This summer, people saw Brood X cicadas emerge from their 17-year slumber, Venus crossing in front of the sun for the first time in 122 years and even, just last week, a so-called "blue moon."
Ah, the wonders of the natural world: In 2003, Mars moved closer to Earth than it's been in 60,000 years. In 1997, the great comet Hale-Bopp streaked overhead. In 1994, another comet, Shoemaker-Levy 9, crashed into Jupiter. And recent years have featured blooms of once-rare Sumatran "corpse flowers," which foul the air with the intense smell of rotting flesh.
"We're in a pretty fortunate era," says Donald K. Yeomans, a senior research scientist who manages NASA's Near-Earth Object Program office. "We're pretty lucky if we take the time to go outside and turn off the tube."
But no matter what you do, you'll never see it all.
Few alive now have ever seen Halley's comet in its prime. And you may never witness a truly massive meteor storm flash across the sky, or see Venus pass in front of Jupiter. There's only a slim chance you'll see a major "fireball event" pummel the landscape, or a nearby star explode in what's known as a supernova.
And unless you go out of your way, you may not even see a total solar eclipse, or a blue moon that really is blue.
For sky gazers, time and space often just don't cooperate.
Like when Halley's last was in Earth's neighborhood in 1985-86 for the first time in 76 years: It wasn't a particularly close brush with Earth, and some found the show disappointing.
"I would say that it was a good comet, but it couldn't live up to the 76 years of expectations that the previous generation laid upon us," says W. Scott Kardel, public affairs coordinator for the Palomar Observatory, located in Southern California and operated by the California Institute of Technology.
In fact, Halley's best near-Earth pass may have been in the year 837, when Yeomans says Chinese accounts had it "stretch[ing] from one horizon to the other because it was so close."
"Most famously, it was seen in 1066 before the Battle of Hastings and it was depicted on the Bayeux Tapestry," Kardel says of the comet, once considered a portent of doom.
1910 also was a vintage year. As Halley's made its approach, poisonous gas mistakenly was detected in its tail.
"Cyanogen is a very deadly poison, a grain of its potassium salt touched to the tongue being sufficient to cause instant death," The New York Times reported on Feb. 8, 1910. If Earth were to pass through Halley's tail, an astronomer predicted, "the cyanogen gas would impregnate the atmosphere and possibly snuff out all life on the planet."
Other astronomers dismissed the doomsday theory. Sure enough, our ancestors survived a trip through Halley's tail on July 18, 1910 — regardless of The Times' account of a baseball game played that day: "Comet day was observed at American League Park yesterday with appropriate astronomical fireworks, the Yankees gushing forth a shower of meteorites in the seventh and eighth innings which put the St. Louis Browns groggy under the strong influence of cyanogen gas."
Halley's is expected to pass a bit closer to Earth in 2061, and even closer 76 years later. Don't worry about the cyanogen gas on that spectacular pass, though. We'll all be long dead anyway.
Luminous Veils? Not Until 2261
Halley's may be the best known of the short-period comets. But there are other comets that the modern world will miss entirely because they're on mismatched cycles.
Astronomers in June 1861, for instance, described a comet that fanned out toward the tail, and a witness described it as enshrouded in six "luminous veils," according to the Web site cometography.com.
But if you'd like to see those luminous veils, you're out of luck. The so-called great comet of 1861 won't return until around 2261.
Still, though the 1861 comet may have been great, it had nothing on 1997's Hale-Bopp and other recent comets.
"We've been pretty lucky: We've seen a number of good comets in the last couple of years," Yeomans says. "Certainly, Hale-Bopp is what we would call a great comet because it was an easy naked-eye object. … Before that, in 1965, there was Ikeya-Seki. That was a great comet."
If you saw Hale-Bopp, treasure the memory. It won't be back until the year 4377.
Comets aren't the only celestial events on cycles longer than human scale.
Only the relatively young among us likely will live to see Venus pass in front of Jupiter, as it will do on Nov. 22, 2065 for the first time since Jan. 3, 1818.
On the other hand, many saw Venus' transit in front of the sun earlier this summer, and experts consider that a more spectacular and easily discerned sight: Through special filters, viewers saw Venus as a black dot slowly pass across the sun, which appeared about 30 times the planet's size.
"It's like a fine French wine for the people who know about it and enjoy it," Jay Pasachoff of Williams College in Williamstown, Mass., who watched the June 8 phenomenon from Thessaloniki, Greece's Aristotle University, told The Associated Press.
Missed it? It will repeat on June 6, 2012. Better catch it then, though. It won't happen again until Dec. 11, 2117.
See the Cicadas, Smell the Corpse
Most cycles in earthbound nature tend to be much shorter than those in space, according to Greg Borzo, a spokesman for the Field Museum in Chicago. Therefore, some can be seen by people willing to plan ahead and travel.
Brood X, the largest brood of cicadas, emerged from the ground this year for just a matter of weeks, as they do every 17 years in parts of the Eastern United States. People who want to see it next time should mark their calendars for summer 2021.
Also keep a nose out for the knee-buckling stench of the "corpse flower," also known as the Amorphophallus titanum plant, which grows wild in Indonesia. Each one typically blooms once a decade or longer, but this year alone has seen cultivated blooms in Storrs, Conn., Chico, Calif., and Nacogdoches, Texas, with another expected in Madison, Wis.
The heavens also have some dazzling short-term cycles.
One of the most prized sights for astronomers is a total solar eclipse. Many professionals have seen them, as they occur periodically in localized parts of the globe. But North American residents have not seen one since February 1979, unless they made travel plans.
"The next total solar eclipse here in the continental U.S. does not occur until Aug. 21, 2017," Kardell says. "The 2017 eclipse will cut right across the heartland. It will be followed nearly seven years later by another one that will be visible in Mexico and the eastern U.S. [on] April 8, 2024."
North American residents have seen a "blue moon." In fact, one took place Saturday, though it wasn't really blue.
Folklore holds that a "blue moon" is the second full moon in a single month, an event that occurs on average every 2 ½ years, according to Tony Phillips, editor of Science@NASA and SpaceWeather.com.
More elusive is a truly blue moon, though they appeared night after night around the world in 1883 after the Indonesian volcano of Krakatoa threw plumes of ash into Earth's atmosphere. On some nights the moon appeared blue, and on other nights it was green. It all depended on the size and uniformity of the ash particles suspended in the air, Phillips says. Under the right conditions, they distorted the wavelength of the moonlight so it appeared blue.
Localized blue moons have since been reported in conjunction with certain volcanoes, forest fires and dust storms, and Phillips records several eyewitness accounts on SpaceWeather.com.
But there are rarer things than solar eclipses or blue moons that could conceivably appear in our lifetimes.
Northern hemisphere astronomers hope they get to see an exploding star, called a supernova.
"The last one that was bright enough to be seen without a telescope was in 1987; it was only visible from the southern hemisphere," Kardel says. "For those of us who live in the Northern Hemisphere, there hasn't been one since the 1600s. … We are likely overdue for another one, yet we cannot predict them and as such have no idea when the next one will be."
The last supernova explosion observed in Earth's galaxy, the Milky Way, was Kepler's star in 1604, according to the web site of NASA's Goddard Space Flight Center.
But the humdinger of Milky Way supernova explosions occurred in 1054. It could be seen by the naked eye for 653 days — during daylight for 23 days — and was estimated to be four times brighter than Venus, according to Chinese records.
"It was visible in the day like Venus, with pointed rays in all four directions," the records say. "The color was reddish-white."
Expanding remnants of the explosion are known as the Crab Nebula, and to this day can be seen through a telescope as a glowing, string-mottled gas cloud.
‘Oohh and Aahh’
Also unpredictable are fireball events, caused when meteorites enter the atmosphere and burn up in the sky.
Fireballs are not uncommon around the globe, but the very largest are legendary. According to Yeomans, the greats include the Tunguska fireball event of 1908, which felled trees in Siberia for thousands of miles around, and Siberia's Sikhote-Alin fireball event of 1947 and the western Connecticut fireball event of 1807, each of which rained down numerous large meteorites that fell to Earth intact.
Yeomans considers fireball events among the top three " 'oohh and aahh' types of events" potentially visible to the naked eye, along with total solar eclipses and meteor storms.
The great meteor storms of 1799, 1833 and 1966 looked like shooting stars emanating from a single point in the sky 1,000 times an hour, Yeomans says — far more than the still-impressive Leonid meteor showers of recent years, when a few hundred blips an hour might appear in a dark, moonless sky.
Yeomans regrets missing the 1966 shower that only was visible in the American Southwest while most people slept. He was in the Northeast at the time.
"Folks in Arizona, if they were alert to the fact that this was a possibility, saw an extraordinary shower," Yeomans said. "But few people did, because it was at 3 in the morning, and how many people are up at 3 in the morning?"