Difficulty: Challenger (Level 2)
This month, skywatchers will get the rare chance to see something that happens only once in a blue moon – in fact, it is the very phenomenon that gave rise to the phrase “once in a blue moon”.
The so-called blue moon is the name given to a full moon that appears for the second time in the space of one month.
This is rare since there are 29.5 days to a full lunation - the time between two successive identical lunar phases, such as from full moon to full moon – while calendar months are between 28 and 31 days. Because these cycles are different, some calendar months have two full moons, which happens about once every two to three years.
This year, the first of the two full moon phases occurred on May 1; on May 31, Earth’s satellite will again appear fully illuminated (see graphic). Because the moon will also be at its farthest distance from Earth the following evening at 405,839km, the full moon will appear the smallest this year.
The somewhat misleading name of “blue moon” has been used for centuries, yet its origins are a mystery. Some believe that the word “blue” may have come from the Old English word “belewe”, which means “betray”. Perhaps people who studied the sky in the past thought the moon was playing a trick on them.
What we do know is that neither the first nor the second full moon shines in a blue light.
The next blue moon will be the full moon on December 31, 2028.
