Neil Armstrong (search), the first man to walk on the moon, has never felt comfortable with the celebrity he achieved. In fact, it puzzles him.

"Friends and colleagues, all of a sudden, looked at us, treated us slightly differently than they had months or years before when we were working together," the Apollo 11 (search) astronaut told "60 Minutes" in an interview to be broadcast Sunday. "I never quite understood that."

Armstrong, 75, rarely grants interviews. He agreed to one last month just before his only authorized biography, "First Man: The Life of Neil A. Armstrong," hit bookstores.

The interview will air on CBS, which, like the book's publisher, Simon & Schuster, is owned by Viacom.

Author James R. Hansen (search), an Auburn University (search) professor and former NASA historian who wrote the biography, was allowed more than 50 hours of recorded interviews with Armstrong in his suburban Cincinnati home.

On July 20, 1969, Armstrong, then 38, stepped onto the moon with the famous words, "That's one small step for man, one giant leap for mankind."

In the years since, he has taught at the University of Cincinnati and served on corporate boards, all the while rejecting interview requests.

In an e-mail response to The Cincinnati Enquirer, Armstrong said he reluctantly agreed to the book deal.

"Many individuals whose opinions I value have urged me to find a way to put my story in print," Armstrong said. "I concluded a biography would be superior to an autobiography.

"I believed the author should have access to my recollections and thoughts although he would not be bound to use or accept them."