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Eyes to the skies: It’s a rare blue moon

Tom Greenwood
The Detroit News

If you’re a stargazer, you’ll be delighted by Friday’s blue moon, the second full moon of the month.

If you’re a werewolf, not so much.

The phrase “blue moon” goes back centuries and nowadays not only refers to a lunar event, but also defines anything that occurs on a rare basis.

In reality, blue moons aren’t all that exceptional, occurring every two to three years: The next full moon is slated for Jan. 31, 2018.

According to astronomers, a normal year is host to 12 full moons, three in each of the four astronomical seasons: spring, summer, fall and winter.

When one of the astronomical seasons has four full moons that extra appearance is referred to as a blue moon.

(Fine print: a moon can also be called blue when it actually appears blue due to fine particles in the air, usually from volcanic eruptions, dust storms or massive forest fires. The particles scatter red light and thus give the moon a blue hue. This type of blue moon is pretty rare.)

Other types of moons include:

■ Black Moon. Think the lunar opposite of a blue moon — two new moons in a calendar month. This is when the moon cannot be seen (except for perhaps a slight sliver of light): hence the phrase, “by the dark of the moon.

Black moons are loved by stargazers because the lack of moonlight deepens the sky for better viewing.

■ Harvest moon: the full moon closest to the autumnal equinox.

■ Hunter’s moon: the first full moon after the autumnal equinox.

■ Blood moon: During a total lunar eclipse, the surface of the moon may appear red due to light refracting through the earth’s atmosphere. The refraction essentially filters out the violet to green spectrum, but has little effect on the red spectrum.

■ Blue moon: an orange colored beer advertised as a craft brew but is produced by MillerCoors.

A final note: In actuality, Friday’s blue moon officially occurred at 6:43 a.m. EST when it reached the crest of its full phase. But “hey diddle diddle,” tonight there will still be good view of any cow “jumping over the moon.”

TGreenwood@detroitnews.com

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