Which observatory in Greenwich, London, marks the location of the Prime Meridian?

The Greenwich observatory associated with the Prime Meridian is the Royal Observatory, Greenwich.

Set on a hill in Greenwich Park overlooking the River Thames, the Royal Observatory was commissioned by King Charles II in 1675. Its original purpose was closely tied to astronomy and navigation, especially the problem of determining longitude at sea. Generations of astronomers used instruments there to establish precise reference lines and time standards.

The meridian through the observatory's Airy Transit Circle was adopted as the international Prime Meridian at the International Meridian Conference in Washington, D.C., in 1884. That decision helped make Greenwich the reference point for global longitude and gave rise to Greenwich Mean Time, the predecessor of modern UTC.

A common modern misconception is that the brass or steel line in the courtyard is exactly zero longitude for GPS. The current geodetic reference meridian lies roughly 102 metres east of the historic astronomical line. The courtyard line and green laser remain powerful symbols of the world's historic zero meridian.