An intelligent person who is not only well-versed in his/her subject, but is also graced with an ability to make that subject come to life for others who may not have as much knowledge. A Sagan knows how to evoke childlike wonder and awe, while not taking himself/herself too seriously. Humanisitic, warmhearted, optimistic, and unpretentious. Not just a sage - a Sagan.
"That Neil DeGrasse Tyson - what an excellent example of a Sagan!"