Weathering the stars: ancient astronomers tuned into the Pleiades for a seasonal forecast.
Today in the Northern Hemisphere we begin summer in June, at the solstice. In the 3rd century BC, however, the poet Aratus of Soli saw the onset of summer--and the time to harvest the winter wheat crop--in May at the first predawn rising of the Pleiades star cluster in Taurus. In the same passage in the Phaenomena, Aratus indicates that the setting of the Pleiades just before dawn in November coincides with the beginning of winter and the time to plow. He also warns, "The Pleiades at their dawn setting will bring a stormy winter."
The Pleiades didn't really predict the weather in the ancient Mediterranean world, however. Rather, they timed seasonal change. Mythologically, the stars were regarded by the Greeks as the daughters of Atlas, the Titan condemned to support the heavens on his shoulders.
Recognizing their seasonal significance, the Roman mythographer Hyginus (1st century BC) asserted that the cluster's stars "enjoy greater honor than other stars because the rising of their sign signals summer, while its setting signals winter." Other peoples around the planet also used the Pleiades as a sign for seasonal shifts.
The Pleiades shine bright enough to attract notice and are distinctively compact. There's nothing quite like them in the sky. Their location along the Moon's monthly path through the stars and on the Sun's annual circuit caught the ancients' attention.
Even in the Southern Hemisphere, where the seasons are reversed, the cluster has seasonal meaning. Anthropologist Gary Urton reported that Pleiades observations are performed at the end of the Festival of St. John in Mismanay, a highland Quechua village in Peru about 30 miles (50 km) northwest of Cuzco. Celebrated on June 24th, this fiesta is associated with the June solstice, which was observed astronomically and ritually in the Inca era.
The Pleiades reappear for the first time in the predawn sky about three weeks before the solstice. As Urton recounts in At the Crossroads of the Earth and the Sky (1981), the villagers assemble at the end of the night on June 24th and examine the Pleiades as the cluster clears a ridge northeast of the village.
They establish whether its stars look "large and bright" or "small and dim" and plan their agricultural strategy accordingly. If the former is seen, the year's crops will be abundant. If the latter, the harvest will be less successful, and planting must occur later to compensate for late and less rain. For Mismanay, the Pleiades aren't just a calendrical event--they're an annual rain almanac.
Another anthropologist, Benjamin Orlove, documented similar Pleiades prognostications from a dozen Andean communities. He wondered how the visual appearance of the Pleiades could be linked to agricultural production--or even change at all. Collaborating with climate expert John Chiang and meteorologist Mark Cane, Orlove learned how the appearance of the Pleiades could be linked with annual variations in rainfall, which in turn affect the growing season of potatoes.
In years when the local people reported more stars in the Pleiades (and therefore a larger cluster) and brighter stars, the growing season began on time and enjoyed adequate rain. When the Pleiades looked under the weather--small, dim, and fewer in number--the rain arrived late, and there was less of it.
With the help of satellite observations, Orlove's team confirmed that rainfall variations and the appearance of the Pleiades are correlated with changes in the amount of high cirrus clouds northeast of the Andean highlands. The irregularly recurring "El Nino" warming of the eastern Pacific Ocean creates cirrus over the northern Amazon basin and also diminishes the flow of moist air and rainy weather from the east.
From high elevation, the Andean farmers often observe 10 or more stars in the Pleiades. El Nino's high cirrus clouds, however, obscure the cluster's fainter stars and make even the brightest stars seem dull and soft.
The Quechua name for the Pleiades is Collca, which means "storehouse" and links the cluster to farming and the food supply. The farmers successfully use the Pleiades to make accurate weather predictions for the coming growing season because their appearance really is related to future weather.
There's evidence that this practice was already in place at the time of the Spanish conquest. The unknown author of the Huarochiri Manuscript, probably written around 1600, mentions the Pleiades: "If they come out at their biggest people say
'This year we'll have plenty.' But if they come out at their smallest people say, 'We're in for a very hard time.'"
This long-standing Peruvian tradition reinforces an astronomical analysis of the Torrean, an odd-shaped building at Machu Picchu, the famous Inca citadel. In 1983 astronomers demonstrated that the sculpted outcrop inside the Torre6n's curved wall accommodates an accurate alignment on the June solstice sunrise through a northeastern window. The Pleiades also appear in the same window, and it's reasonable to imagine official Inca skywatchers observing the solstice and keeping a weather eye out for the Pleiades.
The brightest of the Pleiades is only 3rd magnitude. None of the cluster's stars have the gravitas to anchor the weather news alone. But as a team, the Pleiades present a seasonal forecast in prime time.
E. C. Krupp is more than a fair-weather friend of astronomy at Griffith Observatory.
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|Title Annotation:||Rambling Through the Skies|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 2008|
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