Friday, October 06, 2006

Interesting Moon Facts

October 6th is the “Harvest Moon" – the last full moon before the Autumnal Equinox that allowed farmers to harvest all their crops into the night. Moonlight is just sunlight reflected from the dusty surface of the moon. The only difference is intensity: Moonlight is about 400,000 times fainter than direct sunlight. And it does some strange stuff:

1. Moonlight steals color from whatever it touches. In full moonlight, the flower is brightly lit and even casts a shadow, but the red is gone, replaced by shades of gray.

2. If you stare at the gray landscape long enough, it turns blue. As your eyes become maximally dark adapted, the blue appears. Yet if you look up at the full moon, it is certainly not blue.

3. Moonlight won't let you read. Open a book beneath the full moon. Even is the light is bright enough, you can’t make out the word. This is because moonlight not only blurs your vision but also makes a little blind spot.

This is all because of the human retina, 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, rods take over.

Rods are 1000 times more sensitive and are responsible for our night vision. But, rods are colorblind. Roses at night thus appear gray. And, 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 at night it becomes a blind spot.

As for the blueshift, that is still a mystery.

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