Give us this day our crepuscular rays

Ragged moonbeams and shadows are cast by a cloud covering the gibbous moon last night. Photo: Bob King

Last night a raft of clouds drifted across the gibbous moon and rays of darkness shot across the sky. What I saw were crepuscular rays, more familiar during the daytime when sunlight shines between towering clouds to create a spreading fan of sunbeams and shadows. Dust and moisture in the atmosphere make the beams of light stand out even better.

What was unusual about the moon’s crepuscular rays was how the bright ones blended into the sky, but the “shadow beams” – created by clouds blocking the moonlight – stood out as dark shadows against the moonlit sky. The effect was striking.

Individual cloud turrets create the shadow rays while gaps allow sunlight to pass, creating a splendid show  of crepuscular rays shortly before sunset earlier this month. Photo: Bob King

Crepuscular comes from the Latin word “crepusculum” meaning twilight. They can occur anytime but are more apt to be noticed when the sun is low in the sky or shortly after sunset or before sunrise. What really catches the eye is how they radiate in beautiful fans like crowns of glory or angelic halos.

I wouldn’t doubt that apparitions of the Virgin Mary and the like have something to do with crepuscular rays. The stunning sight is based on a well-known optical illusion. The rays are actually parallel to one another and only appear to converge into a crown the way the rails of a train track narrow to a point in the distance. If we could look down on the clouds from high above, the rays would appear perfectly parallel to one another.

Anticrepuscular rays seen at sunset from the airplane window. I’m looking to the southeast. The bright rays are beams of sunlight passing through gaps in clouds on the opposite side of the plane. The dark rays are where the sunlight was blocked by the clouds. Photo: Bob King

Sometimes around sunset or sunrise when atmospheric conditions are right, you can see crepuscular rays beamed by clouds massed along or just below the horizon reach all the way to the opposite end of the sky called the antisolar point.

A display of anticrepuscular rays seen at sunset from the ground. The beams of light and shadow appear to draw together or converge in the eastern sky. Photo: Bob King

There they converge (illusion again) to form a curious bundle of dark beams that look like a gigantic sunless sunburst. Crepuscular rays seen opposite the sun (or moon) are called anticrepuscular rays. They’re much rarer than the crepuscular variety and well worth your time to seek. Next time you see beams of light and dark spreading up from the western horizon at sunset, try to follow them all the way back to the antisolar point in the east. You might find a lovely set of anticrepusculars waiting for you there.

I was fortunate to see them from my airplane seat window while flying back to Duluth from Colorado earlier this month. They looked peculiar – appearing below my local horizon – no doubt because of my altitude at the time.

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