Once in a blue moon

    千载难逢

    这个短语的字面解释是“当蓝月亮出现”，这是什么意思呢？请看对话：

    A: My old schoolmate John has come up with a business idea and asked me to join him in his startup as partner. What do you say?

    B: John is a brilliant young man and the odds are he will be successful. It's an opportunity that comes once in a blue moon and you should take his offer.

    Note: This idiom means "once in a great while, rarely." Dating back to the 1800's the term "blue moon" was used by the "Farmer's Almanac" to denote the appearance of a third full moon in a season where four full moons occur. Since there are normally 12 full moons a year (one per month), this works out to three per quarter. However, occasionally a quarter will see four full moons. Since blue moons only occur about once every three years, it became becomes a convenient measure of a long period of time, therefore giving rise to the cliche "once in a blue moon."