Numbering
The catalogue numbers observers reach for every night were never meant to be a finding system — they are timestamps in disguise.
Why the numbers are not in any useful order.
Why the sequence makes no sense
Photo: Colon Freld / Pexels
Charles Messier logged his objects in the order he encountered them. M1, the Crab Nebula, came first because he stumbled on it in 1758 while hunting a comet near Taurus. M2 is a globular cluster in Aquarius, nowhere near Taurus. M3 is a globular in Canes Venatici. The sky has not been sorted; a notebook has been transcribed. The numbers are a diary of one man's comet searches, not a map of the heavens.
The same logic — if that is the word — governs the New General Catalogue. John Dreyer compiled the NGC from earlier observers' lists, folding in John Herschel's General Catalogue and a mass of subsequent discoveries. The numbering runs roughly in order of right ascension, which gives it a weak seasonal logic, but only roughly: revisions, additions and the merging of separate lists left the sequence full of local jumps and clusters of entries that belong to entirely different parts of the sky.
The result is that NGC 1 and NGC 2 are a pair of interacting galaxies in Pegasus, while NGC 3 is also in Pegasus but NGC 7 is in Sculptor. Nearby on the page; nowhere near each other overhead.
The same logic — if that is the word — governs the New General Catalogue.
Observers sometimes expect a number to predict difficulty or size. It predicts neither. M57, the Ring Nebula, is a compact planetary nebula that rewards high magnification. M101, higher in number, is a vast face-on galaxy that punishes it. The number is simply a name — arbitrary, historical, and stuck fast by two and a half centuries of habit.
Once you accept that the catalogues are records of discovery rather than indexes of the sky, the disorder stops being frustrating. Every mismatched sequence is a trace of the actual night someone first wrote the object down.