Rare blue moon brightens sky on Friday

In this Friday, Aug. 31, 2012 photo, a small plane passes in front of a "blue moon" over the Westchester County airport in Armonk, N.Y. (AP Photo/The Journal News, Frank Becerra Jr.)

SAN FRANCISCO (KRON/CNN) — Celestial observers should be ready to set their sights on the moon this Friday, when a rare blue will grace the skies.

The July 31 lunar appearance will be the second full moon this month, which is commonly referred to as a “blue moon.”

While such a lunar event has been named a “blue moon” Earth’s satellite will most likely not appear blue at all. Observers will probably see it as gray or silver like any other full moon.

Typically, when a moon does take on a bluish hue, it’s because of smoke or dust particles in the atmosphere, such as ashes from a wildfire or volcanic eruption.

The dark blue tone of an evening sky could also affect what the moon’s coloring looks like.

So when the phrase “once in a blue moon” was coined, it meant something so rare you’d be lucky (or unlucky) to see in your lifetime, according to NASA’s National Space Science Data Center.

The most recent blue moon that truly took on a blue hue was in Edinburgh, Scotland, in September 1950. Astronomer Robert Wilson of the Royal Observatory observed the event and concluded that the moon was blue in color because the satellite’s light was traveling through a patch of clouds that had particles of smoke and ash from forest fires burning in Alberta, Canada.

Those particles crossed the Atlantic Ocean and hovered over Scotland during the lunar event, creating a rare spectacle, according to the National Oceanic and Atmospheric Administration.

Today’s modern day usage of the phrase blue moon can be attributed to a Sky and Telescope writer who in the 1940s tried to clear up a confusing definition published by the Maine Farmers’ Almanac. According to the U.S. Naval Observatory in Washington, when a season has four full moons, the third one is called a blue moon.

This lunar event will not be seen again until January 2018.

In addition to the blue moon, skywatchers may also get another celestial treat this week. The Delta Aquarid meteor shower is set to peak around July 27-30. However, the full moon is expected to impact the viewing of the meteor shower.

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