From Mike Brown (via Boing Boing):
Here’s another experiment to try. Go outside on a bright sunny day and start watching your shadow. Walk along until you find a place where the shadow of your head is falling on grass. Focus on your head shadow while you continue to walk, letting the background grass blur in your vision. You will gradually notice that there is a diffuse glow around the shadow of your head. It won’t be around any other part of your body, and you won’t see the slightest hint around anyone else’s head. Point out your halo to any else and they will see precisely the same thing: a halo around their own heads and nothing around yours.
He’s writing about phenomena that happen when the moon is full and there are no shadows to be seen, but digresses to discuss your “halo” from a bright, sunny sky. You can see much the same from an airplane as you descend to land on a sunny day.
If you’ve ever seen the Earth’s terminator pass your location and noticed the “lights-out” that occurs, this little trick of light will give you a kick.