The 51-52 Century has been a significant time period in the chronology of Doctor Who. It also seems to have captured the imagination of Steven Moffat.
- The 51st Century is the start of "The Great Breakout", an expansionistic period where mankind headed for the stars. First mentioned in the Classic Who episode The Invisible Enemy.
- The 51st Century is the era where K-9 was created.
- The 51st Century is when humans begin early expirements with Time Travel.
- The Time Agents are established in the 51st Century.
- The 51st Century marks the rise and fall of the villainous Magnus Greel from the Classic Who episode 'The Talons of Weng-Chiang'.
- The epsiode 'The Girl in the Fireplace' (written by Steven Moffat) took place in the 51st Century.
- The 'Silence in the Library' and 'Forest of the Dead', which introduced us to River Song (also written by Steven Moffat) took place in this era.
- The 'Time of Angels' and 'Flesh and Stone' (written by Steven Moffat) also take place in this era.
- The Stormcage Containment Facility, where River Song is imprisoned, is located in the 51-52 Century.
So every time you hear a Doctor Who episode mention the 51st or 52nd Century , you should take note. It's probably relevant.