Incan El Niño
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In the central Andes of South America, a single harvest can make the difference between feast and famine. Farmers dating back to the Incan Empire learned how to read the sky to figure out the best time to plant their crops. One of their ancient rules of thumb has now been confirmed by modern science. Hi I'm Dave Thurlow for the Mount Washington Observatory.

This time of year is the beginning of winter in South America. Each night during the middle of June, mountain villages across Peru and Bolivia closely watch the northeast sky for the Pleiades constellation. The Incans worshiped this cluster of stars. If the Pleiades are dimmer than usual, that's a bad sign. It means there will be less rain than usual for the growing season to come. If the stars shine brightly, that means abundant rain.

Why would the stars go dim before a drought? Well it turns out that high, thin cirrus clouds often sweep across the Andes in the summer before an El Niño kicks in. These clouds may be too thin to see with the naked eye, but they're enough to cause a noticeable dimming of the stars at night. The villagers correctly figured out that cirrus was a sign of the drought that El Niño would later bring to their land, even though they didn't know anything about El Niño itself.

Villages across the Andes still watch the stars in June and use their brightness as a forecast tool. If the stars are dim, they'll delay their potato planting a few weeks so that the dry spring is less likely to damage their crop.