I mean, if something is truly important, there will be a reference to it in the Sherlock Holmes stories, right? (At least those of us who seem to want to write endlessly about that great detective definitely hope so.) A quick check-up on the matter proves it out:
Eclipses are important, because if you look at the cases of Sherlock Holmes and see the people they are associated with . . . wow.
Sherlock Holmes, of course, references eclipses on the subject of his own mind: "Should you care to add the case to your annals, my dear Watson, it can only be as an example of that temporary eclipse to which even the best-balance mind may be exposed." (This, about "The Disappearance of Lady Frances Carfax.")
Watson also uses it to show how much Irene Adler meant to Holmes: "In his mind, she eclipses and predominates the whole of her sex." (In "A Scandal in Bohemia, of course.")
And Inspector MacDonald gets eclipses themselves explained to him by Professor Moriarty: "I had a chat with him on eclipses . . . he had out a reflector lantern and a globe and made it all clear in a minute. He lent me a book . . ." (During The Valley of Fear.)
Sherlock Holmes. Irene Adler. Professor Moriarty.
If those aren't the big three of the Sherlock Holmes universe, who is?
(Yes, Mycroft, but he gets "Jupiter is descending today" and "A planet might as well leave its orbit." to show his planetary importance.)
Which makes it all the more interesting when one finds the one other reference to eclipses in the Canon Holmes and what it refers to: "The Lord St. Simon marriage, and its curious termination, have long since ceased to be a subject of interest in those exalted circles in which the unfortunate bridegroom moves. Fresh scandals have eclipsed it . . . ."
John H. Watson uses the word, regarding the scandalous bigamy of St. Simon's bride. Note that Watson also featured a bigamy-involved case in the first novel he wrote of Holmes's detection, A Study in Scarlet. And while that scandal in Bohemia wasn't about bigamy, it was about multiple romantic relationships in conflict. So why might that topic have been of "eclipse-level" importance to Watson? As someone who has written of the possible six wives of John H. Watson, I definitely have my own theories.
But let's not eclipse the eclipse with Watson's little relationship issues. It's kind of a big deal, as the Sherlock Holmes texts definitely convey.