Archeologists in China have unearthed what they believe is an ancient "celestial calendar." The 23 rectangular wooden pieces were found inside a 2,200-year-old tomb in China's Wulong District. They are etched with Chinese characters connected to an astronomical calendar known as Tiangan Dizhi. It was established during the Shang dynasty, which ruled from 1600 BC to about 1050 BC. This is the first time written slips linked to the ancient calendar have been found in a tomb.