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In the first line of this 12th-century conductus:

Sol oritur occasus nescius

what does nescius refer to? Maybe diagramming the sentence is all I need, because I don't follow the grammar.

If the idea is that the Sun rises not knowing when or whether it will set, then I'd expect the sentence should be Sol oritur occasum nesciens. If the idea is that when the Sun rises, no one knows when or whether it will set, then I'd expect Sol oritur occaso nescio (although I think first of nescio as the verb "I don't know").

But the word is occasus, which could be a noun or an adjective. If a noun, that would tend to put it into a different case, as above. So maybe it's an adjective modifying Sol. But then what does the sentence mean—The Sun rises, having already set, without knowing how it rises? That could make sense, given that the rest of the poem runs through religious paradoxes involving a single thing appearing in two forms that somehow interact with each other. I don't know the idioms and grammar well enough to say.

Here the predicative adjective nescius is taken as governing genitive (occasus) rather than accusative (occasum). In fact, this is also found in Classical Latin: e.g., nescia mens hominum fati sortisque futurae (Verg. Aen. 10, 501). So notice that in your example occasus is to be analyzed as an objective genitive (in parallel with fati sortisque futurae, which is also objective genitive with nescia. NB: hominum is subjective genitive with mens in Vergil's example).

To add a bit to Mitomino's excellent (and correct) answer:

Vowel length, so unhelpfully ignored in most Mediaeval manuscripts, is the key here.

Sōl orītur occāsūs nescius

In other words, this is the genitive singular, not the nominative!

Nescius, like some other words referring to knowledge and memory, can take its topic in the genitive. That's what's happening here. The sun rises, ignorant of its own setting.

EDIT to add: it's not always clear whether an -us noun is second or fourth declension, especially one you haven't seen before. But when a noun is formed by putting -us on the supine stem, meaning "act of ___ing", the result is always in the fourth declension.