Moon blues

摘要

Last year I was watching the sun set in Crete, as it went through all shades of red in the final minutes. Then I glanced at the moon and saw it had turned blue. How was this possible, and is it the basis of the saying "once in a blue moon"? 1. The chances are that this blue moon was an illusion - the afterimage of the sun created by your eyes and brain.By staring at the bright red disc of the setting sun for a significant amount of time, say more than 20 seconds, the red photoreceptors in your eyes become desensitised. When you switch your gaze to a white object like the moon, the after-image of the sun is seen in its complimentary colour - cyan or light blue.