An alert reader submitted this question via Facebook:
“[W]hat the heck is up with: ‘One light year, is the distance light, moving at 299,792 kilometres per second travels in a year, or about 10 trillion kilometres.’?”
A most excellent question. <bowing head> Let us parse.
Using the inexplicably outmoded but still useful technique of sentence diagramming (which I have always itched to investigate but never before had opportunity), we find that the simple subject is "light year" (sometimes written as “light-year," which I would advocate here, for reasons you will shortly understand), and that it has been modified by the adjective "One.” So far, so good.
The simple predicate is also easy-peasy--it's "is." Hooray!
No, don’t argue, children. There is no other choice. In this sentence "travels" is obviously a noun, although more specifically it is the second half of the compound noun “second-travels” and (who was the copyeditor???) therefore in dire need of a hyphen. I don't pretend to know what "second-travels" are, but would venture to guess that this compound noun designates the trip one makes to a specific spot after having visited it once (and only once) before.
Similarly, although the word “moving” appears here in classic verb form, it is unquestionably being employed as an adjective (i.e., “causing strong emotion”). Unfortunately, a little interference seems to have inserted itself between this adjective and the noun that it modifies. This may be random noise, probably interstellar. (If you listen to a sample here, you can see how easily this might happen.)
I suspect that the noun being modified by “moving” is the aforementioned “second-travels” (don’t they sound like fun?), although one might argue that it is, instead, the adjective grouped within this phrase (“kilometres per”)—modified in turn by the quantifier “299,792” (which strikes me as rather a long way to go for a second-travel, or even for a first!)—which would mean that it is functioning here more as a noun. But for the sake of argument (’cause we love that stuff), we’ll diagram it as an adjective.
Oh, the diagram! Let’s get back to it.
Our next discovery is a delightful one: We have a subject complement! (I love when that happens.) Here, it is “the distance light.” (We may have to start docking that copyeditor’s pay, as this would have been more immediately graspable had this additional compound noun also been hyphenated: “distance-light.”) Yet again, I am not clear on the meaning. Fortunately, my purpose is to decode the grammar, not to divine the sense. I’m an editor, not an astrophysicist. But I rather imagine a "distance-light" is a kind of sidereal lighthouse. Quite poetic!
What we have next is a somewhat unfortunate pile-up of appositive phrases. I am not sure how “distance-light” (sing.) can be equated with “second-travels” (pl.), but the structure of this sentence demonstrates very clearly that these terms correspond. “Second-travels” is modified quite spectacularly, first (as mentioned earlier) by “moving at,” then by “kilometres per” (which are in turn quantified by “299,972”) and finally by the prepositional phrase “in a year.”
At this stage, we are offered an alternative. Instead of equating “distance-light” with “second-travels,” we are given the option (such is the function of the “or”) of drawing a parallel with “kilometres” (modified by “trillion,” quantified by “10,” in turn qualified by “about").
How very mysterious! I have no clue how 299,972 kilometres can be equated with 10 trillion kilometres. We may need to consult a science person—preferably someone who’s good with this deep-space stuff.
Judging by the grammar alone, we find that the secret may lie in the sneaky insertion—heavens, a third (or is it fourth?) unhyphenated compound, this time an adjective! (Time to fire that copyeditor!)—of “per.” And that gives us all the information we need. Indubitably, in the world of astrophysics, “kilometre [n.]” does not equal “kilometre-per [adj.]”.
Lastly, we discover that . . . Well, we discover, rather shockingly and anticlimactically, not only that this sentence makes no sense at all, but that it fails to conclude! It’s true. Just look at the diagram, above.
On the whole this is a rather good thing, as not only have we run out of patience, but our diagram has run off the page.
But wait. It has struck us like a lightning bolt (or a meteorite) that there is an easier solution. What happens if we commit a simple comma shift? How much confusion (and silliness) might have been avoided!
“One light year is the distance light, moving at 299,792 kilometres per second, travels in a year, or about 10 trillion kilometres.”
Given my druthers, I might have rewritten this sentence (actually, there's no “might” about that), but now, all of a sudden, it makes sense.
See how important it is to keep track of your commas?
Thank you, alert reader!