Commentary on adjectivals that answer the question, "When did something happen?" (or..."When will it happen," etc.)
Example: "Our trip is in the summer."
"In the summer" here is a prepositional phrase functioning as a adjective modifiying "trip." It's a phrase that indicates time (or: 'when').
In the sentence you quote, wwh, there's a more complex construction. There is an adjective clause that is introduced by the adverb 'when'--and that's completely legal. We might expect subordinate clauses beginning with 'when' to modify a verb, as in:
"Joe moved against the wall when the crowd entered."
In the above, the clause clearly functions as an adverb modifying 'moved.'
In the sentence you posted, the subordinate clause simply functions as a predicate adjective that modifies 'orbit.' Adjective clauses can begin with 'when,' although we probably hear a lot more adverb clauses beginning with 'when.'
Romance is when you're living in a dream world.
Edit: wwh: You make a good point. I want to add that I don't especially like the sentence you pasted, but I think it's grammatical.
