Do note, there is now a fee to enter the courtyard to stand on the Prime Meridian Line.
It's a classic photo opportunity: have your photo taken standing on the Prime Meridian Line at Greenwich. Head for The Royal Observatory and in the courtyard is a metal strip where you stand over the line and can be in the the eastern and western hemispheres at the same time. Greenwich was chosen back in 1884 as the Prime Meridian of the world, Longitude Zero (0° 0' 0"). Every place on Earth is measured in terms of its angle east or west from this line (longitude), just as the Equator divides the northern and southern hemispheres (latitude).
Another fun freebie while you’re there is to watch the red Time Ball on top of Flamsteed House drop at 1 p.m. every day. At 12:55 p.m., the time ball rises half way up the mast. At 12:58 p.m. it reaches top, and at 1pm exactly, the ball falls, and so provides a signal to passing ships and anyone else who happens to be looking.
See the Greenwich tour for more details.