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'Half-Blood' lunar eclipse delights stargazers in South America, Europe, Africa, Asia

ABC News logo ABC News 7/17/2019
a large skyscraper in a city: The moon over London during a partial lunar eclipse, July 16, 2019. © Jamie Cooper/Rex/Shutterstock The moon over London during a partial lunar eclipse, July 16, 2019.

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.

a view of a city with tall buildings in the background: The moon over London during a partial lunar eclipse, July 16, 2019. © Jamie Cooper/Rex/Shutterstock The moon over London during a partial lunar eclipse, July 16, 2019.

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.

(MORE: A total solar eclipse seen over South America)

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.

a large mountain in the background: The Stromboli volcano erupts, July 3, 2019, as seen from the nearby island of Panarea, Italy. © Fiona Carter/Twitter via AFP/Getty Images The Stromboli volcano erupts, July 3, 2019, as seen from the nearby island of Panarea, Italy.

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.

(MORE: Total solar eclipse wows crowds across the US)

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.

(MORE: Longest total lunar eclipse of 21st century wows star-gazers for over 100 minutes)

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.

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