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Purple Sunsets

28 October 2008

A Purple Twilight over the Plateau of Giza
The photo shows a purple twilight over two of Giza’s mighty Pyramids, the Pyramid of King Khephren (right) and the Pyramid of King Mycrinus (left).
Photo by Aymen Ibrahem, Senior Astronomy Specialist

Currently, Northern Hemisphere observers are experiencing a fascinating celestial phenomenon: sunsets and evening twilights are tinted in hues of purple, rather than the common reddish hues.

A powerful volcanic eruption took place in Alaska, on 7 August 2008. The Kasatochi Volcano hurled more than a million tones of volcanic ash into the atmosphere. Now, patchy clouds of this volcanic debris are drifting in the Northern Hemisphere, giving rise to fascinating sunsets of subtle unusual hues. Interestingly, the precise mechanism of producing this purplish glow is not yet fully understood.

When sunlight passes through the dense air layers near the horizon, the shorter wavelengths of light (violet and blue) are efficiently scattered by particles suspended in the atmosphere. This renders sunset beautiful yellow and red hues.

When the sky is hazy, with a high concentration of particles that are slightly larger than air molecules, e.g., smoke or dust, violet, blue and yellow are scattered. Therefore, only the longer wavelengths (orange and red) reach the observer's eyes, rendering the Sun disk an orange-reddish hue.

Red sunsets are often observed from shores, due to the high concentration of salt particles in the air over the sea. These particles effectively scatter shorter wavelengths of light, producing red sunsets. Dust and ash particles blasted into the atmosphere by volcanic eruptions can also cause red sunsets.

BA Senior Astronomy Specialist Aymen Ibrahem observed the “purple sunsets”, and produced interesting images of the colorful spectacle.

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