Rarer than a blue moon?

Last updated on September 16, 2023

Rarer than a Blue Moon? Unveiling the Enigmatic Etruscan Vase Moonrise Mirage

Have you ever witnessed the mesmerizing sight of a moonrise mirage? Imagine gazing towards the eastern horizon, where the vast expanse of the Atlantic Ocean meets the sky, and catching a glimpse of not just one, but two lunar images. This phenomenon, known as the "Etruscan Vase" or "Omega" moonrise, is a rare occurrence that has captivated skywatchers for centuries. In this article, we will delve deeper into the intricacies of this elusive optical illusion and explore the science behind its enchanting display.

The ethereal beauty of the Etruscan Vase moonrise mirage was recently captured by photographer John Stetson on March 12th, 2009, off the coast of Cape Elizabeth, Maine. This was John's second encounter with this elusive phenomenon, having witnessed it just a month prior. Intrigued by its rarity, he pondered whether such consecutive sightings are truly "rarer than a blue moon."

The striking visual effect of the Etruscan Vase moonrise mirage is achieved through the interaction of light with Earth's atmosphere. As the moon rises, its rays undergo a lengthy journey through the atmosphere, causing them to scatter and interact with particles in the air. This scattering process selectively removes blue light from the moon's rays, resulting in the blue skies we observe during the day.

To fully comprehend the intricacies of the Etruscan Vase moonrise mirage, let us examine its composition. The mirage consists of two distinct lunar images: an upper image that appears erect and rises akin to the "real" moon, and a lower image near the horizon that is inverted and gradually separates from the upper image before descending into the ocean.

The genesis of this captivating mirage lies in the interplay of temperature and air density gradients within the atmosphere. A layer of warm air, heated by contact with the ocean's surface, resides above a cooler layer of air. The transition between these two layers creates a density gradient, causing rays from the rising moon to refract downwards and then back upwards. This refraction, which occurs due to the variation in air density, deflects the moon's rays towards the denser and colder layer.

This particular mirage is closely associated with another captivating atmospheric phenomenon: the green flash. The region between the two lunar images exhibits significant vertical magnification and color separation, which extends the otherwise invisible green rim at the upper limb of the moon into a brilliant green flash. The third image captured by John Stetson beautifully showcases this distinctive feature.

The allure of the Etruscan Vase moonrise mirage lies not only in its visual splendor but also in its enigmatic shape. The mirage resembles an elegant vase or the Greek letter "Omega," lending it its namesake. However, it is worth noting that no physical Etruscan Vase has been discovered to precisely replicate the shape of this ethereal mirage.

In conclusion, the Etruscan Vase moonrise mirage is a truly extraordinary spectacle that infrequently graces our skies. Its formation relies on the intricate interplay of temperature and air density gradients, resulting in the captivating display of two lunar images and the occasional appearance of a green flash. As skywatchers continue to marvel at its beauty, we are reminded of the immense wonders that await us in the ever-changing tapestry of our atmosphere. So keep your eyes to the sky, for you never know when you might witness a celestial phenomenon rarer than a blue moon.

Etruscan vase Moonrise Mirage - Version II by John Stetson, Maine - March 12th 2009.

©John Stetson, shown with permission.

John saw the first 'Etruscan Vase' or 'Omega' moonrise on February 9th looking eastwards over the Atlantic Ocean from Two Lights State Park in Cape Elizabeth, Maine. This is his second, caught near to the very next full moon! "I wonder if this occurring twice in consecutive months happens more than once in a blue moon (if I may mix moon metaphors)."

The moon was once again reddened by the long passage of its low rays through Earth's atmosphere which preferentially scatters away blue light to form our blue skies.

The mirage consists of two lunar images images. The upper image is erect and rises as the 'real' moon does. The lower image near the horizon is inverted, slowly separates from the upper one and descends into the ocean.

The mirage is produced by a layer of warm air heated by contact with the ocean surface. The warm air is topped by cooler air. The resulting temperature and air density gradients refract downward rays from the rising moon back upwards. Refraction through a density gradient always tends to deflect rays towards the denser (colder) layer.

This is the mirage associated with classic green flashes. The third image in John's sequence shows well the region between the two moon images where there is considerable vertical magnification and colour separation. The region stretches the normally invisible green rim at the moon's or sun's upper limb into a brilliant green flash.

Has anyone found an Etruscan Vase that really looks like the mirage shape?!

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Reference Atmospheric Optics

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