Pollen time is nearing

These images by Pierre-Paul Feyte were taken during a conjunction of the Moon and the Pleiades cluster (lower image). They show a colourful oval and bright-patched pollen corona around the Earthlit crescent moon and another around Venus. Images ©Pierre-Paul Feyte.

"Two years ago [April 19, 2007 ~20:45UT], as I was taking a few pictures of a moon-Pleiades-Venus conjunction with my village on the foreground, I noticed a corona surrounding the moon. Processed pictures show the same faint - but also coloured - corona.

To me, this is related to the massive cloud of pollen released by the Landes Forest (For�t des Landes) in South-Western France which lies between my location and the Atlantic Ocean 150 kilometres away. The For�t des Landes is one of the biggest artificial forests in the world, and the largest of Western Europe. It's easy to spot the green triangle-shaped 1.5 millions square hectares wood on Google Earth . Now just imagine the billions of pollen seeds released by all those pine trees, all of the same size.

"The same size" - That is the key to generating beautiful ringed coronae. Light waves are scattered at the periphery of particles and interfere to form diffraction patterns - coronae - around bright light sources. The corona size increases as the particle size decreases and so scattering by a cloud of particles of various sizes produces only an aureole without rings. Here the monosized pollen has produced a distinct lunar corona with at least two rings.

The corona is oval with bright areas on the rings because the pollen grains are non-spherical and are specially oriented in the air. They have air sacs to help the wind disperse them widely and the sacs also serve to orient them.

The crescent shaped moon has added its own distortion to the corona shape.

Venus has a similar but much fainter corona and long exposures would be needed to reveal the rings.

Now is the start of the Northern Hemisphere pollen season. Look out for pollen coronae. They are smaller than those from the water droplets of clouds and are best seen by just shielding the sun (or moon) behind the edge of a post or building.


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