You don’t take an interest in astronomy for long before you stumble across Messier and his catalogue. He produced it as a public service to astronomers, identifying all the fuzzy blobs in the sky that were permanent, and hence could be disregarded if you were on the search for something (say a new comet). Most of what he picked up were nebulae – or clouds – of one sort or another.
But he was based in Paris, and thus only had a limited amount of sky he could cover. He couldn’t see right down to the south celestial pole, and his M-numbers missed such things as our two nearest galactic neighbours, the Magellanic Clouds. A comparable catalogue for the southern sky was needed.
And so we have C-numbers, named after Patrick Moore, the noted British TV astronomer. There’s an oddity I’m sure you’ve spotted – Moore doesn’t begin with C. As it happens his full surname is Caldwell Moore, which thus gets around the problem of having two catalogues beginning with M. (We’ll leave the fact that he was based in Sussex and could thus see even less of the southern sky than Messier for another day.)
Anyway, here we have a nice structure – one catalogue broadly for the top half of the celestial sphere, the other for the bottom half. It maps nicely on to a grid. Many of the nebulae have popular names or nicknames often from fanciful interpretations of their shape. So we also have a list of thematic words, some for the top half of the grid, some for the bottom.
Looking further, the lack of astronomical imagination means that several names are duplicated in the catalogues. If we choose three with odd lengths we can have them straddling the middle row of a grid, and hence either side of the ecliptic.
Now we could just clue them (wordplay only?) but it seems a shame not to use the catalogue numbers in some way. I’ve even managed to choose a Messier entry that shares its number with a Caldwell entry with a different name. What have we got that would introduce the numbers?
There are 110 Messier objects and 109 Caldwells. My immediate thought for a familiar and similarly-sized scale was the chemical elements. When I was growing up you knew that number 92 was the last, really, but they’d knocked various bits together and got all the way up to lawrencium at 106. These may (as Lehrer says) be the only ones where news has come to Harvard, however many have since been discarvered. (There are updates to the song that get you all the way to oganesson, if that’s your thing, and I have an app that contains a version in Japanese.)
The right sort of magnitude, then, and a category that is very definitely allied to numerical interpretation. So clue the element with the appropriate number, and wait for enough cross-checking to reveal the other catalogue. After all, everyone knows the Crab Nebula goes north up the eastern side of the UK.
This still feels rather a daring leap (I’m writing this before any blogging or other feedback has reached me) though I suppose once you’ve got MOLYBDENUM (or whatever) then it’s likely you’re on the lookout for items in the periodic table. The ones crossing the ecliptic had the added twist that they were unannounced double clues.
I also decided to try and help by giving some idea of the catalogue names – but doing that by omitting letters in wordplay probably didn’t alleviate the process. So, all in all, quite a difficult puzzle, I feel.
And then as I was finishing the grid CRAB popped up in the wrong hemisphere. I decided to take that as a good omen, but I might have been wrong there. We made a late change to a clue on the working proof, and I didn’t check that it had been correctly made, so there was a word missing. It was not such as to render the clue insoluble, but it can’t have helped.