Glitter Paths

Lunar Glitter Path, Lake Neuch�tel, Switzerland - Imaged by John Sergneri on March 13, 2009. ©John Sergneri, shown with permission.

Glitter patches (or paths) are the myriad of individual glints - instantaneous reflections of the moon or sun - from facets of the undulating and shifting liquid surface that happen at that moment to have the right position and tilt.

Glitter paths, GPs, seem to have horizontal striations but this tells us little about the wave directions. To create a path the waves do not need to be coming towards you. Foreshortening makes undulations of all sorts appear to have horizontal striations when they are viewed close to the horizon at shallow angles.

What is notable about this GP?
(1) It is very narrow, only the width of the Moon. (2) Its width is constant. (3) There is a brighter patch where the still water Moon reflection would be. (4) It is bent!

The GP width depends on the altitude of the moon or sun and the slopes (not the absolute height) of the waves. When the slopes are zero, i.e. still water, we see only a mirror reflection of the moon. Very gentle waves with small slopes give a narrow glitter path that is brighter where the reflection would be. Here at Lake Neuchâtel the waves were very gentle. The GP broadening is consequently very small and its width is set by the greater (constant) angular width of the moon. As waves get steeper the glitter path broadens.

Sea and ocean glitter paths often look wider closer to the shore than at the horizon because the waves near the shore are steeper and even breaking.

Bent paths? A 'normal' GP lies at the intersection between the horizontal plane of the water and a vertical plane passing through the moon and your eye. Such GPs can appear tilted in images because of lens distortions.

The Neuchâtel GP has a real 'dog leg'. David Lynch** says "In my experience this is rare, While I haven't done any calculations, I suspect it is due to some sort of asymmetric slope distribution of the waves, perhaps the result of wind blowing locally on the surface, the asymmetry being related to the direction the wind is blowing relative to the vertical plane containing the sun, observer and glitter."

John Sergneri later added support: "The winds on Lac Neuchâtel are known to blow in circles, shifting around the lake, which drives the local sailors crazy. Sailing on the lake is known to be a challenge due to the ever shifting winds. Often, a local effect happens when the wind comes down off the Jura due to temperature differences, one can see the patches of the lake disturbed while all else is calm."

Light on water has many subtleties. Look at reflections, look on overcast as well as sunny days- there are puzzles a plenty.

** My thanks to David Lynch of Color & Light in Nature, for his analysis of the Neuchâtel sighting. Errors and omissions are mine.