I used to think Planet Earth was a pretty big place. That was before I gained a little perspective.
A few months ago, my husband and I were driving to a friend’s house. While looking up at a streaky afternoon sky, I was jabbering on about how far away the moon was, since it was already visible. (I had only recently started reading up on the subject, and the whole thing was freaking me out, quite frankly.)
When I mentioned how big our galaxy is, my husband paused for a moment, then proclaimed, “We are Who-ville!”
And that’s a very good way to wrap your mind around it. Remember the book Horton Hears a Who by Dr. Seuss? Our Earth (which seems so very, very huge to us), is equivalent to a speck on a clover being carried around by an elephant on top of something else that is the size of our Earth.
Except, actually, we’re much, much smaller than that.
Size is relative, of course. To a human standing at the edge of the Grand Canyon, the Grand Canyon is enormous. Ditto for someone at the foot of Mt. Everest, or someone flying over the Pacific Ocean. To our own personal scale, the earth is simply gigantic, and living here, it’s very easy to get myopic about the big picture. Since we have no personal context for the kind of distances involved in outer space, they are as difficult to process as the concept of eternity.
In his fan-freaking-tastic book A Short History of Nearly Everything, Bill Bryson (who should write all science books, ever, from here on out) illustrates why school textbooks weren’t able to show us anything remotely close to scale when it comes to distances in space. Stop to visualize this in your mind, as it goes along.
On a diagram of the solar system to scale, with Earth reduced to about the diameter of a pea, Jupiter would be over a thousand feet away and Pluto would be a mile and a half distant (and about the size of a bacterium, so you wouldn’t be able to see it anyway). On the same scale, Proxima Centauri, our nearest star, would be almost ten thousand miles away.
Here is the famous “Pale Blue Dot” photo, taken in 1990 by Voyager I (which at that point had been hurtling away from us for 13 years.) The photo shows Planet Earth as a tiny speck against the background of deep space.
When I first started delving into all of this, the reported distances in space sounded so completely absurd. How do they even know all of this? I wondered. Aren’t they just guessing?
Here is what I discovered.
A long time ago (in 1676, if you must know, and I must), it was determined that light did NOT move instantaneously (as it appeared to), but that it had a finite speed. Scientists spent the next 299 years narrowing that speed down, until they finally pinpointed it: light travels at 186,282 miles per second.
(I know: Sounds Ridiculous. That speed is impossible to wrap your mind around. Astonishingly enough, humans have been able to make matter move at very, very close to that speed, in the Large Hadron Collider – about which I’ve previously blogged, here.)
Anyway, once they had it pinned down, scientists started using the speed of light to gauge extreme distances in space. During the Apollo program, we planted retroreflectors on the moon and by aiming lasers at these, we were able to determine the distance to the moon, based on how long it took the light to come back (which was 2 1/2 seconds, so, around 237,674 miles.)
A “light year” is simply the distance that light could travel in a year, which comes out to 5879 billion miles (that bears writing out: 5,879,000,000,000 miles.) Thus, every time you see a single “light year” designated for space distances, it stands for nearly 6 trillion miles.
In A Short History of Nearly Everything, Bill Bryson explains the following: if you want to go just to the edge of our own solar system, and you board a rocket ship going 35,000 miles an hour (which is the speed of our Voyager 1 and 2 spacecrafts which, I must tell you, might get their own future post), it would take you a decade or so just to reach Pluto (the farthest used-to-be-a-planet in our system.)
And at those same speeds, (35,000 mph, remember), to get to the Oort cloud, which is at the edge of our solar system? Well, that would take you another 10,000 years.
(This sounds insane, but I checked the math, as best I could. Bryson’s right.)
After the Oort cloud, you reach the rest of our galaxy (the Milky Way), which contains every star you can see in the night sky. Of course, we can’t plant retroreflectors on stars, partly because most of them have a surface temperature of at least 5000 degrees, and partly because they are so freakishly far away.
To calculate distances to stars, scientists use the “parallax shift.” That’s the effect that you see if you hold your thumb up and look at it with one eye closed, then the other – objects in the distance appear to move, in relation to your thumb. Because of Earth’s revolution about the sun, scientists can measure how much the more distant stars appear to “move,” in relation to closer stars. This gives them approximate distances. (This method only works for objects that are up to 100 light-years from Earth. Beyond that, we have to use the brightness of stars, their motion, the space-time continuum, and a lot of other things that I don’t have room to go into, here.)
The closest star to us is Proxima Centauri – and it is 4.2 light years from Earth. (In other words, multiply 6 trillion by 4.2, and you’ll get the rough mileage.)
And now I’m out of time, and I’ve only gotten you to the edge of our own galaxy. And here’s the truly, truly insane thing. With instruments like the Hubble Telescope (which I blogged about here), we have been able to take pictures of not just our galaxy, but many, many others. As in, scientists estimate – are you sitting down? – 125 billion. Entire galaxies. That’s some of them pictured, above.
I will leave you with one final figure: the edge of the observable universe (the parts of the universe that we can see with our instruments, from Earth) is estimated to be 46.6 billion light years away. (Don’t bother trying to work out the mileage on that. Your calculator will explode.)