By 1910 the sight of Halley's Comet wasn't a surprise. As early as 1705 astronomer Edmond Halley identified its orbit and predicted its reappearance every 75 to 76 years. He correctly guessed its arrival in 1759, and the comet dazzled the night sky again in 1835. Though telescopes weren't yet strong enough to track its distant progress, everyone expected another sighting in 1910. That wasn't the scary part [source: Weissman].
What stirred everyone into a tizzy was the new discovery that Earth was actually on track to pass through the comet's 15.5-million-mile (25-million-kilometer) tail. Making things worse, a recently-discovered technique known as spectroscopy was used to analyze the composition of the comet, and scientists learned something unsettling — namely that the tail contained a toxic gas known as cyanogen. While most astronomers weren't actually that worried, a nervous public latched on to the claims of people like French astronomer and author Camille Flammarion, who claimed the poison would smother all life on the planet. It was a recipe for mass hysteria.
Churches held prayer vigils, and enterprising con-men sold comet pills to ward off the poison's effects. Even wilder theories emerged suggesting that the comet's gravity might throw off Earth's tides and cause the Pacific to empty itself into the Atlantic. Despite the doomsday predictions, however, nothing happened. Halley's Comet came again in 1986, and with any luck, we'll all see it again in 2061 [source: Clark].