The observatory is built on top of a hill overlooking Greenwich Park.
This makes it easily visible from the river Themes, where the ships could see the ball drop and synchronise their clocks to the observatory.
The observatory defined where the location of prime meridian, or longitude zero. Most tourists come here, stand astride the prime meridian line, and get the photo taken.
Since the position of the prime meridian is an arbitrary standard, there has actually been several different meridians. Over time, the prime meridian has moved towards the east as extensions were made to the observatory building. This photo is looking south, so the meridians move towards the left of the photo.
The first meridian is
The most obvious: George Airy’s meridian.
Modern GPS systems use a coordinate system called WGS-84 (World Geodetic System) which has established a new prime meridian a bit further east of the observatory – roughly from where this photo was taken.