A blood moon is a phenomenon that occurs during a total lunar eclipse when the Earths moon is in the Earths shadow, causing it to turn a reddish hue. The red color is caused by sunlight passing through the Earths atmosphere and being scattered, with the red component of sunlight being preferentially filtered and diverted into the Earths shadow where it illuminates the eclipsed moon, making it appear red or "blood" color. The exact color that the moon appears depends on the amount of dust and clouds in the atmosphere, and how red the moon appears can depend on how much pollution, cloud cover, or debris there is in the atmosphere. For example, if an eclipse takes place shortly after a volcanic eruption, the particles in the atmosphere will make the moon look darker than usual. It is important to note that a blood moon is not a term used in astronomy, but rather a popular phrase that refers to a total lunar eclipse.