History
What did the 1884 International Meridian Conference decide?
The Washington conference selected Greenwich as a common zero meridian. It did not impose every modern time-zone border on the world.
Longitude needs a starting line
Latitude has an obvious middle at the equator. Longitude does not, so maps and navigation need an agreed zero meridian. By 1884, Greenwich was already widely used on nautical charts and by navigators.
Delegates chose a common prime meridian
Representatives meeting in Washington in October 1884 chose the meridian through Greenwich as the initial meridian for longitude and a universal day. The choice made global coordination easier because it aligned with a practice already in widespread use.
It was not a global clock-law machine
The conference did not draw today’s civil-time borders or compel every country to use an identical local clock. Those are national and regional choices. The important legacy is the shared geographical reference from which offsets and modern UTC-based coordination are described.