The Royal Observatory was founded in 1675 by Charles II. It features the only public planetarium in the UK, a 4.5 billion-year-old meteorite and famously, the prime Meridian. This is the longitude line that divides the Earth, into two hemispheres. If one uses directions of East and West from a defined prime meridian, then they can be called Eastern Hemisphere and Western Hemisphere.
Greenwich Mean Time (GMT) was at one time based on the time observations made at Greenwich (until 1954).