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The Blood Moon of April

Updated on April 2, 2015
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Why does the moon turn Red?

As we know the moon shines by reflecting sunlight. A total lunar eclipse occurs when the earth moves between the sun and moon. This effectively blocks much of the glow of the moon and produces a reddish glow earning it the moniker of a “Blood Moon.”
The principal of Rayleigh scattering explains why the moon turns red during the lunar eclipse. In fact, it is the same process which gives us colorful sunsets. Rayleigh scattering is named after Lord Rayleigh a British physicist.

John William Strutt, better known as Lord Rayleigh, the Third Baron Rayleigh developed a lengthy complicated equation explaining the scattering of solar radiation by the atmosphere. It is a difficult equation to explain the process, but the important thing take away is that the scattering is inversely proportional to the fourth power of the wavelength.

This means in layman's terms that the longest wavelengths are in the red/orange spectrum are scattered the least and the shortest wavelengths in the blue/violet range scatter the most. This is why the sky is blue and the sun takes on a reddish hue as sunrise and sunset.

As the sun’s radiation passes through the Earth’s atmosphere during a lunar eclipse, allowing red light to pass through and be reflected by the earth producing a blood moon. .

People who are avid eclipse watchers say that if you look closely at the very beginning and at the end may see a blue band on the moon's face this ring is caused by our ozone layer scattering the red spectrum light and allows some blue light through. Because of this, at the height of the eclipse when the moon turns red we are actually seeing every sunrise and sunset on earth

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How do we rate eclipses?

The Danjon Scale is how scientists measure appearance and brightness of a lunar eclipse. The Danjon Scale was created by Andre-Louise Danjon a French astronomer in 1921 it was believed the brightness of a lunar eclipse was related to the solar cycle.

The Danjon Scale brightness provides a useful tool for measuring the appearance and luminosity of the moon during a lunar eclipse. An eclipse's rating on the Danjon Scale is traditionally denoted by the letter L and measures the brightness of the eclipse on the 5 point scale below.

Determination of the value of L for an eclipse is best done near mid-totality with the naked eye. The scale is subjective, and different observers may determine different values. In addition, different parts of the Moon may have different L values, depending on their distance from the center of the Earth's umbra.

Many factors can affect the appearance of the Moon during a lunar eclipse. The Moon's path through the Earth's umbr is important, but so too are the current conditions of the earth's atmosphere. While the Earth's shadow blocks any direct light from striking the Moon during a lunar eclipse, some light is refracted through the Earth's atmosphere giving a Moon a red hue.

The amount of light refracted affects the brightness of the moon at mid-eclipse, and this depends on several factors. For instance volcanic eruptions are one of the most significant. These eruptions fill the air with ash resulting in deep dark red lunar eclipses. one of the most significant of this effect. The eruption at Mount Pinatubo in early 1992 caused the lunar eclipse in December of that year to be rated a 0 on the on the Danjon Scale..

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