Two thousand years ago, give or take a few hundred years, Seleucus of Seleucia in Mesopotamia and Wang Chong in China concluded separately that the moon was responsible for the tides. They may not even have been the first; there may be a line of bright celestial thinkers stretching back into deep antiquity: In each age the connection made, and lost again in disbelief. I walk slowly along the beach, gazing steadily downward along the wrack line, and think about how long it would take me to claim such a far-fetched sisterhood between sea and sky.