As far as the Eye Can See

How far can we see? Most people would say just a few miles, but they're looking in the wrong direction. Every clear day we can see the sun, and that's about a hundred million miles away. The planet Saturn, rising about midnight, is over a billion miles away. But much further away than the planets are the stars.

The nearest star is four light years away, as far as light itself can travel in four years. Light travels about six trillion miles each year, so the nearest star is over twenty-four trillion miles away. The brightest star in the nighttime sky, Sirius, is over eight light years away, over fifty trillion miles. Other stars are hundreds of trillion miles away. But we can see even further than that.

Our own galaxy, the Milky Way, has many galactic neighbors. Its nearest neighbor, the Andromeda galaxy, is easy to find, and looks like a dim star to the unaided eye and like a ghostly, glowing cloud of light when seen through binoculars. It is over two million light years away. That's twelve million trillion miles, the furthest thing we can see with our unaided eyes.

To find the Andromeda galaxy, start at the Great Square of Pegasus, an almost perfect square of stars rising in the east at about 10:00 P.M. From the upper left star of this square, move two stars to the left, to a bright star in the constellation of Andromeda. From that, move two stars up. That's it. That dim "star" you just found is the Andromeda galaxy. In binoculars its glow is bigger than six full moons, so it's easy to find. People were still living in caves when the light we see tonight left this galaxy. So when someone asks you how far you can see, say, "Oh, about twelve million trillion miles". And you can show him where.

(8/13/97)