For many of us, allergies kick in this time of year. Tree pollen is often the culprit. These tiny “male” granules often ride the wind to pollinate nearby flowers and in the process land in our eyes and get stuck in our noses. Although the grains are harmless, our immune system goes on the attack anyway, releasing histamines that make for teary, itchy eyes.
But like nearly everything in life pollen also has an up side. The powdery particles are so tiny they diffract the light of the sun and moon to form pollen coronas. Few skywatchers pay them much attention to them probably because we’re rubbing our eyes so much. But they’re totally amazing, appearing like a psychedelic bullseye of color centered on the sun or moon.
Several days ago I happened to look up toward the sun and noticed some color up there. So I stuck out my thumb at arm’s length and carefully blocked the solar disk. After a few seconds I discerned three vividly-colored, concentric rings glowing against the blue sky.
Unlike typical cloud coronas, which are caused by tiny water droplets, these rings were oval, not circular. That and the fact that not a single cloud sullied the blue were certain signs I was seeing a pollen corona. Pollen coronas are oval because the pollen particles are elongated rather than spherical like water droplets.
Next, I found a tree to do the sun-blocking for a better look. Key in seeing pollen coronas is NEVER looking directly at the sun itself. Always hide it, then allow your eyes a little time to adapt to the bright glare. You can also see this time of year around the moon around the time of full.
You’ve probably seen a ring around the sun or moon. That’s a halo and caused by light that’s refracted or bent as it passes through six-sided ice crystals. Coronas are much smaller, only about a quarter the size of a typical halo so you have to look more closely. In a corona, light waves scatter off — get diffracted by — extremely tiny droplets of water in clouds or minute pollen particles. Some of those waves reinforce one another to make bigger waves (brighter light), others cancel each other out to create dips in brightness. Since light is made up of many colors, each of which is diffracted a little differently, the end result is a series of nested, multi-colored rings.
Halos form by refraction of light and coronas by diffraction. In both, light does amazing things when it interacts with little stuff that floats in the air. How visible a pollen corona is depends on how much and what types of pollen are blowing by as well as the transparency of the sky. I’ve noticed that a slight haziness in the air reduces contrast. Other days I’ve looked and seen only a bright white aureole around the sun but no corona.
The best thing is to keep looking. If the sky is a deep blue, block the sun using a rooftop, power pole or tree and see if any colored rings are present. Do the same when searching for pollen coronas around the moon. As a side bonus, view the solar ones in a pair of binoculars. I saw another something amazing this way — hundreds of strands of spider silk glimmering in sunlight as they blew across the field of view.
Oh, and bring some tissue to keep those runny eyes clear — good luck!