There has been a lot of interest recently in an upcoming series of lunar eclipses that begins April 15. These are usually described as "four blood moons" and taken by some to prophesy upcoming disasters.
This NASA graphic shows the months with total lunar eclipses between April 2014 and September 2015. This tetrad of total lunar eclipses features eclipses on April 15, Oct. 8, April 4, 2015 and Sept. 28, 2015.
The total lunar eclipse of April 15 will begin a so-called tetrad series of eclipses that is making the rounds online as a potential harbinger of doom, due in part to a recent book on the four blood moons that makes the dubious claim.
Astronomers rarely if ever use the term blood moon. When they do, they are usually using it as an alternate name for the Hunter's Moon, the full moon that follows the Harvest Moon, usually in late October. The Hunter's Moon, like the Harvest Moon, rises slowly on autumn evenings so that it shines through a thick layer of the Earth's atmosphere, and is colored red by Rayleigh scattering and air pollution. [Four Blood Moons: Lunar Eclipse Tetrad Explained (Video)]
This NASA graphic depicts the position of the moon in Earth's shadow during the total lunar eclipse of April 15, 2014 at 3 a.m. ET. It is the first of four consecutive total lunar eclipses, a tetrad, between April 2015 and September 2015.
A lunar eclipse is something quite different. It occurs when the moon passes through the Earth's shadow.
The Earth's shadow consists of two parts: a dark inner core called the "umbra," and a lighter outer part called the "penumbra." Rather than being truly dark, the inner shadow is usually tinted orange or red by light passing through the ring of atmosphere surrounding the Earth.
Depending on the atmospheric conditions on Earth in the band of atmosphere through which the sun's light is passing, the umbra may take on a range of colors from light coppery-red to almost total black. The light illuminating an eclipsed moon is coming from thousands of sunsets and sunrises around the Earth. During some eclipses, these sunsets and sunrises are clear, and much light passes through; during others, clouds may block the light, causing a dark eclipse.
What makes the moon turn dark and red? Find out in the full SPACE.com infographic here.
Credit: Karl Tate, SPACE.com Contributor
On rare occasions, the light reaching the moon is exactly the color of blood, but there is no way of predicting this in advance. So there are no grounds to call any particular lunar eclipse a blood moon until it actually shows its color.
This NASA graphic shows where the total lunar eclipse of April 14-15, 2014 will be visible from. The lunar eclipse coincides with April's full moon and is the first of four total lunar eclipes (a tetrad) between April 2014 and September 2015.
Because the moon's orbit is slightly tilted with respect to the sun's path across the sky, most of the time the moon passes above or below the Earth's shadow, and no eclipse occurs. Sometimes it passes only through the penumbra and produces what is called a penumbral eclipse, a moon so lightly shaded that the casual observer might not even notice a difference ( via space.com ).