User blog comment:Sepublic22/Should Knighton be considered one of the Sixteen Realms?/@comment-27507964-20160309023133/@comment-27747281-20160309163812

I doubt Knighton is on the same planet as Ninjago. We've already established stuff like space travel and whatnot, which means there must be orbital pictures of the entire ocean surrounding Ninjago, meaning if Knighton existed, they would've seen it.

Granted, no one knew of Chen's Island, the Lighthouse Prison, or that switch to raise the Island of Darkness, but they were all places associated with antagonistic beings who would want to hide their islands (and it's confirmed Clouse used magic to hide Chen's island, and if Samukai went to the trouble of chaining the Leviathan to protect Julien, hiding the island with a spell when he can resurrect the dead shouldn't be too hard).

Knighton, on the other hand, is not ruled by anyone villainous, so I see no reason why they would end up being hidden from Ninjago for so long.