This site lists many Dawnguard add-on random encounters that occur "during an eclipse."
Why do I want to know? I'm trying contract Sanguinare Vampiris ASAP. In an earlier play-through, I came upon a vampire attack at Pelagius Farm, on my way to Whiterun from Riverwood. I manipulated the vampire into afflicting me. I'm trying to get this to occur again, but another vampire isn't spawning there. Perhaps I came upon it during an eclipse? I was also level 33 at the time rather than level 1, but I've buffered up to level 26, so that may matter too. Now that I think about it, I could possibly get a vampire to attack Riverwood too.