by the Greeks. It has 4–7 masts and is able to sail against the wind due to the usage of tanja sails. These ships reached as far as Madagascar by ca. 50–500 AD and Ghana in the eighth century AD. Northern European Vikings also developed oceangoing vessels and depended heavily upon them for travel and population movements prior to 1000 AD, with the oldest known examples being longships dated to around 190 AD from the Nydam Boat site. In early modern India and Arabia the lateen-sail ship known as the dhow was used on the waters of the Red Sea, Indian Ocean, and Persian Gulf. China started building sea-going ships in the tenth century during the Song Dynasty. Chinese seagoing ship is based on Austronesian ship designs which have been trading with the Eastern Han dynasty since the second century AD. They purportedly reached massive sizes by the Yuan dynasty in the 14th century, and by the Ming dynasty, they were used by Zheng He to send expeditions to the Indian Ocean. Water was the cheapest and usually the only way to transport goods in bulk over long distances. In addition, it was the safest way to transport commodities. The long trade routes created popular trading ports called Entrepôts. there were three popular Entrepôts; the Malaka in southwestern Malaya, Hoi An in Vietnam, and Ayuthaya in Thailand. These super centers for trade were ethnically diverse, because ports served as a midpoint of voyages and trade instead of a destination. The Entrepôts helped link the coastal cities to the "hempispheric trade nexus". The increase in sea trade initiated a cultural exchange among traders. From 1400 to 1600 the Chinese population doubled from 75 million to 150 million as a result of imported goods, this was known as the "age of commerce". Mechanical engineer Ma Jun (c. 200–265 AD) invented the south-pointing chariot, a wheeled device employing a differential gear that allowed a fixed figurine to point always in the southern cardinal direction. The mariner's astrolabe was the chief tool of Celestial navigation in early modern maritime history. This scaled down version of the instrument used by astronomers served as a navigational aid to measure latitude at sea, and was employed by Portuguese sailors no later than 1481. The precise date of the discovery of the magnetic needle compass is undetermined, but the earliest attestation of the device for navigation was in