In fact, when talking about time, `s' is also official. While `sec' may be seen frequently, it's nonstandard.
Lightyears are not the standard unit for measuring astronomical distances, of course; that distinction belongs to the parsec (pc). That being said, `ly' is a perfectly good abbreviation and much better than writing it out. The original poster certainly understood what it meant, at any rate.
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I like the one where the Toyota Yaris eats the vacuum cleaner spider: http://www.youtube.com/watch?v=skhaeXwfh3s
In fact, when talking about time, `s' is also official. While `sec' may be seen frequently, it's nonstandard. Lightyears are not the standard unit for measuring astronomical distances, of course; that distinction belongs to the parsec (pc). That being said, `ly' is a perfectly good abbreviation and much better than writing it out. The original poster certainly understood what it meant, at any rate.