Today I did my usual morning hill climbing, around 5:00 a.m., and I was especially struck by the various effects of light on the snow-covered town at that pre-dawn hour. Although the darkness of the winter night still prevailed, there seemed to be a surprisingly intense illumination everywhere. The whiteness of the snow on lawns and streets surely caused some of this brightness, as did the golden glow of the streetlamps. There also seemed to be an unusual number of lights glowing in windows, especially for that early hour, and several houses were adorned with all-night Christmas lights. What surprised me the most, however, was the brightness of the sky. The sun would not rise for another 90 minutes, yet the sky was suffused with a soft shine. On this frosty, snow-white morning, I felt like I was exercising in a universe made mostly of light.