My most recent flight was at nighttime. I was sitting in a window seat a bit behind the wing. On the rear side of the wing almost at the tip was a very bright flashing light–the light you might see by looking up at an airplane viewed against a dark sky.

As I was mindlessly staring towards the now-distant ground, something odd caught my eye: several of the brighter lights on the ground where flashing exactly in sync with each other.

At first, I thought they must be part of the same system on the ground, a tall cell phone tower, a runway, a series of traffic lights flashing due to the late hour….

As I watched, I noticed the lights were slightly different colors, in no clear geometric pattern and very far spread out over many, many miles.

I wondered if maybe I was seeing a reflection of a single light, maybe being reflected off of a bunch of lakes, for example–maybe even the flashing light on the plane being reflected.

Upon closer observation, something else very strange became apparent. Instead of the lights on the ground being exactly in sync with the flashing light on the plane, as would be expected if I was just seeing a bunch of reflections, the lights on the ground were instead perfectly OUT of sync with the flashing light on the plane: when the light on the plane was on, the lights on the ground were dimmer; when the light on the plane was off, the lights on the ground were brighter.

Very puzzling.

With a chuckle, I realized that in fact the lights on the ground were not flashing at all. Rather, my pupils were dilating and contracting each time the bright light on the wing flashed–when it turned on, my pupils contracted, making the lights on the ground seem dimmer; when it turned off, my pupils dilated, making the lights on the ground seem brighter.

The observer, when he seems to himself to be observing a stone, is really, if physics is to be believed, observing the effects of the stone upon himself.  —Bertrand Russell

Update: Neither the flashing light on the wing nor any of the “flashing” lights on the ground were green or “across the bay at Daisy’s house”–thanks for asking.