They are very faint and difficult to detect. They weren't even observed directly when they were discovered. They were originally discovered when astronomers noticed that they occulted light of background stats.
That's so strange? 'Occluded' would be a suitable word for that meaning-wise though right? When did astronomy decide to use such a close sounding but different meaning word? Did occult mean what it does now when they started? Language is so strange
The sciences are full of new-latin and neo-greek formations like Biology and Astronomy. So reaching back the the original meaning of occult "to hide (from)" isn't that big a stretch. Especially when "occlude" comes from a word meaning "to shut (away)" and an eclipse isn't shutting the sun away, just hiding it. The Mahdi isn't just standing behind something, he's hidden away in another plane. That's why he's the occluded imam.
But also occlude has a chemistry meaning when one substance gets hidden inside another, so maybe it was too overloaded to be a good word for that.
I'm not sure about the history of these words, but astronomy also uses the noun form: "occultation" [1], for which there's not an obvious equivalent for "occlude."
> Did occult mean what it does now when they started?
A word can have more than one meaning. The first definition on merriam-webster.com covers the definition used in astronomy:
occult (v.): to shut off from view or exposure: cover, eclipse [2]
The adjective form might be a source of derivation for the meaning you're alluding to:
occult (adj.): (1) not revealed: secret; (2) not easily apprehended or understood: abstruse, mysterious; (3) hidden from view: concealed [2]
And finally, the paranormal meaning that people are more familiar with today:
occult (n): matters regarded as involving the action or influence of supernatural or supernormal powers or some secret knowledge of them -> used with the [2]
Again, I don't know the history of these words. If I had to hazard a guess, I'd bet that the noun form, "the occult", is derived from the adjective form since "the occult" refers to supernatural phenomena, which is naturally hidden from view, concealed, not revealed, secret, not easily apprehended or understood, etc (because it's not real).
Edit: Another guess. If you think about the history of astronomy, it was originally intertwined with religion and astrology. Perhaps these words date back to a time when "the occult" and astronomy weren't entirely separate. Anyways, I agree. Language is strange.