# How Long Would it Take to Walk a Light-Year?

Our experts tackle the big questions that keep you up at night

Foot Work Eduardo Schumacher

If you started just before the first dinosaurs appeared, you’d probably be finishing your hike just about now.

Here’s how it breaks down: One light-year—the distance light travels in one year, used as the yardstick for interstellar distances—is about 5.9 trillion miles. If you hoofed it at a moderate pace of 20 minutes a mile, it would take you 225 million years to complete your journey (not including stops for meals or the restroom). Even if you hitched a ride on NASA’s Mach 9.68 X-43A hypersonic scramjet, the fastest aircraft in the world, it would take about 95,000 years to cover the distance.

You’ll need to bring a big bag, too; walking such a distance requires substantial supplies. The average adult burns about 80 calories per mile walked, so you’d need two trillion PowerBars to fuel your trip. You’d also produce a heap of worn-out shoes. The typical pair of sneakers will last you 500 miles, so you’d burn through some 11.8 billion pairs of shoes. And all that effort wouldn’t get you very far, astronomically speaking: The closest star to the sun, Proxima Centauri, is 4.22 light-years away.

I thought this was a great little question and answer column! I hope this puts it more into perspective for some people who might not get the distances involved in interstellar travel. I had a 30 year old friend ask me once how the space shuttle 'dodges' stars on its way into orbit. Please tell me high school is better now!

I love photos from space, and used to think about the view when I walked by the 8x15' map on the wall at work. Bored, I calculated how high my view would be from a scale 3' away. I later taped a piece of paper showing how high various things were, to scale. I think the shuttle (at 200mi) was 1.4 inch high. The moon would have been in the parking lot.

I love little factoids like this.
===========
If a globe is 12” dia…- 12” =8000 mi diameter-- 1”=667 mi
What ---- How high ------ Above globe surface
breathable air limit ~3 mile ( 12-14,000’) = .004” (sounds rather fragile!)
space begins= 100 mi ( 80-100 mi) = .15”
space shuttle= 250 mi avg (190-350 mi)= .37”
geosynchronous satellite== 22,000 mi = 33”
moon= 250,000 mi = 31 Feet ( =375”)

On a related and equally fun subject, how fast are you going while you sleep in your bed tonight (and dream of walking a light-year)?

- The Earth is 25,000 miles around and it takes about a day to go once around so its rotational velocity is 25,000 / 24. That means things on the Earth's surface near its equator are rotating about 1,000 mph.

- The Earth's orbital velocity around the Sun is about 67,000 mph.

- Our sun rotates around the center of the Milky Way galaxy at about 490,000 mph. Even at that speed it still takes something like 250 million years to go once around the center - a cosmic year.

- Our Milky Way galaxy is moving through the universe in the direction of the constellation Hydra at about 1,340,000 mph.

- (Maybe the new atom smasher will someday tell us that our universe is moving through something else at such a such speed.)

So, adding all those up, you are moving not quite 2 million miles per hour while you sleep, or about .3% the speed of light.

Three follow-up bonus questions. How much time dilation is that? How fast are you really going if you drive East going 60 mph? What happens if you roll the window down and stick your head out?

### June 2013: American Energy Independence

Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.

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