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Current astronomy: go with the flow and take a dip in the celestial river.

Go with the flow and take a dip in the celestial river.

IT'S STILL A LITTLE EARLY FOR THE spring flood, but there's high water on February's all-sky map. From its source near Rigel, at Orion's left foot, Eridanus, the River, meanders through the starry floodplain. Cascading over the southern horizon, the celestial current flows out of sight.

According to the ancient Greeks, the River's mouth was the star Acamar (Theta Eridani). There it emptied into the abyss and perhaps fed Oceanus, the world-girdling waters. Later, when 16th-century European explorers sailed far enough to map Achernar, a more southern and brighter star, it replaced Acamar as the "End of the River." Canopus, east of Acamar and sharing a name with a famous port in Egypt's delta, reinforced the association of Eridanus with the Nile. The Catasterismi, the oldest collection of Greek constellation myths, mentions their affiliation.

Other famous rivers also were said to be terrestrial counterparts of Eridanus, but the Roman mythographer Hyginus reiterated the Nile's connection with Eridanus. He also entertained the belief that Eridanus just keeps rolling along as a celestial version of the river Oceanus beyond the horizon.

Eridanus was a cosmic current that mirrored antiquity's understanding of the circulation of the world's waters. Somehow the streams, springs, ponds, lakes, ocean, and seas were all connected, and rivers were part of Earth's plumbing. From the high ground of the starry watershed, Eridanus transported celestial water downriver to the world's southern edge.

People didn't recognize celestial rivers just in streams of stars, however. Sometimes they saw running water in the frothy band of the Milky Way. Hindu mythology makes a heavenly Ganges out of the Milky Way's fluid course. Its waters fall from the zone of the north celestial pole, pour across the sky, and splash down upon the head of the god Siva, enthroned upon the summit of Mount Sumeru, the world-axis mountain. Rippling through his hair, the river splits into four branches that rush down the mountain's sides toward the four cardinal directions. These waters feed the Ganges, which irrigates India's great central plain, nurtures its life, and, in Hindu belief, purifies everything.

Two thousand years ago the Chinese called the Milky Way the "Heavenly River," and it was regarded as the sky's version of the Yellow River, which flows through north central China and cradled civilization there as early as the Bronze Age's Shang dynasty around 1500 BC.

One Chinese star tale puts a seasonal bend in the Heavenly River's bed. Stranded on opposite banks of the Milky Way, the stars Vega and Altair are a pair of lovers known as the Weaver Maid and the Cowherd. Their failure to perform their work in heaven prompted the Queen Mother of the West to separate the two. She cut a channel through the sky and diverted the Heavenly River between them. Their only chance to meet falls in late summer, after the summer rains, when the river level has fallen. A late storm will, however, delay their rendezvous for another year.

According to the Shih chi, composed around 90 BC during the Han dynasty, the Milky Way's stars are abundant when water is plentiful and few during drought. Rains will wash the air clear for a bright Milky Way, while drought generates dust, which diminishes its light. The ancient Chinese also saw additional hydrological meaning in the Milky Way, which flowed, they believed, into Earth's encircling ocean waters and helped generate the tides.

In Andean Peru, the Milky Way is also seen as an essential element of the hydrological cycle. Quechua people call it Mayu, or "River," and say it has two branches. These correspond to what are called the summer Milky Way and the winter Milky Way in the Northern Hemisphere, though in the Southern Hemisphere, of course, the seasons are reversed. Fed by the oceanic waters that surround Earth, both branches flow into the sky from an invisible, subterranean source in the north. The two currents meet in the southern sky, near the Southern Cross, and the force of their confluence creates foam that is visible in the rich star clouds of the southern Milky Way.

Anthropologist Gary Urton encountered these traditional ideas among the villagers of Mismanay, a small Quechua settlement about 50 kilometers northwest of Cuzco, the old Inca capital. He collected a great deal of additional data and reported his analysis in his first book, At the Crossroads of the Earth and the Sky, in 1981. Bernabe Cobo, a Jesuit priest, documented a similar conception of the Milky Way among the Inca in 1653.

In this part of the Andes, the primary watercourse is known as the Vilcanota. It flows from the southeast to the northwest, where its waters are believed to join the sea. There they are thought to meet the submerged source of the Milky Way, and from the northern horizon they are recycled through the sky to fall as rain, which replenishes the river.

Orion's half of the celestial river is the Southern Hemisphere's summer Milky Way, and when it crosses overhead, it arcs from the southeast to the northwest. It is said to mimic the Vilcanota, and it is accompanied by rain. Winter, on the other hand, is the dry season, and its Milky Way, dominated by Scorpius, crosses the zenith from the southwest to the northeast. Linking seasonality with the intercardinal configurations of the Milky Way, the Quechua see the cosmos as an engine of hydrological renewal, a recirculation of water pumped by seasons and carried by the sky.

Sinuous and fluid like a river, the Milky Way inspired some to see its seasonally distinctive appearances at work in one of the planet's most important processes, recycling water via rain. Of course, we now know the fresh waters of the world are not really resupplied by any celestial river. In fact, the Sun and its seasonal variation drive the water cycle, not the stars or the Milky Way.

Although hydrologists now handle studying the circulation of the world's water supply, an astronomer first sorted out the real mechanism of the cycle. Edmond Halley, who calculated the orbit of the famous comet that bears his name, also experimentally determined the Mediterranean Sea's annual evaporation and matched the loss with the water fed to it by rivers. At roughly the same time, physicist Edme Mariotte demonstrated the relationship between rainfall and the level of groundwater by monitoring the rise in the water table in the seeping basement of Paris Observatory.

In the late 17th century, rainy days, cloudy nights, and leaking observatories helped precipitate the flood of scientific knowledge.

E. C. KRUPP goes with the flow at Griffith Observatory in Los Angeles.
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Author:Krupp, E.C.
Publication:Sky & Telescope
Date:Feb 1, 2004
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