Unusual astronomical effect can be seen only once in 2.5 years.
On Friday, the entire planet will be able to observe the rare phenomenon — the Blue Moon. This is an optical effect due to unusual refraction of light rays reflected from the lunar surface.
In astronomy, the term "blue moon" is used to determine the second full moon in a calendar month, which happens about once every 2.5 years. Originally, it was formed by the English idiom «Once in a Blue Moon» (born Once in blue moon). It denotes a phenomenon that happens very rarely, and roughly corresponds to the Russian "When cancer at Mount whistle."
In reality, during the "Blue Moon" is a familiar companion color — yellow or ashen. However, in some regions of the world can see the moon, painted in blue color, this is due to the volcanic activity, or clouds — light blue spectrum, which is the most intense scattered in the Earth's atmosphere.