Imagine the unwashed folk who lounged around the fire with their cups of foul ale in January 1000, wondering what the new millennium might bring. Perhaps they hoped for a better way to sharpen knives or soothe a throbbing tooth. But computers and genetic engineering? Inconceivable. Equally futile are most of our own speculations about third-millennium technology.
At least we can be reasonably certain about predicting what will occur in the heavens over specific periods. True, events can take us by surprise. Some comets, for example, have such incredibly slow orbits-5,000 to 100,000 years-that every visit close to the sun seems like the first time around. But others, like Halley's, follow shorter, more predictable periods. Its next appearance in 2061 should be a winner-the comet's bright, fuzzy head will lie favorably in the northern sky and its tail may span half the sky.
