This month brings May flowers down here and a rare planetary gathering up there. All five naked-eye planets will be readily visible in the same patch of sky, creating a striking assembly that will not be equaled again for decades.
Many cultures have regarded past planet bunchings, or conjunctions, with alarm. History records panic among segments of society during the close conjunctions of 1186, 1365, 1504, 1524, and 1962. In 1974 a best-selling book warned of the "Jupiter effect," predicting that a 1982 planetary alignment might trigger powerful earthquakes. Despite conjunctions coming and going without incident, many people still fear the consequences when the bright planets all crowd on one side of the sun. Even rational minds may wonder whether the combined gravity of those bodies could have a disruptive influence on Earth.
The answers are reassuring. During the tightest conjunction of the past 5,000 years—when every naked-eye planet crammed into a 4 1/2 degree bit of sky on February 26, 1953 B.C.—Earth's average, three-foot ocean tides rose by less than an additional 1/100 inch. Not surprisingly, every attempt to link conjunctions with earthly phenomena has failed. The daily tides triggered by the sun and the moon are thousands of times as powerful and far more variable, changing by a factor of two depending on whether the sun and the moon form a line with the Earth or lie at right angles to each other. There is a slight statistical increase in the number of certain earthquake aftershocks during periods of maximal lunar-solar tides, but even that effect is very weak.
Mercury, Venus, Mars, Jupiter, and Saturn gather together for good viewing this month—but only NASA can show you this whimsical lineup.Photograph courtesy of NASA/JPL
So why do the superstitions persist? One reason may be that multi-planet conjunctions are a lot more common than most people realize, especially if you include planets hidden behind the sun. In the past decade alone, five-planet gatherings occurred in February 1992, January 1994, December 1995, February 1997, January 1998, and May 2000. If one randomly coincides with a major natural disaster or human calamity, the naive may overlook the laws of chance and fall for astrology instead.
Mathematicians have a more rational concern—the effect of planetary interactions on the long-term stability of the solar system. A century ago, French mathematician Henri Poincaré showed that the gravitational interplay of just three celestial bodies is ultimately chaotic and unpredictable. And recent analyses indicate that such skirmishes were much more intense around the time the planets were forming. According to a simulation run by planetary astronomer Renu Malhotra of the University of Arizona, Neptune was flung outward by 700 million miles while Jupiter edged some 20 million miles sunward. "The magnitude of the chaos has decreased since the solar system's youth," Malhotra says reassuringly. Anything strongly unstable would have escaped or suffered a fatal collision long ago. But the modern solar system is still fundamentally chaotic, meaning that scientists cannot in principle predict the future locations of the planets. Over billions of years, we may yet lose Pluto or even Mars.
For the short term, one forecast is easy: It will be 38 years before all five bright planets visibly assemble again. But you can see them right now, hovering in the twilight around 9 p.m. As May opens, Mercury hangs lowest, with dazzling Venus just above; Mars and Saturn are a bit higher, and bright Jupiter is perched on top. The scene changes nightly: Mars and Venus nearly merge on the 10th, the crescent moon joins Venus and Mars on the 14th, and Jupiter approaches Venus at month's end. Our own planet will likely be a very different place when next they meet.
Sky & Telescope's calendar of celestial events is a good place to follow this month's planetary gathering: http://SkyandTelescope.com/observing.