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Once in a Blue Moon

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"Once in a Blue Moon"

For the first quarter of 1999, it means 'two-thirds of the time.'

February 26, 1999: We've all heard the saying "it happens only once in a blue moon," generally used to describe an uncommon occurance. But for the first quarter of 1999, using the phrase "once in a blue moon" could also mean "two out of three times."

Right: Artist Duane Hilton's portrayal of the March 1999 "Blue Moon" as viewed between White Bark pine trees in California's Eastern Sierra.

In astronomical terms, a 'blue moon' really doesn't have anything to do with color. Instead, it is the term used to denote the second full moon that occurs within a given calendar month. Because it takes the moon about 29 days to circle the Earth once in its orbit, it is possible that two full moons can occur within the same calendar month. Such was the case in January 1999, when the moon was full on the 2nd and the 31st, making the full moon on the 31st a 'blue moon.' On average, this takes place once every two and a half years. This time, however, we don't have to wait over 2 years for another blue moon. A second blue moon will appear this March, with the moon displaying its full-phase on the 2nd and the 31st of March.

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Although a 'blue moon' doesn't really look blue, there have been times when the moon does seem to have a blue color. This can be caused by dust particles in the atmosphere, which scatter light. The effects of this dust on the light coming from the moon can cause it to appear bluish in color. Fine dust particles are ejected into the Earth's upper atmosphere after large volcanic eruptions, for example. The eruption of the Krakatoa volcano in 1883 gave us one such 'blue moon'. For about 24 months after this volcano exploded, the dust it spewed into the upper atmosphere caused the moon to appear green and blue when viewed from around the world.

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There is little scientific significance to either an astronomical blue moon or a bluish-looking moon. But astronomers are interested in how the moon reflects different colors. The moon generates no light of its own, but reflects light from the Sun. By studying how the moon reflects this light, scientists can explore the chemical composition of the lunar surface.

Right: A Violet Moon. Checking out the Galileo spacecraft's cameras during its December 1992 flyby of Earth's Moon, controllers took this dramatically illuminated picture through a violet filter. The view looks down on the Moon's north polar region with the Sun shining from the left at a low angle and the direction toward the moon's North pole toward the lower right. More information.

Studying the moon can be tricky, because the moon is too bright to be photographed with large, highly sensitive telescopes on the ground or with the Hubble Space Telescope. The moon's brightness can potentially damage such sensitive optical instruments. Less sensitive telescopes on the ground and on satellites, however, have given us some stunning images of the full moon. The moon can also be photographed using different light wavelengths, such as ultraviolet.


Above: An Ultraviolet Moon. One of the more interesting space-based images obtained of the moon was taken from the Astro-2 observatory, lofted aboard the space shuttle in 1995. Although the moon appears blue in both images, the left-hand image is actually taken in the ultraviolet, which is higher in energy than blue or violet light.

The oddity of having two blue moons within three months is not of much interest to scientists, but it is a notable event to many others. Ever charming and compelling, the blue moon will likely continue to serve as inspiration for songwriters and poets for generations to come.
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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Author: Dr. John M. Horack
Curator: Bryan Walls
NASA Official: John M. Horack