For as long as there has been human imagination, the stars above have been thought to presage and preside over the seemingly inexplicable. But mysticism hasn't always jibed with the science of things.

When it comes to potatoes, however, the stars are perfectly clear. Or, in some years, they're exceedingly fuzzy. Either way, it all makes sense to the farmers high up in the mountains of South America.

For centuries, Andean villagers have looked to the stars each June for clues about rainfall several months hence -- during the growing season -- so they could decide when to stick their spuds in the ground.

Only now has modern science figured out how they did it, and it turns out the Andean imagination isn't so wild. Somewhat unwittingly, the farmers have deciphered a stellar connection with atmospheric conditions, all tied to upcoming effects of El Niño.

Benjamin Orlove, a researcher at the University of California, Davis, explored the ancient Andean ways, looked at potato yields, measured rainfall and then studied ocean temperatures. His work appears in the January 6 issue of the journal Nature.

From about June 15 to 25, a group of Quechua-speaking farmers observe the brightness of stars in the Pleiades, which it turns out is affected by low-density, high-altitude clouds that are not visible to the human eye. When these cirrus clouds are present in June, El Niño conditions often prevail, Orlove and his colleagues found, and that portends drought for the Andes during the subsequent October-to-May growing season.

"The rainfall in El Niño years is about two-thirds of what it is in La Niña years," Orlove told space.com. "Potato yields fall by a similar amount."

El Niño is a periodic warming of water in the tropical Pacific Ocean. Warmer water fuels intense thunderstorms that pump energy high into the atmosphere, altering large-scale wind currents and weather patterns around the globe.

The opposite oceanic condition, La Niña, was found by the researchers to be related to clear Andean night skies in June, later driving increased rainfall during the growing season. The farmers, of course, already knew that, or at least they knew the parts they needed to know.

The decreased visibility related to El Niño is caused by an abundance of high-altitude water droplets that distort incoming light rays. Though the droplets have formed clouds, they're not the typical puffy, visible variety. (Any backyard astronomer knows the frustration of a seemingly clear night that turns out to provide lousy viewing.)

"Think of looking through water at the bottom of a stream or lake," Orlove explains. "How obscured an object is depends on the clarity of the water and the distance of water through which you are looking. Air works similarly."

Long before Orlove figured out the science of the Andean method, farmers had used it to determine the planting time for potatoes, their most important crop and one that is particularly vulnerable to drought at planting time. By delaying planting four to six weeks when drought is expected, farmers increase the odds that their crop will sprout during months that typically experience more rainfall.

Orlove expects that the divining of the Pleiades has been going on for four centuries. He notes that the Incas, who were conquered by the Spaniards in the 1500s, worshipped the cluster of stars.

The ongoing work by Orlove and his colleagues seeks to integrate traditional and modern scientific knowledge to make forecasts, and it may help scientists study regional effects of the El Niño cycle.

Regarding the groups of farmers that were studied, more data is needed to come up with a precise calculation of how often the method is accurate. But Orlove offered this: "The forecasts are accurate around two-thirds or three-quarters of the time."