This is a frame from a car commercial that’s been running lately on TV. The scene doesn’t last long, and if you blink you could miss it. The first time I saw it something just registered as “wrong” so I rewound the DVR to take a closer look. Indeed, the commercial showed a rainbow in the same direction as the setting Sun. This can’t happen, as can be seen from the following graphic:

This graphic shows the geometry for a rainbow that forms at sunset or sunrise on open flat terrain. Under those ideal conditions one can observe a rainbow with 180° of circumference. That’s the maximum. In practice, the bow you see is usually <180° because parts of it are hidden by objects around you. Further, if the Sun is above the horizon the bow will be lower in the sky and display less of its arc. If the Sun is more than 42° above the horizon, the entire rainbow will be “below the ground” and none of its arc will be visible.

From an elevated location one can see more than 180° of bow. In fact, under ideal conditions a full 360° bow is possible. However, in all cases the bow will appear opposite the Sun in the sky. A bow in the same direction as the Sun, as shown in the commercial, is physically impossible.

It doesn’t surprise me that this error occurred — people that script and create commercials are not usually trained in science. What concerns me more is that, despite their lack of scientific training, nobody involved with the production caught this error. I mean, all you need is to have seen a few rainbows to pick up on the geometric pattern. You don’t need to understand the optics behind it. But then, I’m an avowed skygazer.

In all fairness, perhaps the producers were thinking about a corona? Coronae appear as a circular ring around the Sun or Moon, and are caused by ice crystals instead of rain. Coronae are typically 22° in diameter vs. the 84° of rainbows, but the bow in that commercial looks larger than 22°. Also, the sequence of colors in a corona is the reverse of what you see in rainbows. So it’s still bad science.

This isn’t the first time I’ve felt compelled to comment about bad science in the media. See, for example:

Call me pedantic, which I am when it comes to science. Teaching science for 30 years does that to you. Grading all those papers and lab reports sharpens your ability to spot mistakes. That’s why these examples of bad science in the media catch my eye. Bad science provides what educators call a teachable moment, so you’ll see them on occasion in Sky Lights.

At least they got the rainbow color sequence right — red on the outside of the bow and violet on the inside.

Next Week in Sky Lights ⇒ Puzzling Atmospheric Apparition