I've seen "Sun up" as an alternative to "Sunrise" on the Internet on several occasions, so I've been wondering why my English teacher claimed it to be an incorrect word when I was writing a diary entry as a school assignment pretending to be a black slave working at a plantation during the 19th Century racism conflicts.
I used the word this way: "As I started to work at sun up, I..."
I got B/A on my Essay so I didn't feel like arguing on the matter, but my teacher didn't explain properly WHY it was the wrong choice of word. Is it because it's more American English, perhaps, and not a U.K English standard? Or is it even wrong at all?
I'm an 18-year-old Norwegian and I don't know too much about English usage, however I have a certain interest, and I'd like to know if this usage of word was incorrect, or if she was wrong in her claim of it being so.
I got great help last time I consulted this newsgroup. I'll sit back and wait patiently for a reply.

B. Osdal