2--Once in a blue moon. It doesn't mean that the moon is blue.
 
1--So let's listen to this American English conversation to find out how it is used:
 
2--Where did you say your brother lives?
 
1--He lives all the way in Minnesota, so I never get to see him.
 
2--I only get to see him once in a blue moon.
 
1--Once in a blue moon is a common way of saying that something does not happen often.
 
2--But what is a blue moon?
 
1--We typically see a full moon once a month.
 
2--And seeing two full moons in the same month is rare.
 
1--The second full moon in a particular month is called the blue moon.
 
2--Once in a blue moon is an informal phrase.
 
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注解：
 
1.common常见  
2.particular特指的  
3.informal非正式的  

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译文：

1--你听过这个美式俚语么？
2--Once in a blue moon（千载难逢），它的意思可不是月亮是蓝色的。
1--下面让我们来听听这段美语对话，看看这个短语的用法：
2--你兄弟住在哪？
1--他住在明尼苏达州，但是到处搬来搬去。所以我没去看过他。
2--我只难得有一次拜访过他。（once in a blue moon）
1--once in a blue moon意思是某事不经常发生，这是一个很常见的用法。
2--但是什么是蓝月亮呢？
1--一般来说，我们一个月会看到一次满月。
2--在一个月中出现两个满月是很罕见的。
1--某个特别的月份中的第二个满月就叫蓝月亮。
2--Once in a blue moon（千载难逢） 是一个非正式短语。