Here comes the Blue Moon

 

On March 31, skywatchers can view the second Blue Moon of 1999 Yf they say the mone is blewe
We must believe that it is true.
             Rede Me and Be Not Wroth (1528), author unknown

Mar. 30, 1999: The second Blue Moon of 1999 can been seen this Wednesday night when the moon becomes full at 2250 UT (1450 PST). To see the moon simply go outside shortly after sunset. It will be shining brightly just above the eastern horizon. By midnight it will rise high in the southern sky and illuminate the landscape with bright moonlight.

Right: This original art by Duane Hilton shows the Full Moon illuminating the landscape around a volcanic cindercone in Big Pine, CA.
 

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The moon won't appear to be blue on Wednesday. It's called a "Blue Moon" by astronomers simply because it will be the second full moon in the month of March. The moon has assumed a bluish color at times in the past, for example after the eruption of Krakatoa in 1883 filled our atmosphere with obscuring dust, but such events are unrelated to the term "Blue Moon" as it is used today.
  Left: The rising Blue Moon on Jan 31, 1999. This blue moon appears red because of scattering by atmospheric dust on the horizon, for the same reason that sunsets often have a reddish tinge. This photo, courtesy of Phil Harrington, was taken at the prime focus of a 4" f/9.8 refractor telescope, using Kodak Gold 200 color print film with a 1/60th second exposure.

According to research by folklorist Philip Hiscock the term "Blue Moon" is at least 400 years old, but its popular meaning has shifted many times. The earliest known references to a blue moon were intended as examples of obvious absurdities. If a 16th century person asserted "That's as likely as a Blue Moon", they meant that it simply couldn't be. As time passed the expression evolved to mean something that rarely or never happened. Hence the expression "Once in a Blue Moon" which is still popular today. A second, modern definition of a Blue Moon as the second full moon in a calendar month was apparently introduced to popular culture by a mistake in the magazine Sky & Telescope 53 years ago. (The author recommends this month's excellent article in Sky & Telescope on the history of the Blue Moon.)

Below: The first full moon of March 1999 as photographed by Fred Ruszala from Colchester, CT using a 3" Unitron refractor at 48X with Kodak RG ASA 200 film and a 1 sec. exposure. The telescope was mounted on an AP 600E mount tracking at a lunar rate.
  The two contemporary definitions of blue moon, the second full moon in a calendar month (the astronomical definition) and something that rarely happens, are not equivalent. Astronomical blue moons happen fairly often, once every 2.5 years on average. By the end of March there will have been two blue moons in 1999 alone. The first occurred on Jan. 31 and the second takes place this Wednesday. Years like 1999 that include two blue moons come along approximately once every 25 years. The next Blue Moon will arrive in November, 2001.

With Spring beginning in the northern hemisphere and the nights growing warmer, this week should be a pleasant opportunity to view the second Blue Moon of 1999. While Blue Moons may not be as rare as commonly thought, they can be like all full moons a sight of rare beauty.

 

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For more information, please contact: Dr. John M. Horack , Director of Science Communications

Author: Dr. Tony Phillips Curator: Bryan Walls NASA Official: John M. Horack