What is blood moon?

A lunar eclipse lasts for hours as the moon slowly orbits through the Earth's shadow, but the eclipse we witnessed on Saturday was the briefest in the history. If the moon passes through the edge of Earth's shadow, it's called a partial eclipse. Passage through the dense centre of the shadow is a total lunar eclipse. When the entire moon enters the umbra, nearly all the sunlight is blocked from hitting it. This is called totality, and it's during this stage that the darkened moon will glow red — which is why lunar eclipses are sometimes called blood moons.

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 in the evenings so that it shines through a thick layer of the Earth's atmosphere, and is coloured red by Rayleigh scattering and air pollution.

The first Blood Moon eclipse in a series of four happened on the night of April 14-15, 2014. The second one took place on the night of October 7-8, 2014. The third one has just happened on April 4, 2015 and which was visible for a few across the globe. A series of four total lunar eclipses is called a tetrad. Astronomy had not heard the term Blood Moon used in quite this way before this year, but now the term has become widespread in the media. The origin of the term is religious, at least according to Christian pastor John Hagee, who wrote a 2013 book about Blood Moons.

The 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. Thus the term blood moon can be and probably is applied to any and all total lunar eclipses. It’s only in years where volcanic activity is pronounced that the moon’s face during a total lunar eclipse might appear more brownish or gray in colour. Usually, the moon looks red.