'Once in a Blue Moon' Happens on Friday -- Why?

On Friday, much of the world will have the opportunity to observe a Blue Moon: A somewhat rare occurrence that doesn't have anything to do with the moon's color.

On Friday, much of the world will have the opportunity to observe a Blue Moon: A somewhat rare occurrence that doesn't have anything to do with the moon's color.

During most years, the Earth experiences 12 full moons, one in each month. But some years, such as 2015, have 13 full moons, and one of those "extra" lunar displays gets the label of Blue Moon.

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The lunar or synodic month (full moon to full moon) averages 29.530589 days, which is shorter than every calendar month in the year except for February. Those extra one-half or one-and-one-half days accumulate over the year, causing some years to have 13 full moons rather than 12. [Video: What's a Blue Moon, Is It REALLY Blue?]

To see what I mean, here is a list of full-moon dates in 2015: Jan. 5, Feb. 3, March 5, April 4, May 4, June 2, July 2, July 31, Aug. 29, Sept. 28, Oct. 27, Nov. 25 and Dec. 25. In 2016, the first full moon falls on Jan. 23, and each calendar month has only one full moon.

The expression "once in a blue moon" has a long history of being used to describe rare events; but it was also used in the Maine Farmers' Almanac to describe the third full moon in a season that has four (normally, a three-month season will only have three full moons).

In 1946, Sky & Telescope magazine published an article that misinterpreted the older definition, defining a Blue Moon as the second full moon in a calendar month. This has become the most recent and perhaps most widely accepted definition of a Blue Moon. And hence, the full moon on July 31 is referred to as a Blue Moon, because it was preceded by the full moon on July 2. By this definition, a Blue Moon occurs roughly once every 2.7 years.

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The full moon appears to last for at least the length of one night, but technically speaking, it is an instantaneous event: It occurs when the sun, Earth and moon fall close to a straight line. It takes place at the same instant everywhere in the world, whether the moon is above or below the horizon.

The full moon on July 31 occurs at exactly 6:43 a.m. EDT (1043 GMT).

So, when you look at the Blue Moon on Friday morning, don't expect to see a different color scheme (although it is possible for the moon to appear to have a bluish hue). Just be aware that the so-called Blue Moon is a byproduct of the contrast between the calendar month and the lunar month.

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2015 Full Moon Calendar 'Blue Moon' History and What Can Actually Turn It Blue | Video Original article on Space.com. Copyright 2015 SPACE.com, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

2015 gets an "extra" full moon, an event known as a Blue Moon.

Moon phases influence the behavior of all living things, including humans. Lunar power is due to two primary forces: gravity and light changes.

Lions and other predators attack more during the week after a full moon. “The first hours of the night are darkest during the week following a full moon, and the lions are hungriest at that time because of the low predation success during full moon nights,” according to Noga Kronfeld-Schor, who led a study on moonlight’s affect on various species. The paper is published in the latest Proceedings of the Royal Society B.

“Doodle bug larvae (Myrmeleon obscurus) dig funnel-shaped traps into the sand at the bottom of which they lurk for insect prey,” Kronfeld-Schor told Discovery News. “These funnels are rebuilt every day; around full moon the pits are large, during a new moon -- small. Perhaps the higher probability of catching prey during moonlit nights is worth the investment.”

Bats, such as this vampire bat, tend to decrease their activity during nights when the moon is bright. “The reduction in activity probably reflects predation avoidance,” Kronfeld-Schor explained. The bats would simply be more visible to predators. On islands where bats have few predators, nocturnal species come out in force, as usual, at night.

Earth’s biggest reproduction event is the “mass spawning” of corals in the Great Barrier Reef. Co-author Oren Levy of Bar Ilan University told Discovery News that the moon “may choreograph sex among more than 130 species of corals” on the same night. “The event appears to be triggered by the level of lunar irradiance.”

Nightjars, which are insect-loving birds, increase their bug chomping on bright moonlit nights. They also “avoid activity on dark nights,” according to Kronfeld-Schor. These birds primarily rely upon eyesight to catch prey, so they can see better when there’s a full moon.

The moon pulling upward on water while the earth exerts a downward pull affects tidal changes. The moon’s gravity is responsible for 56 percent of earth’s tidal energy, with the sun influencing tides too. Filter feeders like oysters feast at high tide, with food washing in.

Some animals, such as spiny mice, reduce or increase their body temperature in response to moonlight levels. “Common spiny mice (Acomys cahirinus) reduce activity and body temperature, and aggressive intra-specific encounters are higher in response to light at night,” Kronfeld-Schor said. “Moreover, during full moon nights, their foraging activity is lower.”

The moon is like nature’s flashlight for nocturnal predators such as the aye-aye. The aye-aye is a type of lemur native to Madagascar. Primates that are normally active during the night will actually shift their entire daily schedule in response to lunar movements. During new moon periods, for example, these primates may become fully diurnal (activity during the day), “compensating for the lack of nocturnal activity,” Kronfeld-Schor said.

Charles Darwin, according to Kronfeld-Schor, was one of the first researchers to note that “human innate fear of darkness is an adaptation for avoiding risk of predation by nocturnal predators.” Back in the day, our ancestors fell victim to such animals and quickly learned that full moon and other brighter nights work to human advantage. To this day, many people keep a nightlight on for comfort’s sake.

Plants, bugs, and animals have all evolved ways of adapting to lunar cycles. Artificial lighting is disrupting those adaptations, however, to the point that scientists now refer to it as “light pollution.” This image of a highway, for example, was taken at 9:30 p.m. Light emitted from freeway illumination as well as from traffic keeps the surrounding area well lit all night long. Species, including humans, subjected to this light will therefore be in full moon mode each night, regardless of lunar phases.

Kronfeld-Schor warns that “artificial illumination is becoming widespread while its consequences to humans and ecological systems are poorly understood.” This scientific team, as well as others, hope to learn more about light pollution’s impact on us.