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This week's full moon will be blue -- sort of

This month's full moon is a "blue" one by one definition, but not according to the traditional definition of a blue moon.

The moon will be full at 6:43 a.m. Friday, so it will be at its brightest in our night sky Thursday night into Friday morning. July's full moon is known as the Buck Moon or the Thunder Moon.

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Because it is the second full moon this month -- the first was July 1 -- it can be considered a blue moon under the modern definition of the term. That definition can be traced to a Sky and Telescope magazine article from 1946, a children's almanac in 1985, and the game Trivial Pursuit, according to EarthSky.org.

But the traditional definition was always the third full moon in a season that contained four full moons. A normal season contains three full moons, of course, each with a name appropriate for the time of year. (In July, thunder is common, of course, and it's also when bucks often begin to grow antlers.) So when a fourth full moon appears in a season, it disrupts those naming conventions.

Friday is an inopportune time for a full moon in one sense because it will outshine the Delta Aquarid meteor shower, which peaks in late July. But the timing also means the moon will be a thin crescent by the time the more dramatic Perseid meteors peak Aug. 11 and 12. 

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