The Harvest Moon of 2006 rises on October 6th, and if you pay attention, you may notice a few puzzling things:
1. Moonlight steals color from whatever it touches. It’s a bit like seeing the world through an old black and white TV set.
2. If you stare at the gray landscape long enough, it turns blue. The best place to see this effect, called the “blueshift” or “Purkinje shift” after the 19th century scientist Johannes Purkinje who first described it, is in the countryside far from artificial lights.
3. Moonlight won’t let you read. Open a book beneath the full moon. At first glance, the page seems bright enough. Yet when you try to make out the words, you can’t.
So what do we make of it all? The answer lies in the eye of the beholder. The human retina is responsible.
The retina is like an organic digital camera with two kinds of pixels: rods and cones. Cones allow us to see colors (red roses) and fine details (words in a book), but they only work in bright light. After sunset, the rods take over.
Rods are marvelously sensitive (1000 times more so than cones) and are responsible for our night vision. According to some reports, rods can detect as little as a single photon of light! There’s only one drawback: rods are colorblind. Roses at night thus appear gray.
If rods are so sensitive, why can’t we use them to read by moonlight? The problem is, rods are almost completely absent from a central patch of retina called the fovea, which the brain uses for reading. The fovea is densely packed with cones, so we can read during the day. At night, however, the fovea becomes a blind spot. The remaining peripheral vision isn’t sharp enough to make out individual letters and words.
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