No cigar, no lady on his arm, just a guy made of dots and lines

Next time you see a constellation ride across the sky, try to picture to yourself just how far away those stars are. It's humbling, and does a good job of giving life some perspective. But how far away are they, you ask...how can we be so sure? It turns out that this kind of measurement forms the cornerstone of modern astronomy, and the technique to do it has been known for centuries. It was used to determine the size of the solar system itself in 1672, and first turned to distant stars in 1838. The method is parallax, and it can best be tried at home if you have a finger and a distant object. Close one eye, and take your finger and hold it in front of the distant object. Keeping the finger still, Open the closed eye and close the open one. Note the position of the finger, which should have moved. If you can measure the difference in angle between the two places you saw your finger, you have measured the distance to it, assuming you know how far apart your eyes are. A good picture of the process can be found at Wikipedia, or by searching for "parallax".

Anyway, this technique was used in1672 to find out the distance to Mars, which in turn allowed us to figure out the size of the entire solar system. You will notice that this discovery had to wait for the era of colonization, since it required Giovanni Cassini to send a colleague to French Guiana in the Americas while he stayed in Paris. This is also one of the many, many examples of the advantages of seniority in the sciences, as it will shock you to no end to learn that 17th century Paris was considered a more comfortable place to reside than a boat to South America. In any case, the measurement established the distance to Mars, and in doing so the distance to the sun as 140 million kilometers, or 86 million miles. It turns out that it is actually 150 million kilometers (93 million miles), but what's 7 million miles between friends. For comparison's sake, the moon is 239,000 miles away, or about 1/400th the distance.

It would prove to be over a century and a half before the distance to the next nearest stars were measured. In 1838, Friedrich Bessel measured the distance to the star 61 Cygni, finding it to be 11.4 light years away, or 3.5 parsecs (or about 60 trillion miles; nearly one million times further than the sun). Parsecs, as the name suggest, are the distance at which a star has a parallax angle of 1 second of arc. A star 3.5 parsecs away has a parallax angle of 1/3.5=0.28 arcseconds, or about 1/100,000 of a degree! The same year, two other astronomers measured the distances to the stars Vega and Alpha Centauri, the latter of which is the closest star to Earth besides the sun, located at about 1.3 parsecs, or 4.4 light years away. In all of these cases, the star whose distance was being measured was considered "close", like your finger in the experiment above. The "distant" stars are actually much farther away: the star Deneb, among the 25 brightest in the sky, is over a thousand parsecs away, and the furthest visible to the naked eye are a couple times further than that. With telescopes, we can see bright stars located most of the way across the galaxy, but more on that next week.

Today, we can measure the distance through parallax to hundreds of thousands of stars, of which we know the distance to about 7,000 to within 5 percent thanks to the Hipparcos satellite. This may seem like a lot, but it represents only our local neighborhood in the galaxy. Still, these distance determinations are critical, since they provide us with unambiguous measurements that span light years. Next week, we'll extend our reach a bit further extending outward to cover the entire galaxy.

Most of what I wrote here can be investigated further by going to wikipedia and following the links there. For basic science, it really is an invaluable tool. Give it a whirl!