How far away are the 100 closest stars to the Earth? Or the 100 closest galaxies? How about the size of the galaxy in which we live? Or the extent of the entire Universe? How would you even go about answering that question?
How do we know that the nearest star to our Sun is 4.2 light-years away? It’s not as though we can stretch out a long tape measure between here and Proxima Centauri. Barring drastic advances in space travel, if we’re going to figure out the distances to the stars, we’re limited to measuring them while not leaving the Solar System.
Let’s try an experiment. Hold up a finger several inches in front of your face and focus on something across the room. Now close one eye and notice where your finger appears to be. Without moving your head or your hand, close that eye and open the other one. Notice anything different about your finger? It appeared to move! If you rapidly alternate which eye is open you’ll see your finger appear to dance back and forth in front of your face. Every time you switch eyes, you’re seeing your finger from a different angle. To your left eye, the finger is off to its right while the opposite is true for your right eye. Now, continue to alternate eyes while moving your finger away from your face. You’ll notice that your finger won’t bounce back and forth as much when it’s further away. Now try it with an object on the other side of the room. The more distant the object, the less it moves as you alternate eyes.
This effect is known as parallax and it’s one of several ways our brains interpret distance. Having two eyes lets us view everything from two different angles. By comparing how objects appear in both our eyes, our brain can figure out whether an object is close or far. Successfully picking up a glass without missing would be a lot harder if you only had one eye.
But we can do a lot better than just saying whether an object looks close or far. We can actually calculate precisely how far it is. If you know how far apart your eyes are and you measure the angle over which your finger appears to move, you can construct a triangle. The base of the triangle is the distance between your eyes and the opposite angle is the one you just measured.
Remember in high school trigonometry when you wondered what use all of it was? Well, here you go: you can use those two numbers, and a simple equation, to calculate the height of the triangle which, in this case, is the distance to the object.
What ultimately limits us is how far apart our two viewing angles are. Try the alternating eye trick with a far away flag pole or even a cloud and you’ll note that your eyes don’t see those things much differently. You need to move your eyes farther apart for that to work. Since that sounds like an unpleasant solution, what if we just stand at two different locations? This is the whole basis of triangulation and is something surveyors have been using for a very, very long time.
If you pick two locations that are sufficiently far apart, you can start to measure some seriously large distances. By standing on opposite sides of the Earth, you can measure the distances to the Moon, Sun, and planets. But even that’s not enough to measure the distances to the stars. Instead, we let the Earth do our work for us by taking us around the Sun! As the Earth travels on its orbit, we constantly see the stars at slightly different angles. If we take a picture of a field of stars and then wait six months to take another, we see those stars from two locations which are as far apart as the Earth’s orbit is wide – roughly 200 million miles!
Interestingly enough, this was for a long time used as an argument against the idea that the Earth went around the Sun. Ancient Greek scholars knew that if the Earth moved, they should see the stars shifting back and forth. They didn’t, so clearly the Earth was stationary. It didn’t occur to them just how far away the stars actually were! It wasn’t until 1838 that German astronomer Friedrich Bessel successfully made the first stellar parallax measurement and deduced the distance to the star 61 Cygni as roughly 10 light-years. This is no easy task. Even for the closest star to our Sun, measuring the parallax angle is equivalent to measuring the width of a human hair from roughly 70 feet away!
By measuring stellar parallaxes, astronomers have mapped out the distances to hundreds of thousands of the closest stars. To get really accurate measurements, you need to get above the turbulence of the Earth’s atmosphere. This led the European Space Agency (ESA) in 1989 to launch the Hipparcos satellite and map stars out to 1600 light-years away. But even that is a small percentage of our entire Galaxy. To probe deeper, ESA will launch the Gaia satellite in late 2012. Gaia will provide an unprecedented galactic census of some one billion stars and let astronomers map their positions right to the core of the Milky Way Galaxy.
The power of parallax measurements, however, ends pretty much at our Galaxy’s edge. Beyond that, the angles become much too small for even the most sophisticated instruments to measure. To determine galactic distances, and beyond, astronomers have a whole host of other tricks up their sleeves. But they all depend upon the parallax measurements of the closest stars for calibration.
Amazingly, the journey to the edge of the Universe starts by measuring the tiniest of angles right here at home.