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Where is the Northern Coalsack?

June 2012: Pinpointing this dark nebula can prove exceptionally difficult.
stephen_james_o_meara_new
I recently got an email from Steve Bookout of Newton, Iowa, a confessed “dark nebula junkie.” He found many published positions for the Northern Coalsack, a dark nebula in Cygnus the Swan, which has been driving him “bonkers.” He wondered if I might help him and other Astronomy readers find the real Northern Coalsack.

Conflicting coalsacks
Admittedly, I too was once confused over the nebula’s whereabouts. The first star chart I saw it on as a child depicted the Northern Coalsack as a tiny “hole” in the Milky Way, a few degrees west-northwest of Deneb (Alpha [α] Cygni); I never did see that one. Many years later, to my chagrin, someone told me that must have been a mistake because the Northern Coalsack is a 5°-wide hole in the Milky Way north-northeast of Deneb, close to the Cepheus border. And, yes, I could definitely see that void under a dark sky with my unaided eye.
Digitally-removing-stars
Digitally removing the stars from the night sky reveals the dust patterns in the Milky Way more clearly, especially the official Northern Coalsack — between Alpha (α), Gamma (γ), and Epsilon (ε) Cygni — as well as the other large "coalsack," Le Gentil 3, nearly 10° north-northeast of Alpha Cygni. Stephen James O’Meara
Later, however, I noticed that in H. A. Rey’s 1952 book, The Stars: A New Way to See Them, the author depicted two large coalsacks in Cygnus: one just southeast of Deneb, and another between Sadr (Gamma [γ] Cygni) and Epsilon (ε) Cygni. “The dark blotches in the Milky Way near Deneb,” Rey explained, “are not holes but tremendous clouds of cosmic dust, coal sacks so called, hiding the stars behind them.” He didn’t mention the other one north-northeast of Deneb. My search had led me back to where I started: confused!

Pin the tail
Generally, all of these objects are “coalsacks.” Brave mariners sailing the southern seas centuries ago applied that name to any dark vacancy in the Milky Way they saw with their naked eye. Italian historian Peter Martyr d’Anghiera (1457–1526) first formally described the dark void near the Southern Cross as a coalsack. But what about the Northern Coalsack?
Milky-Way-is-littered
The Milky Way is littered with dark obscuring clouds of dust commonly referred to as "coalsacks." Of special note is the Northern Coalsack in Cygnus. Chris Schur
English astronomer Sir William Herschel (1738–1822) may have been the first to catalog this “hole in the heavens,” as he put it. It became a commonly known feature by the late 19th century, as British author Thomas Hardy alluded to it in his 1882 novel, Two on a Tower; in it, Swithin St. Cleeve points out the bleak region to his lover, Lady Constantine: “You see that dark opening in [the Milky Way] near the Swan? There is a still more remarkable one south of the equator, called the Coal Sack, as a sort of nickname that has a farcical force from its very inadequacy.”
 
But where in the Swan did Swithin see it?
 
Garrett P. Serviss told us clearly in his 1888 Astronomy with an Opera-Glass: “Between the stars α, γ, ε,” he explained, “is the strange dark gap in the galaxy called the Coal-Sack, a sort of hole in the starry heavens.” Richard Hinckley Allen confirmed this position in his 1899 Star-Names and Their Meanings; he then went on to formally call this “almost vacant space” the “Northern Coal-sack.”
 
Alas, Serviss muddied the waters a little in his 1910 book, Round the Year with the Stars, when he said, “Around and above the head of the cross there are dark spaces, which are specially impressive when the eyes are partly averted from them.”
 
As always, send your thoughts and comments to someara@interpac.net.
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