It's a weird question, but I gotta ask. Let's say I keep sneezing a lot and want to explain to someone that it's because of the sun. So in English I'd simply say "It's the sun." as an explanation. So how do I say something like this in Japanese? Which word for "sun" do I use? 日、太陽、天日? Is saying 「日ですから。」correct?
"The sun" itself is 太陽, so you could say:
太陽のせいです。 (It's because of/due to the sun)
EDIT: Although not incorrect grammatically speling, the following sentence does not sound natural (please see the comment by @Naruto):
太陽でくしゃみします 。( [I/You] sneeze due to the sun, sounds like "[I/You] sneeze using the sun").
However, I don't see the connection between sneezing and the sun. If you can provide more context beyond "It's the sun", we will be able to assist you further.
Also note that both せいで and で are used only when the outcome is something negative (like becoming ill, having a sneeze or such). You can't use them for positive outcomes.