A prime meridian is a meridian that is defined as having a longitude of 0º. Internationally, the prime meridian passes through the Greenwich Observatory in London, in the United Kingdom. When combined with the 180th meridian, the prime meridian forms a great circle that divides the Earth into an eastern hemisphere and a western hemisphere. Throughout history, many prime meridians have vied to become the international standard; unlike the equator, a prime meridian is arbitrary. The dominance of Great Britain in 19th century science and technology led to the London meridian being adopted.