Lunar eclipses and solar cycles
posted: May 7, 2014

On 15 April, a total lunar eclipse was visible from the America's and the Pacific. Some websites were buzzing about the "Blood Moon", as if werewolves and witches were going to team up and unleash a veritable apocalypse upon us. Fortunately, reality was a lot less dreadful and a lot more beautiful.

Total lunar eclipses occur when Sun, Earth and Moon (in that order) are on one line. The full moon then passes through the shadow cast by the Earth (sketch above). The dark shadow would normally make the moon completely invisible. However, Earth is surrounded by an atmosphere which refracts and scatters the light from the Sun that passes through. As the blue light is scattered more by the atmosphere than red, the resulting effect is a faint, reddish illumination of the moon's surface, even during the midst of totality. The effect is the same as the one that causes the red hues during sunset and sunrise here on Earth, and some bloggers eloquently wrote that the eclipsed moon gets illuminated by the sunsets and sunrises from all over the Earth.

Picture underneath is a screenshot from the livestream by Griffith Observatory, taken near mid-totality (07:46UT).

In 1921, André-Louis Danjon created a luminosity scale "L" to quantify the brightness of the eclipsed moon. "L" could vary between "0", meaning that the Moon was almost invisible, and "4" which meant that the eclipsed moon had a bright orange or copper red hue. From a study of descriptions of 70 lunar eclipses going back till 1823, he found that during the first two years after a solar minimum the eclipsed moon was rather dark and grayish. In subsequent years, the luminosity gradually increased during the solar cycle, with nothing particular happening around solar cycle maximum. Then, around the next minimum, the brightness of the eclipsed moon abruptly decreased back to very low L-values.

For this study, Danjon left out 3 eclipses from 1884 and 1885. Indeed, the eruption of the Krakatau in August 1883 caused very dark eclipses, whereas normally bright eclipses were expected (maximum of solar cycle 12). It highlights a weakness in the relationship discovered by Danjon, as the deduced L-values are influenced not only by strong volcanic eruptions, but can also be impacted by high altitude dust and aerosols (desert sand, wildfires, air pollution,...) and clouds. Also the depth of the immersion of the moon into the Earth’s shadow has its influence.

All these factors have an important impact and make it very difficult to validate Danjon's initial assertion. In fact, there's now general agreement that the brightness and color of lunar eclipses are dictated only by the contents and cloudiness of the mid and upper Earth atmosphere. The Sun is considered as a passive light source, with no measurable influence of the solar activity on the eclipse's color. So despite the 15 April eclipsed moon was reported to be bright (L=3), this is rather a hint at a relative clear Earth atmosphere at the time of the eclipse and not a result of some solar cycle effect. The next eclipse is on 8 October 2014, and it will be the second of 4 consecutive total lunar eclipses (a so-called "tetrad"). So, there will be plenty of opportunities for further evaluation of this "Blood Moon"!

Further reading - The Danjon paper (in French!) can be found here. More recent discussions on the effects of clouds and air pollution can be found at Sky and Telescope (2011), and in this paper (2009).