I'd honestly rate this as the most common misconception of the fanbase. Above Ork miracle field and blam-happy commissars, and every other meme which has ascended to fanon without any real basis. The spelling of the great beacon of Terra.

People (me included, often) spell it "Astronomicon", like Necronomicon, or panopticon. But it isn't; it's Astronomican, with an 'a'. The "-ican" suffix is definitely less common in English than "-icon", and particularly in 'old' words that suggest Greek or Roman roots, so it's an understandable mistake.

Words ending in "-ican" are very few, and mostly adjectives: republican, American, Gallican, Mohican, basilican; other than basilican, they specify group membership and can also be nouns meaning a person who is in that group. None of these have any visible connection to the Astronomican. The other nouns are a couple birds (pelican, florican), a type of dance (hoolican), a food (pemmican), a natural dye (indican), a weird misspelling (jerrican), and one other: barbican.

A barbican (from Old French: barbacane) is a fortified outpost or gateway, such as at an outer defence perimeter of a city or castle, or any tower situated over a gate or bridge which was used for defensive purposes.

By process of elimination, this is the word the original author had in mind. And it has interesting lore implications, because this is exactly the role the Golden Throne was meant to play for the Webway Project. "The barbican to the stars" is a pretty excellent way to gloss "Astronom- -ican". So, at least to whoever (Doylistically) named the Astronomican, it was actually meant to be the name of the Human Webway Gate, not the beacon as distinct from the Golden Throne. (Conflating the two is also a common misconception, so congratulations everyone, your misconception is justified. Still wrong, lore moved on, but justified.)

There's also some connection to its modern meaning as the beacon itself, in the rest of the name. "Astrolabe", the tool for navigation for ancient sailors, is an obvious relative, and glossing it as "Astro-" (stars/astrolabe) "nomic" ('of law', from Greek nomos), "-an" (mostly-meaningless nounification suffix) also works pretty well. But for that purpose "-on" would have worked better and felt more natural, so it seems pretty clear that while they didn't write it into the setting bible (if that even existed), originally the word Astronomican was created from the "barbican" meaning.