One of the less well-known constellations that inhabit the far northern sky is Draco, the dragon. The tip of the dragon's tail lies between the Little Dipper and the "pointer" stars at the end of the Big Dipper's bowl; the tail then curves eastward between the two dippers, bends a bit toward the star Polaris, then curves back on itself to end with the dragon's head, marked by a quadrilateral of four moderately bright stars near the brilliant star Vega, now high in our northern sky. Several old cultures apparently saw the shape of a dragon in these stars; the ancient Babylonians saw these as representing the dragon Tiamat, who fought the god Marduk for control of Heaven, whereas the Greeks saw the dragon Ladon, who guarded the golden apples of Hesperides and who the hero Hercules slew during the course of fulfilling one of his twelve labors.

The brightest star in Draco is the star known as Gamma Draconis (sometimes called Eltanin), which marks the southeastern corner of the dragon's head. This appears to be a relatively large star, or "sub-giant," and it is located approximately 100 light-years from our solar system. While there is nothing especially unusual about Gamma Draconis from a scientific standpoint, it nevertheless has played a rather notable role in our quest to understand the surrounding universe.

Over two thousand years ago the Greek scientist and philosopher Aristotle pointed out that, if the Earth were moving around the sun, then we should be seeing a reflex motion of this, or "parallax," in the stars. (Since this motion was not observed, Aristotle used this point to argue for an Earth-centered universe.) Many centuries later, after scientists like Nicolaus Copernicus and Galileo Galilei had shown that the sun is indeed the center of the solar system and that the Earth and other planets must orbit around it, astronomers concluded that the stars are too far away for this parallax motion to be obvious, but that it might be detected if careful observations could be made.

One of the first scientists to attempt this observation was the British astronomer James Bradley, who tried this in 1728. Bradley selected Gamma Draconis, since it passed directly overhead from the latitude of the Royal Observatory in Greenwich, and by means of a special telescope mounted through a chimney made careful measurements of this star's position over the course of a year. Somewhat to his surprise, Bradley found that the star's position was marking out a rather large circle – significantly larger than expected -- and, moreover, he discovered that all the other stars were also marking out similar circles in the sky.

According to an apocryphal story that may or may not be true, Bradley discovered the solution to this problem one day while sailing on the Thames River, when he noticed that the apparent direction of the wind changed when his boat changed direction. Since light travels at a finite velocity -- which had been determined half a century earlier by the Danish astronomer Olaus Romer -- one has to "look ahead" of a star's position in order to view the light coming from it, and since our -- i.e., the Earth's -- direction of motion changes during the course of a year, the direction we have to look toward changes accordingly. The effect is analogous to trying to collect vertically-falling rain through a pipe while traveling; one has to tilt the pipe forward in order for the rain to fall vertically through it.

This phenomenon is now called the "aberration of starlight," and Bradley's observations offered the first conclusive demonstration that the Earth is indeed in orbital motion around the sun. Somewhat ironically, this was not the parallax motion that Bradley was looking for; that effect -- which is much, much smaller than the apparent motion due to aberration -- would not be detected until over a century later. It is parallax, incidentally, that allows us to compute the distances to other stars.

The phenomenon of aberration of starlight is thus completely intertwined with the speed of light. Some two decades ago astronomer Walt Sanders at New Mexico State University, together with a couple of graduate students, took a series of photographs of quasars -- extremely brilliant nuclei of very distant galaxies -- over the course of a year, and measured the positions of these against the positions of stars that appeared on the same photographs. The quasars are located so far away that their light takes billions of years to travel to Earth; the light from the stars, meanwhile, takes at most a few thousand years. Sanders' team determined that there was no change in the positions of these objects relative to each other over the course of a year, meaning that the aberration phenomenon was the same for both types of objects; in other words, the speed of light appears to have remained constant over the entire age of the universe (or, at least, for the past several billions of years).

Gamma Draconis, a rather nondescript star in a constellation that represents figures from some of humanity's oldest surviving legends, thus -- almost entirely coincidentally -- ends up becoming an object that has challenged, and that has helped verify, some of our most basic assumptions of how the universe operates. Such is often the nature of science, and this shows that we should always be open to the knowledge that nature gives to us.