I often watch the moon and the sky at night from a second-floor west-facing window. The different phases of the moon as it cycles through the crescent quarters night by night grow more familiar. On a cloudy San Francisco evening, I may not see any moon. Or only for moments during breaks in the clouds blown to the east by the tides of the onshore evening winds. Monthly the moon hides out of sight below the horizon, ready to start the next cycle. On a clear night, the full moon is a bright, shiny silver dollar in the sky.
The other day I saw something new, a pale glowing halo around a nearly full moon. Looking closer, I saw this halo was a circular rainbow, a spectral spectrum. What is this? Does it have a name?
I had to look it up. Yes, that lunar phenomenon is a moon halo formed by the refraction and diffraction of moonlight by ice crystals in the upper atmosphere. The thin, wispy cirrus cloud I saw earlier in the day refracted the moonlight, bending it into that glowing halo. Then the ice crystals, like tiny prisms in just the right place, diffracted the moonlight into the faint rainbow spectrum on the inner edge of the halo. As in much of life, it is rare good fortune for everything to line up just so. This marvel was a wonderful reminder of the natural world’s complexity and elegance.
I roused my wife and daughter, who both love nature as much as I, and we watched together from a back porch. The photos I took didn’t do justice to this striking interplay of moonlight and ice crystals. Dynamic and charmingly intricate moments like these connect us more deeply with the natural world, evoking a sense of wonder. We watched for a while and then noticed the halo slowly dissipating. I thought about the ethereal, passing beauty of life and nature.
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