What is the difference between 早 and 早晨? E.g. 早晨好 What is the difference between 早 and 早晨?  I hear people say 早晨好 sometimes.

Does it refer to a different time in the day?   They both say morning in my dictionary.
早 is not morning, it means "early" in general. 我早知道了！吃早饭。

早上 is morning in general. 早上好！

早晨 is also morning, but earlier, at daybreak, in the very early morning hours.

The day is divided into twelve 时辰, where 寅时 is before dawn, the 凌晨.

Then comes the 卯时 between 5 and 7, the 黎明, the dawn.

辰时 is the next two hour period between 7 and 9, the 早晨.

After 9 is 上午, but 早上 refers to the greater morning period in general.