An orbit is when one object in the universe (for example a planet or a star) goes
round another one without touching it.

Is this an idiom? Something about it bothers me. Is it an ellipsis for "an orbit is created when......"?
Is there a better way to say it?

I think I'd write it more like: An orbit consists of one object in the universe (for example a planet or a star) going round another one without touching it.

Scratch that. An orbit is the path one object in the universe (for example a planet or a star) follows as it goes
round another one without touching it.

Is there a better way to say it?
Thoreau didn't think so:
"Our truest life is when we are in dreams awake."

... goes round...

I'd say 'goes around', but.

Dear Wordminstrel: Glad to see you posting again. I like the Thoreau quote, no problem.
The thing that jars me is " A thing is when".

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.

I just want to add one more observation about adjectivals: In some writings about grammar, you will read that adjectival clause markers are:

who
whom
which
that
where


...but that classic list is incomplete. When reading about descriptive grammar, you'll find mentioning of 'when' as an adjectival clause marker, too, among other subordinate candidates.

I think similar objections have been posed, for fifty years at least, regarding "That's why..." Something to do with predicate adverb issues and the verb "to be", as I recall. The preferred (read "correct") construction was "The reason is that..."

"The reason is that..."

Or either that or "The reason is…", one.

The problem with prescriptive grammar is it doesn't always reflect the grammar as it's generally spoken. Although in very formal writing prescriptive grammar would be the better choice, I don't think it should always be the end. For example, if I said, "That's why we shouldn't go!" and, if I am generally and easily understood to be speaking a grammatical sentence, then that construction is acceptable.

I wouldn't have any problem with a speaker in a formal situation developing an argument, and pointedly ending it with the construction: "That is why we should not ________________." Now, had it been a formal argument in writing, I'd lean toward "The reason is _________________."

I don't mean to imply that simply because we can describe a grammatical construction, it is therefore acceptable in all situations. I do mean that when constructions are generally used and understood by most of the population, those constructions are acceptable in formal speech and even to some degree in formal writing. For example, consider the split infinitive. I don't think too many people worry these days about splitting it in formal writing as long as the split doesn't sound awkward and, in fact, facilitates the reading of the sentence.

I ran into an interesting argument for prescriptive grammar in writing (emphasis mine) recently. When one is speaking one can add many meaningful elements non-linguistically, through facial expression, tone of voice, body language, etc., that cannot be used in written language. Therefore it is important that well understood and universal rules be followed to ensure proper communication.

And I generally agree, Faldage. But, if you know the rules and if you're sensitive to how the language is spoken and understood, you can break them for rhetorical effect.

Also, some of the rules need to be occasionally questioned if they consistently plug up the plumbing.

...by entering:

*is when*

Just thought the number of hits was interesting.

Important Edit:I just learned something important about Googling. In the search above, I entered this exactly:

*is when*

...in an attempt to find every listing that included 'is when'--but what I didn't do was to put 'is when' in quotes. Therefore my hits were way off--I looked through the sites and found 'is' and 'when' often split up.

So, here's a correction. By entering:

*"is when"*

...you only get 1,990,000 hits.