|Peder Krøyer, Summer evening on Skagen's beach, 1899|
In Sir Montagu Pollock's 1903 classic book Light and Water: A Study of Reflexion and Colour in River, Lake, and Sea, he explains how the varied wavelets act like mirrors pointing slightly toward or away from the viewer, thus stretching the moon's reflection downward.
He lays out the following general rule: "In the rippled water the combined effect of all the images of the luminous point P, formed by the reflexions in countless wavelets, is a vertical line of light."
Pollock makes another observation about the sun or moon's reflection on calm water:
"The width of this streak of light also depends largely upon the height of the sun or moon above the horizon. As the sun mounts in the sky it gets wider and vaguer."
Thanks, Tim Atkins for the Kroyer photo!
Download Pollock's book for free on Archive.org
More about water, light, and reflections in my book: Color and Light: A Guide for the Realist Painter