Lunar Corona - OPOD

Last updated on September 16, 2023

Lunar Corona: A Spectacular Atmospheric Phenomenon

Have you ever looked up at the night sky and been captivated by the beauty of a lunar corona? This enchanting optical phenomenon occurs when sunlight or moonlight is diffracted by clouds or fog droplets in the atmosphere. The result is a stunning display of rings surrounding the moon or sun, creating a celestial spectacle that leaves us in awe.

When observing a lunar corona, you may notice a large blue ring around the moon with the naked eye. This captivating color is a result of diffraction, where the light waves are bent as they pass through the tiny water particles in the air. These particles, illuminated by an LED flashlight, appear to swirl around the camera equipment, adding to the magical ambiance of the scene.

To capture the beauty of a lunar corona in a photograph, it is important to position the moon at the center of the camera viewfinder to avoid lens flare. The photographer in this particular instance used a Canon 17-40mm lens set at 27mm, with an aperture of f/4.0 and a shutter speed of 1 second. The ISO was set at 2500, capturing the intricate details of the corona with a Canon 5D MKII camera.

The phenomenon of coronae arises from the diffraction of sunlight or moonlight by clouds or fog droplets. Each individual droplet produces its own unique diffraction pattern, and what we witness in the sky is the collective effect of millions of these diffraction glints. It is this collective diffraction that creates the mesmerizing rings we see encircling the moon during a lunar corona event.

The purity and complexity of coronae are influenced by the size and distribution of the cloud or fog droplets. In freshly formed fog or cloud, monosized droplets create coronae with multiple rings, resulting in a striking visual display. However, as fog ages, the droplet size distribution widens and the mean size of the droplets increases. This variation in droplet sizes leads to coronae of different sizes appearing in the sky, often resulting in a more blurred and less defined ring formation.

It is fascinating to note that coronae can be observed not only around the moon but also around the sun. However, lunar coronae tend to be more commonly observed due to the moon's inherent brightness and the contrast it creates against the night sky. The delicate interplay between light, atmospheric conditions, and the size and distribution of droplets all contribute to the enchanting display of a lunar corona.

In conclusion, a lunar corona is a breathtaking atmospheric phenomenon that occurs when sunlight or moonlight is diffracted by clouds or fog droplets in the atmosphere. The resulting rings of light encircling the moon create a captivating spectacle that leaves us in awe of the wonders of nature. Whether observing with the naked eye or capturing its beauty through photography, a lunar corona is a reminder of the intricate and awe-inspiring nature of our atmosphere. So next time you find yourself gazing at the night sky, keep an eye out for this celestial marvel and let it transport you into a world of wonder and beauty.

Lunar Corona ~ An image from Canada by Lauri Kangas.

"We had the temperature plummet last night and an interesting ground fog formed. I have attached a photo of the lunar corona. What was really interesting was the beautiful colour of the large blue ring to the naked eye. I could see the small water particles with my LED flashlight swirling around the camera equipment. I had to centre the moon in my camera viewfinder to avoid lens flare. The lens was a Canon 17-40mm set at 27mm, f/4.0, 1 second, ISO 2500 using a Canon 5D MKII."

©Jim Grant, shown with permission

Coronae arise from diffraction of sunlight or moonlight by clouds of randomly spaced cloud or fog droplets.

Each droplet produces its own diffraction pattern. And what we see in the sky is a sky transform - the collective diffraction glints from millions of drops.

IRIS Mie scattering calculations for monosized drops at left and ones with a 20% standard deviation Gaussian distribution of sizes at right.

Monosized drops from freshly formed fog or cloud give the purest - many ringed - coronae.

In old fog the drop size distribution widens and the mean size increases. Coronae from the different drops have different sizes in the sky. Their overlap can give a ring less blur.

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Reference Atmospheric Optics

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