A Blood-Red Moon
The first—and most common—usage of the term points to a fully eclipsed Moon. If you’ve ever witnessed a lunar totality, you may understand how this term came about and why it has become so popular: As the Earth’s shadow blocks all direct sunlight from falling onto the lunar surface, the Moon usually takes on a reddish color.
You can see the effect in the image below, which was taken during the total lunar eclipse in September 2015.
A Blood Moon gracing the night sky in September 2015.
Deanne Fortnam
Eclipses in your city (with animations)
Why Are Blood Moons Red?
A total lunar eclipse happens when the Moon travels through the Earth’s umbra, which blocks all direct sunlight from illuminating the Moon’s surface. However, some red light still reaches the lunar surface indirectly, via the Earth’s atmosphere, bathing the Moon in a reddish, yellow, or orange glow.
As the Sun’s rays pass through the atmosphere, some colors in the light spectrum—those towards the violet spectrum—are filtered out by a phenomenon called Rayleigh scattering. This is the same mechanism that causes colorful sunrises and sunsets. Red wavelengths are least affected by this effect, so the light reaching the Moon’s surface has a reddish hue, causing the fully eclipsed Moon to take on a red color.
Depending on the composition of the atmosphere, different parts of the light spectrum are filtered out, so the Moon may also look yellow, orange, or brown during a total lunar eclipse.
What you can see during a total solar eclipse
Lunar Tetrads
The second definition of Blood Moon has spiritual origins, but it still refers to an actual astronomical phenomenon: a lunar tetrad, a term astronomers use to describe four total lunar eclipses that happen in the span of two years.
The eclipses in a tetrad occur about six months apart, with five uneclipsed Full Moons between them. Usually, only about one in three lunar eclipses are total, and about four to five total eclipses can be seen from any single location on Earth in a decade (though this ratio can vary by century). This means that lunar tetrads are rare occurrences, leading some to attach special, even religious, significance to these events.
The 2014–2015 lunar tetrad gathered a lot of attention because some religious organizations claimed that the eclipses in the tetrad were a sign of the end times. They called the eclipses Blood Moons after a statement in the Book of Joel in the Hebrew Bible that referred to the Sun turning dark and the Moon turning red before the second coming of Jesus.
Other doomsday prophecies that fell short
A Blood Moon hangs over the Royal Pavilion in Brighton, UK.
©iStockphoto.com/oversnap
How Often Do Lunar Tetrads Happen?
Italian astronomer Giovanni Schiaparelli calculated that the frequency of lunar tetrads varies over centuries. Some have several lunar tetrads, while others do not have any. For example, the years between 1582 and 1908 did not have any tetrads, whereas the next 250 years—from 1909 to 2156—will have 17 tetrads.
The current century—2001 to 2100—will have eight tetrads. The first tetrad of the 21st century took place in 2003-2004, the second was in 2014–2015, and the next will be in 2032–2033, with the following eclipses:
Hunter’s Moon
The third definition of a Blood Moon is unrelated to lunar eclipses. It originates from certain North American traditions, which use the term to describe the Full Moon in October.
It refers to the fact that October is usually the time of the year to hunt game and prepare meat for the coming winter. The October Full Moon is also known as Hunter’s Moon or Harvest Moon.
