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The term Blood Moon once was used in some sects of Christian prophecy to describe a total lunar eclipse that belonged to a tetrad of total lunar eclipses. The most recent Blood Moon – at least by this definition – took place on September 28, 2015. The next one won’t come until April 25, 2032.

Around the year 2013, when the expression Blood Moon first gained widespread popularity, a Blood Moon didn’t refer to just any total lunar eclipse, but to a member of a special series: four totally eclipsed moons in a row, each separated by six lunar months, with no partial lunar eclipses in between. Such a series is called a lunar tetrad. Two Christian pastors popularized the term Blood Moon in their book, in which they discussed the upcoming lunar tetrad (April 14 and October 8, 2014; April 8 and September 28, 2015) in apocalyptic terms.

The four lunar eclipses came and went, and, although the world didn’t end, we gained another dubious meme: Blood Moon.

It’s not that the words “blood” and “moon” never appeared together in the same sentence before 2013, especially as regards eclipses. A full moon nearly always appears coppery red during a total lunar eclipse. That’s because the dispersed light from all the Earth’s sunrises and sunsets falls on the face of the moon at mid-eclipse. For some decades at least, it’s been common for astronomy writers like us to describe an eclipsed moon as blood red. You just didn’t, until recently, hear the same eclipsed moon called a Blood Moon.

And there’s another kind of Blood Moon in astronomy, unrelated to eclipses. In folklore, all the full moons have names. The names typically coincide with months of the year, or seasons. One of the most famous moon names is the Hunter’s Moon. It’s the full moon immediately following the Harvest Moon, which is the full moon occurring most closely to the autumnal equinox.

The Hunter’s Moon, in skylore, is also sometimes called the Blood Moon. Why? Probably because it’s a characteristic of these autumn full moons that they appear nearly full – and rise soon after sunset – for several evenings in a row. Many people see them when they are low in the sky, shortly after they’ve risen, at which time there’s more atmosphere between you and the moon than when the moon is overhead. When you see the moon low in the sky, the extra air between you and the moon makes the moon look reddish. Voila. Blood Moon.

Still, the term Blood Moon in regard to total lunar eclipses might have been given new meaning by the Christian pastor John Hagee, who wrote a 2013 book about Blood Moons.

What is a lunar tetrad? The word tetrad refers to four things in a group. In astronomy, the word is used to describe four successive total lunar eclipses, with no partial lunar eclipses in between, each of which is separated from the other by six lunar months (six full moons).

Nowadays, each of these four-in-a-row totally eclipsed moons has come to be called a Blood Moon. Why? We don’t know for sure, but the answer may lie in the book pictured above.

We’re not experts on prophecy of any kind. But we’ll tell you what we know about the modern definition for Blood Moon that raised many questions over years.

From what we’ve been able to gather, two Christian pastors, Mark Blitz and John Hagee, used the term Blood Moon to apply to the full moons of the 2014-2015 tetrad. John Hagee appears to have popularized the term in his 2013 book Four Blood Moons: Something is About to Change.

Mark Blitz and John Hagee spoke of the 2014-2015 lunar tetrad as representing a fulfillment of Biblical prophecy. After all, the moon is supposed to turn blood red before the end times, isn’t it? As described in Joel 2:31 (Common English Bible):

The sun will be turned to darkness, and the moon to blood before the great and dreadful day of the LORD comes.

That description, by the way, sounds like two different astronomical events: a total solar eclipse and total lunar eclipse. Sun turned to darkness = moon directly between the Earth and sun in a total solar eclipse. Moon turned to blood = Earth directly between the sun and moon, Earth’s shadow falling on the moon in a total lunar eclipse. Solar and lunar eclipses are very ordinary and frequent happenings that have occurred many times in our lifetimes. In fact, every year, there are four to seven eclipses, some lunar, some solar, some total, some partial.

The last lunar tetrad took place over the years 2014 and 2015. The first Blood Moon eclipse in the series of four happened on the night of April 14-15, 2014. The second one took place October 7-8, 2014, and the third one (the shortest total lunar eclipse of the 21st century) was April 4, 2015.

The fourth and final total lunar eclipse of the 2014-2015 tetrad – the last Blood Moon – fell on the night of September 27-28. It was also a supermoon. The eclipse was visible to sky watchers in North America, and the partial phases were seen from various places around the world. Many saw it!

Best photos: September 27-28 total lunar eclipse

When is the next lunar tetrad? There are a total of 8 tetrads in the 21st century (2001 to 2100). The next four total lunar eclipses in a row (spaced six lunar months apart, with no partial lunar eclipses in between) will take place in 2032 and 2033. Click here to learn more about lunar tetrads in this century, courtesy of Fred Espenak.

How common is a tetrad of total lunar eclipses? Depending upon the century in which you live, a lunar tetrad (four consecutive total lunar eclipses) may happen fairly frequently – or not at all.

For instance, in our 21st century (2001-2100), there are a total of eight tetrads, but in the 17th, 18th and 19th centuries, there were none at all. If we include all the centuries from the 1st century (A.D. 1-100) through the 21st century (2001-2100), inclusive, there are a total of 62 tetrads. The last one happened in 2014-2015, and the next one will happen in 2032-2033.

The 2014-2015 tetrad fell on the Jewish feasts of Passover and Tabernacles. That happens a total of eight times in these 21 centuries:

1. 162-163 C.E. (Common Era)

2. 795-796 C.E.

3. 842-843 C.E.

4. 860-861 C.E.

5. 1493-1494 C.E.

6. 1949-1950 C.E.

7. 1967-1968 C.E.

8. 2014-2015 C.E.

Bottom line: The term Blood Moon in Biblical prophecy appears to have been popularized by two Christian pastors, Mark Blitz and John Hagee. They used the term to apply to the full moons of the 2014-2015 lunar tetrad – four successive total lunar eclipses, each separated by six lunar months, with no partial lunar eclipses in between. Nowadays, it seems, the term Blood Moon is being used for any and all total lunar eclipses.

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