Rosy-Fingered Dawn

July 21st, 1998

Along with "wine-dark sea", this has to be one of the most overquoted terms from Homer. It describes a physical phenomenon everybody has seen, but, so far as I know, has never been explained. This is the set of rays reaching out from the rising sun, schematically displayed on the Japanese national flag. The same rays can also be seen after sunset.

I’d like to add my own observation to the pot: the rays extend all the way across the sky. You can’t make them out overhead, but if you face away from the sun, you can see them converging on the horizon. This is an important bit of data; it clearly indicates that the phenomenon extends as parallel lines across a great distance.

All of the explanations I have read assume that the rays themselves are optical effects, and the extended linearity demonstrated by the observation above certainly supports that assumption. In other words, we can reject explanations that the rays are windblown streaks in the upper atmosphere. They just wouldn’t be parallel, and they wouldn’t always radiate from the sun.

Most explanations posit that the rays are shadows; the problems all arise from the source of the shadows. The first explanation, mountains, would mean that nobody on the west coast of a continent would see rays in the evening, and nobody on the east coast would see rays in the morning. The Japanese national flag puts that theory to rest.

The most popular explanation is that the rays are caused by distant clouds. This explanation doesn’t make sense to me. Here’s my reasoning. First, we can get a rough idea of the distance to the shadow-casting agents by timing the appearance of the rays. The earth rotates through one degree every four minutes; at temperate latitudes, that amounts to about 80 miles distance. The rays are most prominent about 30 minutes after local sunset; this implies that whatever is casting the shadow is about 600 miles away. This raises two serious problems. First is the implied size of the shadow-casting agent. To cast a shadow, it should be roughly twice the angular size of the sun, or about one degree of arc. That corresponds to a linear breadth, at 600 miles, of 12 miles. That’s a very large cloud!

The second problem comes from the curvature of the earth. 30 minutes after local sunset, the rays of sunlight are 52 km above the ground. That’s about 160,000 feet, and there just isn’t much air that high. What’s there to cast a shadow onto?

Now, there is one saving grace in all this, and that’s atmospheric refraction. The atmosphere bends light slightly. I was able to take advantage of this phenomenon to observe the star Canopus from a latitude of 37 degrees, 30 minutes, which would have been impossible if there were no atmospheric refraction. The 600 mile distance to the shadow-caster would not change, but it would be able to cast its shadow to a lower, denser portion of the atmosphere.