Double Shadows Again

Most recent answer: 01/09/2015

While driving a school bus this afternoon, about 1510 on 01/09/15, going basically east near Marion, Oregon with the sun almost directly behind me, I noticed something odd about the shadow in front of me. The outline of the bus was doubled, not fuzzy, but two distinct shadows, the lighter offsetting the darker by about 4 inches. I was not able to stop to get a picture and it lasted only a minute or so. The conditions were sunny with a few light clouds. My question, of course, what would cause this phenomenon? I have never witnessed anything like it.
- Mark (age 58)
Aumsville, Oregon

We've heard a lot about these double-shadows () but don't have any great ideas about how it would work. In a city, different reflections can give different shadows. It's just possible that one of those light clouds was blocking light from the middle of the sun, leaving light coming in from the two sides. These sides are about 1/120 radian apart, as viewed from the earth. If the part of the shadow you were looking at was around 40 feet away from the bus, that could leave two shadows 40 feet/120 = 4 inches apart. At 15:10 near Marion OR on that date, it would be about an hour and a half before sunset. The sun would be roughly 0.4 radians above the horizon. Shadows would extend horizontally to about the height of the bus times cotangent(0.4 rad) , or around 2.5 times the bus height. So 40 feet sounds a bit far, but then maybe your 4 inch estimate was a bit high. So the idea sounds odd, but it would pretty much fit your description.

Mike W.

(published on 01/09/2015)