This fall marks the 30th anniversary of what has been called “the most widely watched PBS series in the world.” According to one of the show’s co-writers, almost a billion people worldwide have watched “Cosmos: A Personal Voyage” and gained an understanding of humanity’s place in the universe, and the paths taken by early astronomers to achieve that knowledge. For 26 of those years, Norm Wheeler has shown all 13 television episodes of “Cosmos” to his high school science students at The Leelanau School in Glen Arbor.