In 1999, two men with metal detectors unearthed one of the most significant finds of modern archaeology: the Nebra Sky Disc, a 30-cm bronze disc inlaid with gold depicting the sun, moon, stars (including the Pleiades), and arcs that apparently represent sunrise and sunset at the solstices at Mittelberg Hill in Germany, and a holy sun boat symbol, dating from 1600 BCE or earlier. Because the illicit finders sold the disc on the black market, skepticism about its authenticity abounded for several years before scientific investigations confirmed it was a legitimate find and possibly the oldest concrete depiction of astronomical phenomena ever found. (The looters were seized by police in a sting operation in a bar in Switzerland, sentenced to prison, appealed, and got longer sentences.) [more inside]
Signs of Modern Astronomy Seen in Ancient Babylon - "Scientists have found a small clay tablet with markings indicating that a sort of precalculus technique was used to track Jupiter's motion in the night sky." [more inside]
At dawn on the winter solstice, the passage and chamber of the megalithic passage tomb at Newgrange are illuminated for 17 minutes by a shaft of sunlight entering through the roofbox above the entrance. The builders of Newgrange achieved this precise alignment over 5,000 years ago, 1,000 years before Stonehenge. You can watch the sunrise illumination on a live webcast between 08:30 and 09:30 UTC on Sunday, December 21st.
The Thirteen Towers of Chankillo in Peru may be the Western Hemisphere's oldest known full-service solar observatory, showing evidence of early, sophisticated Sun cults, according to archaeoastronomy professor Clive Ruggles. The 2,300-year-old complex featured 13 towers running north to south along a ridge and spread across 980 feet to form a toothed horizon that spans the solar arc. Last year, another ancient observatory was discovered in Peru by Robert Benfer. The Temple of the Fox is 4,200 years old, making it 1,900 years older than the Chankillo site, but wasn't a complete calendar.
Ancient observatories from space Satellite images of Angkor Wat, Chichen Itza, Chaco Canyon, Stonehenge, Teotihuacan, and others. The observers, observed. High res images available.
The Chaco Culture National Historical Park (flash) encompasses a whole canyon's worth of buildings that appear to be designed to elaborately showcase the movement of the sun and the moon. And the website for the park is pretty well done. Also see the PBS-supported documentary called "The Mystery of Chaco Canyon" from the Solstice Project and a previous Metafilter discussion of archaeoastronomy.