Second Story: Sunbeams

At last week’s Sudden Star Party at Ward Pound Ridge Park, after sunset we saw sunbeams coming from the western sky, without any clouds in the sky.  How can that happen?

We noted the light and dark beams stretched across the sky and coming together at the opposite horizon.

A great source for information about cool optical sights in the sky is the Optics Picture of the Day site:

I can’t copy the pictures from their site, so I’ll include the links to photos to follow with examples to help us understand what we saw……

Sunbeams are common with towering clouds blocking the sun… sunbeams or through gaps in a middle deck of clouds…. sun spilling through a deck of clouds  The rays seem to fan out across the sky, but it’s really just a trick of perspective – like the furrows in the field in the photo below.  And the appear to come back together at the other side of the sky….. sunbeams across the sky  and   rays converging on the other side of the sky

I love the Global Rainbows site.  Yvette Mattern sets up seven high-powered lasers pointing their beams parallel to each other.  Global Rainbow site    Check the view where the laser beams appear to be coming from one point in the distance – it’s actually a photo of the beams coming together in the distance due to the perspective effect. But as the other photos show, the beams are still parallel when you get a close look at them!


Photo of the northeastern United States at 445pm Sat Sept 5th from the NOAA’s geostationary satellite via the image archive at the National Center for Atmospheric Research Sunset was about 715pm that night.

So where are the clouds?  Above is a satellite photo from late that afternoon

The classic book about the physics of optical wonders in the sky, Light and Color in the Outdoors by Minnaert says the clouds that make after-sunset sunbeams can be 120 to 450 miles away.  The billowing clouds in western Pennsylvania in the satellite photo are about 300 miles from our observing site in Westchester County and in the direction where the sun set, they could be the cause of our post-sunset sunbeams, called crepuscular rays.


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