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home for the night. To get to my cabin, I had to drive up the windy mountain road into the forest, past the national forest parking lot, and down to the end of a dirt road, and finally walk along a poorly maintained trail by the side of a seasonal creek. For some time after I first moved in, I tried to remember to bring a flashlight with me to light my way, but more often than not I forgot. On those nights, I had to navigate the trail by whatever light was available or, sometimes, by no light whatsoever.
    The time it took to get from the top of the trail to the bottom, where my cabin was, depended almost entirely on the phase of the moon. When the moon was full, it felt almost likewalking in daylight, and I practically skipped down the trail. The darker quarter moon slowed me a bit, but my mind seemed to be able to reconstruct my surroundings from the few glints and outlines that the weak moonlight revealed. I could almost walk the trail with my eyes closed. I had memorized the positions of nearly all the rocks that stuck up and all the trees and branches that hung down. I knew where to avoid the right side of the trail so that I wouldn’t brush against the poison oak bush. I knew where to hug the left side of the trail so that I wouldn’t fall off the twenty-foot embankment called “refrigerator hill,” named after a legendary incident when some previous inhabitants of the same cabin had hauled a refrigerator most of the way down the trail before losing it over the edge and into the creek at that very spot.
    I had
almost
memorized the trail, but every twenty-nine days I was reminded that there is quite a big difference between memorization and near memorization.
    Every twenty-nine days the moon became new and entirely disappeared from the sky, and I was almost lost. If by luck there were clouds that night, I might be able to get enough illumination from the reflected lights of Los Angeles, just a few miles away, to help me on my way. But on days with no moon and no clouds and only the stars and planets to light the way, I would shuffle slowly down the trail knowing that over here—somewhere—was a rock that stuck out—there!—and over here I had to reach out to feel a branch—here! It was a good thing that my skin does not react strongly to the touch of poison oak.
    These days I live in a more normal suburban setting and drive my car right up to my house. I even have indoor plumbing. The moon has almost no direct effect on my day-to-day life, but still, I consciously track its phases and its location in the sky and try to show my daughter every month when it comes aroundfull. All of this, though, is just because I like the moon and find its motions and shapes fascinating. If I get busy, I can go for weeks without really noticing where it is in the sky. Back when I lived in the cabin, though, the moon mattered, and I couldn’t help but feel its monthly absences and the dark skies and my own slow shuffling down the trail.
    Contrary to the way it might sound, however, back then the moon was not my friend. The two-and-a-half-year-old daughter of one of my best friends—a girl who would, a few years later, be the flower girl at my wedding—would say, when asked about the bright object nearly full in the night sky: “That’s the moon. The moon is Mike’s nemesis.” And indeed, the moon was my nemesis, because I was looking for planets. Astronomers build telescopes in the most remote places they can find—the mountains of Chile, volcanoes in Hawaii, the plains of Antarctica, even in outer space—partially in the hope of escaping the city glare that increasingly permeates the skies. For all that effort, though, we can’t hide from the brightest light that illuminates the night skies and washes out the faintest stars: the full moon.
    As a new graduate student in astronomy at Berkeley, I had never previously considered the moon to be an obstacle. It was still the world that people had walked on early in my childhood,