If the Pleiades are bright in June, plant the potato crop soon; if they are faint and rise late, it's better to wait. Farmers in the Andes have followed this maxim for centuries. Now, scientists have shown that there's some truth to the age-old advice: A dimming of the Pleiades star cluster may actually auger a weather change that affects local farmers, according to a report in today's Nature.
Freshly planted potato tubers are extremely vulnerable to drought, so Andean villagers try to time the planting to coincide with the start of the rainy season in October. To fine-tune the date, they keep their eyes on the Pleiades star cluster as it rises over the eastern horizon in the June predawn sky. If the stars are faint and become visible a day or two later than normal, the villagers believe a drought must be coming, and they postpone planting the potato crop by 4 to 6 weeks. Is this mere superstition ... or ancient Incan knowledge put to good use?
Ben Orlove, an anthropologist at the University of California, Davis, and his collaborators now conclude that the practice follows wisdom gleaned from careful observations. They compared 8 years of atmospheric transparency measurements over the Central Andes, compiled by the International Satellite Cloud Climatology Project, with crop yields in the same region and found that obscuring high-altitude clouds were more common in years in which El Niño causes drought in the Andes, postponing the onset of rain. It's not a perfect correlation, says Orlove, "but it is more than just an astounding series of coincidences."
"This is a good example of [Incan] protoscience at work," agrees Yale University astronomer Brad Schaefer. "Now I'd like to know how reliable an indicator it is." To answer this question, says Orlove, one would have to plant test potato crops on several possible planting dates and compare yields in successive years--but that's not an experiment he's currently planning to undertake.