By Jim Kaler

Constellations reflect the times. And the first people to invent them skim the best stars. Most of the classical constellations from ancient days are made of bright patterns that represent heroes and familiar animals. Look at Orion, the Hunter, composed of some of the brightest stars of the sky, or at Ursa Major, the Great Bear, which is centered on one of the most beloved figures of the heavens, the Plough or Dipper, depending upon which side of the puddle you live. (Of course there is spring's dim Crater, the Cup, which belies these rules, but it is a distinct exception, and besides, it really does look like a Cup.)

By the time of the next round of constellation-making, in the seventeenth and eighteenth centuries, the bright patterns of the northern hemisphere had been used up. The vast deep southern hemisphere (which the Greeks and their predecessors could not see) was loaded with them, however, and there were still plenty of faint star patterns that the ancients had ignored, all waiting to be organized and named. The culture had also changed. While animals abound in this set of "modern" constellations, we now see a variety of industrial artifacts, among them three similar naval navigational devices (used to determine angles in the sky so as to find terrestrial location): the quadrant, octant, and sextant, in the sky honored as Quadrans, Sextans, and Octans. They are remarkably well distributed: Quadrans lies in the far northern hemisphere, Sextans straddles the equator, and Octans surrounds the southern rotation pole.

Though made of dim stars, Sextans and Octans are reasonably familiar. You will look in vain for Quadrans, however, as it was long ago officially dropped, and thus appears on no modern star chart. No matter, it still exists if you have a sky dark enough to find it. Locate spring's Boötes with brilliant Arcturus. The Herdsman's hand extends northward to a long thin triangle just to the east of the Great Bear's tail. Further east by a bit over an hour of time is a set of faint fifth and sixth magnitude stars of no particular form that in the latter 1600s still somehow reminded French astronomer Joseph Lalande (1732-1807) of a quadrant. Its best-known star is obscure BP Bootis, an "RS Canum Venaticorum" variable. Such stars are made of close pairs whose rotations have been synchronized to their short-period orbits, and thereby exhibit strong solar-type magnetic behavior that includes notable starspots.

While among the departed (many mercifully so) constellations, the ghost of Quadrans survives through a meteor shower that was discovered (in 1825) emanating from it before its official demise, the marvelous Quadrantids. One of the best of the regular showers of the year, it can produce up to 200 meteors per hour, comparable to if not better than the much more famous Perseids. Its lack of popularity is in its timing, as the shower peaks around January 3 (near Earth's orbital perihelion, though that has nothing to do with it), and it's hard to get people to lie outdoors on their backs for hours in the snow.

Meteor showers are caused by bits of fluffy rock that break off comets when they travel close enough to the Sun for their icy matrices to be evaporated. The little meteoroids travel in the comet's path, and if Earth passes close to it, we can be hit with the debris, which burns up high in the air. The meteors' parent body is identified by matching the particles' collective orbit (found by tracing the resulting meteors backwards) with that of a comet. Halley's, for example, produces May's Eta Aquarids and October's Orionids. The originator of the Quadrantids was only recently identified with a near-Earth "asteroid" called 2003 EH1, which is not an asteroid at all, but a now-boiled-out dead comet.

Moving southward, we come to Sextans, invented much earlier by Johannes Hevelius (1611-1687). The luminary, such as it is, is a white class A faint-fourth-magnitude giant 287 light years away. Consistent with its placement within a navigational instrument, this ordinary star, Alpha Sextantis, sits but half a degree south of the celestial equator almost directly south of Leo's Regulus. Precession, the 26,000-year wobble of the Earth's axis, is slowly moving this part of the equator to the north relative to the stars, and therefore makes the star appear to move to the south. A century ago Alpha Sextantis was NORTH of the equator, crossing over near the beginning of 1924.

Sextans is perhaps best known for a wildly contrasting pair of galaxies, one very bright, the other amazingly faint. NGC 3115, discovered by William Herschel, is so obvious that it even has a special name, the "Spindle Galaxy" (which it shares with others). The ninth magnitude Spindle, 27 million light years distant, is a class S0 disk galaxy seen edge-on that has no spiral arms. It was once thought to harbor a supermassive black hole of a billion solar masses that further examination reduced to a mere two million solar, roughly equal to the one that resides at the center of our Milky Way system. The constellation is also home to three small members of the Local Group of galaxies. By far the faintest is the "Sextans Dwarf." Only 280,000 light years away, sprawling over a degree across, this "dwarf spheroidal" galaxy is barely detectable against the night sky. In integrated light it is fainter and less massive than the biggest of our own Galaxy's globular clusters, and greatly under-studied. Such systems are important in learning about star formation and the history of galaxy encounters, in which galaxies collide, merge, and tear tidal pieces from one another.

Now plunge far down to the South Celestial Pole and to the last of our tools, Octans, the Octant (a product of the mind of the Abbé Nicolas de La Caille, 1713-1762), where logic deserts the star names. The Alpha star is oddly only sixth brightest, being beaten out (in brightening order) by Gamma, Theta, Delta, Beta, and, at the top of the pile, of all things, fourth magnitude Nu Octantis, an interesting class K giant 69 light years away. With close to a solar mass, it is in the act of brightening (over the next 100 million years) with a dead helium core. The best known star (or best known of about anything in the constellation) is nearly-sixth magnitude Sigma Octantis, a class F giant 270 light years off. Along with much brighter Polaris (coincidentally a class F supergiant), Sigma pins the sky's apparent rotation axis, Polaris at the northern end, Sigma at the southern, from which comes its little-used proper name, "Polaris Australis." Faint as it is, Sigma, only a bit over a degree off the pole, is not a bad pole star and (because of precession) just a century ago was better at it than Polaris was (at that time) in the north.

Dim and mostly ignored the three may be. But lose your way, and from almost anywhere on Earth you can find one of these venerable devices to lead you home.

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 May/Augusg 2005 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.