On Friday, the entire world will be able to observe the "blue moon." This is a fairly rare event, but the color of Earth's natural satellite is not irrelevant. July 31, there will be a second full moon this month - an event that happens once every 2, 7 years old.
Typically, one year of full moons occur 12 - one for each month. However, in some years, including 2015, thirteen full moon. One of these "extra" full moon is called "Blue Moon».