Entries that are the same object twice, or nothing at all.
When the catalogue entry is wrong, missing, or already there
The NGC contains nearly eight thousand entries. A handful of them are the same object listed twice. A few are the sky position where something was once reported and nothing has ever been found since.
These are the catalogue's soft spots — not errors of measurement so much as errors of circumstance. John Louis Emil Dreyer compiled the New General Catalogue from the notebooks of dozens of observers working across two centuries, in different conditions, with instruments of wildly unequal quality. That anything slipped through twice, or got recorded at all when the seeing was appalling and the comet-hunter was tired, is less surprising than the fact that the error rate is as low as it is.
A duplicate enters the record when the same object is observed independently and neither observer knows the other has already logged it. The two positions are close but not identical — atmospheric refraction, an imprecise clock, a slightly bent declination circle — so Dreyer, working from paper catalogues rather than being able to pull up a finder chart, treats them as two objects. The result is a pair of entries that resolve to the same smear of light. Across the catalogue there are only a handful of such pairings, some obvious on inspection, others requiring careful cross-matching.
A duplicate enters the record when the same object is observed independently and neither observer knows the other has already logged it.
Ghosts are stranger. A ghost entry is a position in the sky where observers have failed, repeatedly and methodically, to find anything at all. Some arose from transcription errors: a sign flipped in a declination, an hour-angle misread, and the real object sits somewhere else entirely while the catalogue position stares into a blank field. Others may trace to genuine atmospheric artefacts — a bright star just out of the field scattering light across the eyepiece in a way that looked, for a moment, like a faint nebula. The observer noted it; Dreyer trusted observers; the entry stood.
The task of auditing these fell largely to a community of dedicated visual observers in the twentieth century, most systematically gathered under what became the NGC/IC Project, a long-running effort to re-examine every entry with modern equipment and coordinates. Some ghosts were resolved: the "missing" object turned up once the position was corrected. Others remain genuinely nonexistent.
For the visual observer this matters practically. Chase a ghost long enough on a good night and doubt sets in — not about the catalogue, but about yourself. Knowing that a handful of NGC numbers are simply wrong, or point elsewhere, converts frustration into information. The smudge you cannot find may not be you.