Two weeks after the weather gods shone down on South America’s mid-winter total solar eclipse, sky watchers in Australia, Asia, Africa, Europe, and South America enjoyed a Half-Blood Moon lunar eclipse Tuesday.
Although it wasn't a total lunar eclipse, the moon still turned red to many viewers as Earth’s shadow engulfed it.
Many eclipse watchers experienced an additional treat as the eruption on July 3 of the Mt. Stromboli volcano off the north coast of Sicily resulted in an additional depth of color due to the atmospheric pollution caused by the dust and ash the volcano has been spewing into the sky.
Dust and other pollutants in the atmosphere scatter light. Shorter wavelengths, seen as blues, are scattered more than the longer wavelengths, seen as reds -- it’s why sunsets and sunrises are orange or red.
During a lunar eclipse, the longer wavelength red light can penetrate Earth’s atmosphere and reach the moon’s surface. The more dust in the atmosphere, the greater the effect -- and with huge quantities of volcanic dust still pouring into the sky, this was an unusually dark blood-red partial lunar eclipse.
So why was this eclipse a Half-Blood Moon lunar eclipse? Well, there are two parts to Earth’s shadow: the outer penumbral shadow and the inner umbral shadow. When the moon enters the penumbral shadow, we see a slight darkening on the surface of the moon -- but no change in color.
It is only when the moon enters the inner umbral shadow that we see Earth’s natural satellite change color -- and the further it enters the umbral shadow, the deeper the eclipse and color.
This lunar eclipse was partial, with a little more than half of the moon entering the umbral shadow -- but this was sufficient to see the moon change color. Combined with the volcanic dust polluting the atmosphere, it resulted in a very special display.
For those in the northern hemisphere, the full moon remains low to the horizon at this time of year. The already red light reaching our eyes and telescopes reflected by the moon was filtered again through Earth’s atmosphere, increasing the depth of the color even more.
Paul Cox is the Chief Astronomical Officer of the astronomy website Slooh.com.