So, can we tell the time of day by using a rainbow?
This shot was taken between 12:13 and 12:27 pm (assumes November 20th).
Okay...I give up?? HOW can you tell time by a rainbow?? Ask a leprechaun?
I'm so glad you asked!
Rainbows only exist in the eyes of those who see them. There is nothing but a mist of water drops at the spot where we see a rainbow. They were first figured out by the Frenchman Rene Descartes in 1637. Here is a good place to see the physics of the rainbow
. The long an short of it is that the sunlight comes from behind us, enters the distant water drop at about a 60 degree angle with respect to the drop's surface normal, reflects off the inside back-surface of the drop, exits the front of drop (not the same place as it came in), and then proceeds to our eyes. There is a precise, color-dependent angle between our eyes, the drop , and the Sun: 40 to 42 degrees. At each passage from air-to-water and water-to-air the sunlight is dispersed into its component colors. The higher energy colors: Blue, Indigo, Violet are more strongly dispersed. That's why the rainbow colors go, in order from the outside to the inside: Red, Orange, Yellow, Green, Blue, Indigo, Violet.
So, in the promo photo, there are two ways to use the rainbow to give the time of day. One is to estimate the angle from horizontal up to the top of the bow. Subtract that from 40 degrees and that gives the Solar altitude. The Solar altitude gives two possible times of day (one for AM and one for PM). The other way to use the Rainbow (and this was what I used) is to recognize that the top of the bow gives the direction to the Sun. The Sun is 180 degrees from the top of the bow. So all you have to do is know where the shot was taken and make a line from there to where the picture shows the top of the bow. In our shot I used the mountain peaks in the distance to make a vector. From that you get the Solar azimuth. There is one time of day for any given Solar Azimuth.
Yes, this was my first rainbow.