So, floating islands are a pretty common trope at least in the media I've consumed, and one thing that sort of stands out - oftentimes, they're meant to be a reveal to a larger world (Either as part of a secret group the protagonists come to seek help from, or more exposition on a situation, or as part of a larger escalation of the conflict of the story.), and the people living on the non-floating ground of Earth aren't usually aware of the floating island, except in legends that they don't know about them until then. At best, the Grounded People believe the existence of the island is a legend, a myth even, even though the Sky People have been around for ages, often millennia.
This however, poses a problem - for this Floating Island is neither at sea on its own (Which still leads to the issue of sailors noticing it when sailing), nor likely at International Space Station levels of height out sight (Or applying the "Move very quickly across the sky" orbital trick to avoiding immediate notice). So, how do they manage to hide themselves from Grounded People when people should just be able to look up at the sky and see it, or potentially notice it through a telescope? Especially on a clear day/night, where cloud cover isn't an option?
Is there a way to hide an floating island from below for any climate situation, and is there an upper limit on when that solution would fail in terms of the technology level of Grounded People?
(I'm presuming it would fail when airplanes and/or spaceship related technologies being up, but I'd like to at least hide the island through the Medieval era tech if possible.).
Bonus if it's a passive way to hide the island itself, and not require active powering of technology to keep it hidden.