When green streaks light up the night sky, the effect is otherworldly--like some toxic spill from the International Space Station.
Solar flares and explosions hurl particles that collide with the Earth's atmosphere, producing energy emitted as photons, or light particles. It takes 100 million photons to make the aurora borealis, or northern lights, visible to the naked eye.
As with rainbow spotting, there are no guarantees. The key ingredients are a cloudless sky, little or no moon, and luck. For the best odds, head near or above the Arctic Circle from October through March. At 78 degrees north, between mainland Norway and the North Pole, Spitsbergen on the Svalbard archipelago is the world's northernmost place reached by regularly scheduled flights (about $200 round trip from Olso). If that's too hardcore, go as far north as you can manage. The Norwegian town of Hammerfest was popularized as a viewing place by Bill Bryson's Neither Here Nor There; Tromsø is a decent-size city with charm 250 miles north of the Arctic Circle. Both cities are stops on the slow-moving Norwegian Coastal Voyage cruise (800/323-7436, coastalvoyage.com, six-night packages from $1,067). Though 110 miles south of the Arctic Circle, Fairbanks, Alaska, has reliable enough viewing to attract scientists annually; flights from Seattle cost around $500 in winter.
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It helps if your destination offers more than the lights, because sitting around in the freezing dark can try the most patient of souls (and in winter, the farther north you go, the less daylight you get). Svalbard is ideal for snowmobiling, dogsledding, and polar-bear viewing. Basecamp Spitsbergen arranges tours, as well as accommodations aboard an old ship embedded in ice or in a trappers' lodge (011-47/7902-4600, basecampexplorer.com, doubles from $165). Many decent Fairbanks motels charge under $100 a night; drive a few miles away from the city in any direction for a chance at clear viewings. Or stay outside town at Northern Sky Lodge, a log B&B with dogsledding tours (907/388-9954, northernskylodge.com, doubles from $75), or Mt. Aurora Fairbanks Creek Lodge, with 270 degrees of sky visible from its deck (907/389-2000, mt-aurora.com, $289 for two with meals). Whichever destination you choose, ask around about when to head out for a look; locals keep tabs on the best viewing times, which can change seasonally.
Every 11 or so years, the northern lights are known to appear way below the Arctic Circle. In 2000 they were visible in El Paso, Tex. Wherever you are during the winters of 2011 and 2012, be sure to look up at night.