What does "悠" mean in "悠久"? "悠久" is often used to describe a period of history (I read it from ), and I wonder how does the adv "悠" work in "悠久".

I've searched, the complete word means,


  悠久，汉语词汇，形容词（褒义）。
  拼音：yōu jiǔ
  词义：长久；久远。


But I can't figure out what does "悠" mean.
悠 has the same meaning as 久 in this case.

It is a kind of word making method in Chinese, which combines two synonymic characters together. Perhaps the terminology tautology can be used here.

For example: 亲吻 拥抱 战斗 早晨 林木