In recent years, the term Blood Moon has become popular when referring to Total Lunar Eclipses. A lunar eclipse occurs when the earth passes between the sun and the moon. This blocks the sun’s rays from reflecting off the moon as normal. However, some of the sun’s rays curve around the earth causing the moon to appear red during a total eclipse. Because of its vivid color, a total lunar eclipse is often referred to by NASA as a Blood Red Moon.
In astronomy, a tetrad is a set of four total lunar eclipses within two years. On average, lunar eclipses occur about twice a year, but not all of them are total. During the 21st century, there are 8 sets of tetrads, 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! Tetrads are rather rare, only taking place fifty-five times since 1 AD.
Four blood moons are going to occur between April 15, 2014 and September 28, 2015. The action started on April 15th, 2014 when the full Moon passed through the amber shadow of Earth, producing a midnight eclipse. So began a lunar eclipse tetrad — a series of 4 consecutive total eclipses occurring at approximately six month intervals. The total eclipse of April 15th, 2014, is followed by another on October 8th, 2014, and another on April 4th, 2015, and another on September 28th, 2015.
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.
My schoolmate Debabroto Raha sent me these images of “Blood Moon” on the night of Lunar Eclipse of April 4th, 2015 as seen from Army mess at Leh, in Ladakh.