Washington, Jul 30 (EFE).– On Friday, July 31, a rare so-called “blue moon” will be seen in the sky, an event that occurs only once every three years or so and which will not recur until January 2018.

As the U.S. Naval Observatory says on its Web page, a so-called “blue moon” is a second full moon within the same calendar month.

Each lunar cycle between full moons lasts approximately 28 days, and so when there is a full moon very early in a given month it is probable that there will be a second one very late in the month.

That is what will occur this month, which had its first full moon on July 2 and will have its second one on the 31st.

Although it’s called a “blue moon,” that has nothing to do with the moon’s color, which remains white and gray.

According to astronomical experts, the moon can – in fact – assume a bluish tinge on very rare occasions due to the effect of dust, ash or smoke in the atmosphere after volcanic eruptions or forest fires.

The most recent blue moon occurred on Aug. 31, 2012. EFE