What is the difference between 太阳 (tài yáng) and 日 (rì) for "sun"? Are they used in different contexts or are they interchangeable?
日 both means sun and day (and is short for Japan, etc.), but the most common meaning of 日 is day or daytime.
E. g. :
- 每日 every day, daily
- 日志 journal, log
- 昨日 yesterday
- 日报 daily (paper)
- and many-many more.
The only examples I can think of where 日 denotes the Sun and not day/daily are:
- 日出 (rì chū) sunrise
- 旭日 (xùrì) rising sun
- 烈日 (lièrì) scorching sun
- 红日 (red) sun
As you can see, the day meanings are derived from the sun meaning, as 日 originally denoted the Sun since earliest times. But the meaning later shifted toward day/daily and as you can see most compounds with 日 now translate as something with day.
太阳 on the other hand unambiguously refers to the Sun as a celestial body, both in scientific and everyday usage. In compounds it sometimes translates as solar but I think that is no surprise for an Italian [if my guess is correct].
I'd like to offer a slightly different perspective from the other answers, and suggest that the most important difference between the two is that 太陽 is a free word, whereas 日 is a bound morpheme, i.e. it cannot appear as an independent word. Think of 太陽 as 'sun' and '日' as sol-. Of course they are not word for word identical with the English forms, but the basic concept is, perhaps, somewhat similar.
I don't agree that 日 is more common as 'day' than as 'sun'; it is pretty much parallel with 月, which is both 'month' and 'moon', and it's really quite difficult to say which is more common. The morphology of 'moon' is also somewhat similar to 'sun' in Chinese, except that 月 is free when it means 'month'. whereas when it means the celestial body, it is bound, and you need the form 月亮. (Correct me if I'm wrong!)
Historically, 日 was both sun and day. (天 was originally different from 日, consisting of two things: 一日and 一夜.) How 太陽 became the word for 'sun' is an interesting question. In a large number of dialects, especially in northern China, the word for sun is still 日頭. It seems that 日 kept its meaning as sun pretty much unchanged in these dialects.
太陽 seems to have been an astrological term for 'sun', with the moon identified as 太陰. Why did 太陽 win out and supplant 日 and all its variants, while 太陰 lost out completely? I'll award 5 *'s to the lexico-mane who can answer that one.
The word used in modern Chinese is nearly always 太阳. In nearly all cases where in English you would use the word
sun you can use 太阳.
The only exception I can think of is 向日葵, which means
sunflower. There may be some other compounds words or derivative words (solar) that use 日 instead of 太阳, but I think generally, it would be would be pretty safe to use 太阳.
日 in modern Chinese means
daytime or a
As a Chinese, I feel 日 is usually used in ancient Chinese or modern Chinese vocabularies from the old days, we use 日 in modern Chinese is because in some cases, we want to simplify the word, like 日光浴，basically means tanning, 日光 means sunshine, and 阳光 means sunshine too.
So, if you say 太阳 in modern Chinese, in most cases, you're exactly fine. however, if you use 太阳 for everything related to sun, that will be too nerdy and scientific... for example, we usually use 太阳光 when we write some technical articles about analysis on radiation of sunlight, in this case, we may use 太阳光 for sunlight.