distance of more than 12,900,000,000,000,000,000 miles. What the heck – round it up to 13 quintillion miles.

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 Antioch, your naked eye might not be able to resolve a puny 15-watt light bulb atop Mt. Diablo, but it sure can resolve the searing beacons on Diablo’s peaks. 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 we 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, reached 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 happened to 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.”