Bill Nye is a Time LordA strange man with a love for showing people strange things (Doctor Who was originally conceived as a Edutainment Show) and for bow ties with a title starting with "The". The children that regularly appear on the show are his companions. His TARDIS is Nye Labs.
Bill nye is the reincarnation of abe lincolnOr was sent back to the 1860s to end the civil war and free the slaves
Bill nye is a terminatorThe theme song has him talking with a woman's voice, terminators can do that plus the abe lincoln thing suggests he went back in time
Bill nye was a companionPossibly when he was younger, and his travels inspired him to educate others.