Many of us have heard of the ‘blue moon’. This is when a full moon appears twice in one calendar month. This is a perfectly predictable astronomical phenomenon that occurs once every two or three years, infrequent enough to serve as a metaphor for rarity, as in the phrase ‘once in a blue moon’.

But there is also a lesser known and rarer phenomenon known as the lunar tetrad that consists of four lunar eclipses in a row in which the moon will take on a reddish hue. The first one will occur tonight and the others will appear at six-month intervals, on October 8 and next year on April 4, and September 28. NASA has put out an FAQ explaining what the lunar tetrad is and why the moon looks red at those times.

A quick trip to the Moon provides the answer: Imagine yourself standing on a dusty lunar plain looking up at the sky. Overhead hangs Earth, nightside down, completely hiding the sun behind it. The eclipse is underway.

You might expect Earth seen in this way to be utterly dark, but it’s not. The rim of the planet is on fire! As you scan your eye around Earth’s circumference, you’re seeing every sunrise and every sunset in the world, all of them, all at once. This incredible light beams into the heart of Earth’s shadow, filling it with a coppery glow and transforming the Moon into a great red orb.

It also talks about the erratic frequency of this event.

“During the 21st century, there are 8 sets of tetrads, so I would describe tetrads as a frequent occurrence in the current pattern of lunar eclipses,” says [NASA eclipse expert Fred] Espenak. “But this has not always been the case. During the three hundred year interval from 1600 to 1900, for instance, there were no tetrads at all.”

This is good clean fun for astronomy buffs and night sky watchers but Christian evangelical leader John Hagee sees this as a sign of the end times and that some major event like the apocalypse will happen in October 2015. And unlike Harold Camping who kept wrongly predicting the end of the world and died before it happened, Hagee has solid evidence, such as this passage from the Bible, Acts 2:19-20 that says: “And I will show wonders in Heaven above and signs in the Earth beneath, the sun shall be turned into darkness and the moon into blood before the coming of the great and awesome day of the Lord.”

Incidentally, the term ‘blood moon’ was not used by astronomers to describe the lunar tetrad and became popularized recently by religious people among whom was (you guessed it) John Hagee. This made it easier to relate it to the Bible, which is your standard religious tactic.

But Hagee is not alone in this craziness. Predicting the end of the world based on this seems to be a cottage industry that has slipped under the radar of most of us but is hotly discussed by the religious faithful.

What gets me is how eager these religious people are to see the end of the world. There is something about massive death and destruction (like the Noah story) that seems to really turn them on.