[shooon]

I completely agree with this idea

Originally Posted by RadioKid

陽 is related to sun and sun-light.
太陽 means exactly "sun" itself.

However, may I explain in another way?

[1)difference between 陽 and 日]
IMHO, 陽 simply means 太陽.
On the other hand, 日 has more meanings than 陽.
日 can mean "sun", "day"(, "Japan",etc). but
陽 only means "sun".
So, I think you can say 日　⊇　陽.

For example, Japanese use both 日光 and 陽光.
(Actually the nuance of 日光 is a little different from that of 陽光.
The latter useally implies warmth, but the former doesn't.)

But 陽 is not always interchangeable with 日 even when they both mean "sun".
You can say 朝日, but not 朝陽.
You can say 太陽, but not 太日.
I don't know why...I'm sorry.m(_ _)m

By the way, I think most Japanese people use both "陽が落ちる" and "日が落ちる".
In this case, (today's) Japanse people don't care of the difference.
(Google Results
日が落ちる
陽が落ちる
)

*We rarely use 旭日 today. Only when to refer to 旭日旗(flag).

[2)difference between 陽 and 太陽]
When you use 陽 in 熟語(陽 ＋the other kanji letter(s)), you should use 陽 not 太陽.
In the other cases, generally speaking, you can use both of them.

e.g.)
太陽がまぶしい vs. 陽がまぶしい : both OK.
太陽がのぼる vs. 陽がのぼる : both OK (someone might say the latter is better, but I personally don't care.)

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Please excuse my English!