Moon Dogs, Paraselenae, Iceland

Last updated on September 16, 2023

Moon Dogs, Paraselenae, Iceland: A Stunning Display of Atmospheric Optics

Have you ever witnessed a mesmerizing celestial spectacle in the night sky? If not, let me introduce you to the captivating phenomenon of moon dogs, also known as paraselenae. These enchanting optical displays occur when ice crystals in the atmosphere interact with the moon's light, creating a breathtaking visual feast for those fortunate enough to witness it. One particularly remarkable sighting of moon dogs took place near Mt Hekla, Iceland, on the 21st of March in 2008.

On that chilly evening, with temperatures ranging between -5 and -7°C, nature treated observers to a remarkable display. Interestingly, there was no presence of diamond dust, the usual culprit behind such atmospheric phenomena. Instead, Ágúst Guðmundsson's collection of images from that night revealed that it was the crystals in high cirrus clouds that produced the stunning spectacle. These delicate ice crystals acted as prisms, refracting and reflecting the moonlight in mesmerizing ways.

The centerpiece of this ethereal display was the moondogs themselves. Moondogs are bright spots that appear on either side of the moon, mimicking its shape. They are caused by the refraction of moonlight through hexagonal ice crystals suspended in the atmosphere. As light passes through these ice crystals, it is bent and split into two distinct beams, resulting in the appearance of these luminous celestial companions.

But the moon dogs were not the only celestial delights on offer that night. Accompanying them was a paraselenic circle, gracefully coursing through the sky at an elevation of 18° above the moon. This circular phenomenon occurs due to the interaction between light and plate-shaped ice crystals. These crystals act as miniature mirrors, reflecting and refracting light to create this stunning halo-like feature.

Adding to the enchantment was a 22° halo encircling the moon, positioned above the paraselenic circle. This halo is formed when moonlight passes through randomly oriented ice crystals, resulting in the bending and scattering of light. The end result is a magnificent ring of light that captivates observers and adds to the otherworldly ambiance of the scene.

But the celestial show did not stop there. High above, a lunar circumzenithal arc graced the sky. This rare and captivating arc occurs when sunlight interacts with ice crystals in the upper atmosphere. The result is a stunning display of colors, resembling an upside-down rainbow arcing across the sky. To make this sighting even more special, the lunar circumzenithal arc was touched by either a rare 46° halo or supralateral arc, further enhancing the splendor of the display.

To witness such an extraordinary celestial event is a truly awe-inspiring experience. The intricate interplay between light and ice crystals in the atmosphere creates a symphony of colors and shapes that leaves observers spellbound. Whether it be the moondogs, paraselenic circle, 22° halo, lunar upper tangent arc, or the lunar circumzenithal arc with its accompanying halo or supralateral arc, each element contributes to the overall magnificence of the display.

It is in moments like these that we are reminded of the beauty and complexity of our natural world. The delicate dance between light and ice crystals in the atmosphere unveils a tapestry of wonder that is both humbling and breathtaking. So, keep your eyes to the sky, for you never know when nature will unveil another extraordinary display of atmospheric optics, inviting you to witness the magic that lies just beyond our reach.

Moondogs near Mt Hekla, Iceland imaged by Ágúst Guðmundsson on 21st March '08. ©�g�st Gu�mundsson, shown with permission

Although the temperature was between -5 and -7°C there was no diamond dust and Ágúst's many images indicate that crystals in high cirrus produced the display. In addition to the bright moondogs a paraselenic circle courses through the 18° high moon. A 22° halo circles it topped by a lunar upper tangent arc. That was not all, high above was a lunar circumzenithal arc touched by either a rare 46° halo or supralateral arc.

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