By Jim Kaler

Long streams of stars attract the eye. Among the most famed constellations made of them are Eridanus, the River, which ends far down in the southern hemisphere in blue Achernar, and the longest, Hydra, the Water Serpent, which wraps its slithery body a third of the way around the sky with only its head sticking up into the northern celestial hemisphere as if from a deep southern ocean. To see a whole such figure in a single glance, turn your eyes to the far north, where shy Draco the Dragon, hides between the Bears and Hercules, waiting to be dragged out. It could almost be called the English Constellation, so rich is its historical connection with the land, and is among the central figures in the story of the Earth's motions.

Every skygazer is familiar with the celestial poles, which lie above the terrestrial rotation poles and about which the sky seems to turn. The northern one is beautifully marked by Polaris, which serves as the ends of both the Little Dipper's handle and the tail of Ursa Minor, the Smaller Bear. But there are other poles of equal importance. As the Earth rounds the Sun, it -- and its apparent solar motion against the stars -- carves out the great circle of the Ecliptic, which defines the dozen constellations of the Zodiac. The perpendicular to it goes to the north and south ecliptic poles. Since the Earth's rotational axis is tilted by 23.4 degrees to the orbital axis, the ecliptic poles lie 23.4 degrees from the celestial ones. Out of sight for northerners, the southern one (the SEP) falls in Dorado not far from the Large Magellanic Cloud. The North Ecliptic Pole (NEP), however, circumpolar from everywhere north of the Tropic of Cancer, is eminently visible. Admire it in Draco wrapped within the Dragon's eastern curl midway between Zeta and Delta Draconis.

Why should we care? Because the ecliptic poles are central to the location of the celestial ones. As the Earth spins, it throws itself slightly outward at its equator, the equatorial diameter about 40 kilometers greater than the polar. The Sun and Moon, riding at and near the tilted ecliptic path, pull on the bulge, which because of the Earth's spin, causes the rotational axis to wobble, or precess, like a top. As a result, the celestial poles slowly move in a circle around the ecliptic poles with a radius of 23.4 degrees, taking 25,800 years to make the journey. The motion has been known for more than 2000 years since Hipparchus of Nicaea discovered it in the second century BC.

But from our perspective, it's the sky that seems to be moving. Polaris, now about 3/4 of a degree off the pole, will keep getting better until around the year 2100, when it will be just a quarter of a degree away. Over the long haul, though, Polaris is a temporary marker. Thirteen thousand years ago, it was 47 degrees away from the pole, passing nearly overhead at the latitude of New York! Looking back into the past along the circle of precession, among the most famed of ex- pole stars is Thuban, Alpha Draconis, which almost perfectly marked the rotation pole during the time of ancient Egypt of 2800 BC. Just fourth magnitude (3.65), this white class A0 giant, 300 light years away, is easily found between Ursa Major's Mizar/Alcor pair and Gamma Ursae Minoris in the Little Dipper's bowl.

Thuban's Alpha status comes not from brightness, but from location. Draco's luminary is actually Eltanin, Gamma Draconis, a second magnitude (2.23) orange K5 giant that shines within the Dragon's lopsided head. The star's position 51.5 degrees north of the celestial equator took its daily path directly overhead as seen from London, and gave it the popular name "Zenith Star." To the English astronomer James Bradley, it was the perfect star for the first measure of parallax (hence the star's distance), since when it slid perfectly above, it was unaffected by atmospheric refraction, which raises stars above their true positions.

As the Dragon dragged itself across the Zenith, Eltanin then became something of a paradigm for how science often works: you go out to find one thing and accidentally stumble over another. Instead of measuring parallax, in 1728 Bradley announced the discovery of a much larger shift caused by the "aberration of light." Driving into the rain on a windless day makes the drops seem to come from a point in front of you; the faster you go, the lower the point gets. Light behaves the same way. Gamma Draconis's position was constantly being shifted 20.5 seconds of arc in front of the zenith in the direction of the Earth's motion. The aberrational shift was not only the first actual proof of Copernican theory, but allowed for a measure of the speed of light! The phenomenon must be taken into account in any measure of star positions.

But Bradley was not yet done. Further observation revealed a smaller 17 X 9 second of arc wobble with an 18.6 year period called the "nutation." The lunar orbit, tilted by 5 degrees to the ecliptic plane, precesses as well, which causes the orbital "nodes" (the points where the Moon crosses the ecliptic) to go around through the Zodiac, taking the 18.6 years for a full circuit. (Nodal position is crucial in the calculation of eclipses, which can take place only when the Moon is near one of them.) Nutation is the result of a slight variation in the sum of the gravitational torques induced by the Moon and Sun on the bulge. Over 100 more subtle nutational terms, all caused by irregularities in the lunar and terrestrial motion, are known.

While the Dragon is not famed for many celestial showpieces, a few do stand out. Try, for example, fourth magnitude (4.13) Nu Draconis, the faintest of the stars of Draco's head, a striking double made of nearly identical white class A dwarfs just over a minute of arc apart and that can be split with binoculars. Not quite 100 light years away, the eastern member is a spectroscopic binary, making Nu Dra a triple system. Well away from the dust of the Milky Way, Draco also breathes fire into several fine telescopic galaxies that include the lovely spiral NGC 5985 (among a small group some 100 million light years away) and, at about half that distance, edge-on NGC 5907, the "Knife-Edge Galaxy."

But for the best of Draco's sights, go local. In 1785, William Herschel announced the discovery of the first of his "planetary nebulae," NGC 7009 (the Saturn Nebula), the generic term coming from its disk-like appearance. (The Ring Nebula in Lyra was already known, but was not put into that category until later.) Among his planetaries was Draco's spectacular northern nebula, NGC 6543. Roughly 3000 light years away, and better known now as the "Cat's Eye," it has a profound place in nebular history. In 1786, Herschel wrote that he noticed a bright spot in the center. Four years later he reported it as a central star, making it the first known planetary nebula nucleus, which was "involved in a shining fluid, of a nature unknown to us." Many decades later, in 1864, Sir William Huggins solved the riddle. Looking through his spectroscope at the nebula, he found not the absorption lines that appear in the spectra of stars, but emissions, which through the work of such scientific giants as Gustav Kirchhoff proved that these nebulae were made not of stars, but of radiant gases.

We know now that the planetaries are the ejecta of giant stars that are lit by the glowing coals of the remaining stellar cores. Too far north to be observable by the restrictions of the 100-inch telescope at Mt. Wilson, in later years the Cat's Eye was among the first known to be surrounded by a giant halo of earlier leavings. In our own time, it was the first of its kind to be observed by the Hubble Space Telescope. Paraphrasing his words, one observer noted that if he'd known they were this complex, he would have gone into another line of work. We still do not know the origins of the nebular structures. Given this marvelous jewel, maybe we can still (if you will excuse one last time) drag it out of the Dragon. To complete the circle, the Cat's Eye is almost dead on the North Ecliptic Pole, a mere 10 minutes of arc away, making it the only "Pole Nebula," which allows you to admire both of them at the same time!

Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in the August/December 2010 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.