Dr Who is (L-R) William Hartnell, Patrick Troughton, Jon Pertwee, Tom Baker, Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann, Christopher Eccleston, David Tennant and Matt Smith (Photo courtesy: BBC)

Saturday 23 November is The Day of the Doctor, the 50th anniversary of the world's longest running science fiction series, with a special new episode due for simultaneous broadcast in over 70 countries worldwide at 19:50 GMT.

Writing in Entertainment Weekly, Doctor Who expert Dr Piers D Britton, who wrote a book on Doctor Who titled TARDISbound, offers the following facts for Doctor Who fans' delectation.

1. Doctor Who show-runner Steven Moffat did not initially want 11th Doctor Matt Smith to wear his now-iconic bow tie.

2. All the Silurians seen since 2010′s The Hungry Earth are played by the same three actors.

3. Paul McGann is technically the longest-serving Doctor, though he appeared only once on television, in 1996. Tom Baker is, of course, the longest serving on television, having starred in more Doctor Who shows than any other actor. McGann, incidentally, has recently made a second appearance as the Doctor (see video).

4. All of David Tennant's suits (including the jackets) are made using off-the-rack pants.

5. The Impossible Astronaut (2011) was the first episode filmed in the US in which the actors playing the Doctor and his companions actually participated in shooting; earlier, Daleks in Manhattan featured footage shot in New York, which was then digitally blended with the Welsh locations in which David Tennant and Freema Agyeman were shooting.

6. The TARDIS has a six-sided control console because it was designed to have six operatives.

7. Rose (2005) was the first episode named for a companion (though the title of the first episode, 1963′s An Unearthly Child, does refer to the Doctor's earliest companion, his granddaughter Susan).

8. The ancient race of aliens The Weeping Angels were inspired by a carved figure in a graveyard that Steven Moffat used to see when he went on family holidays. The graveyard was marked "dangerous", which is what attracted Moffat's interest.

9. Two of the actors playing the Doctor have married actresses who had continuing or key roles in the series: Tom Baker was briefly married to Lalla Ward, who played the Time Lady Romana, in the early 1980s, and David Tennant is now married to Georgia Moffett, who played the Doctor's daughter, Jenny (and is, coincidentally, the real-life daughter of the Fifth Doctor, Peter Davison).

10. Both Peter Capaldi and Karen Gillan not only had Doctor Who roles before they were cast as, respectively, the Twelfth Doctor and companion Amy Pond, but actually appeared in the same episode, The Fires of Pompeii, in 2008.

In preparation for The Day of the Doctor, the BBC has released a special mini-episode called The Night of the Doctor on YouTube, starring Doctor number eight, Paul McGann. Watch it below: