题型：阅读选择 题类：常考题 难易度：困难
浙江外研版2018-2019学年初中英语九年级中考模拟卷二（含听力音频）
Have you ever heard someone use the phrase "once in a blue moon"? People use this expression to describe something that they do not do very often. For example, someone might say that he tries to keep away from sweets because they are unhealthy, but will eat chocolate once in a blue moon. Or someone who does not like to. Go to the beach might say, "I visit the shore(岸边) once in a blue moon." While many people use this phrase, not everyone knows the meaning behind it.
The first thing to know is that the moon itself is never actually blue. This is just an expression. The phrase "blue moon" actually has to do with the shape of the moon, not the color.
As the moon travels around the earth, it appears to change shape. We have certain names for certain shapes of the moon. For example, when we can see a small part of the moon, it is called a crescent moon. A crescent is a shape that looks like the tip of a fingernail(指甲). When we can see the moon appear in the sky as a thin crescent, it is called a new moon. When we can see the whole moon, it is called a full moon. Usually, there is only one full moon every month. Sometimes, however, there will be two full moons in one month. When this happens, the second full moon is called a "blue moon".
It's predicted(预言) that there will only be 15 blue moons over the next 20 years. As you can see, a blue moon is a very unusual event. This fact has led people to use the expression "once in blue moon" to describe other" very unusual events in their lives.
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