When I was four years old, if you had asked me to name the most famous people in the history of the world, I would have said Alexander the Great and Neil Armstrong. Probably in that order, because even though I didn't know what he was famous for, I assumed that Alexander the Great wouldn't have been called that if he wasn't supposed to be the best. Still, it didn't seem right. Whatever this Alexander had done, surely it couldn't compare to becoming the first person to walk on the moon.

I was four years old in 1978. That year saw the publication of a book called The 100, in which a guy named Michael Hart took it upon himself to rank the hundred most influential people ever to have lived. His choice for #80 was John F. Kennedy, solely for his role in committing the resources of a global superpower to landing a human being on the moon. Neil Armstrong, Hart argued, didn't affect the course of history; had he never lived, some other test pilot would have taken that small step for a man and giant leap for mankind. But without Kennedy, reaching the moon might still be a fanciful goal.

Maybe that's true. Still, Neil Armstrong always held an important place in my mental landscape. The planet Earth formed 4.6 billion years ago; for four billion of those years, it has harbored life. Multicellular organisms arose a billion years ago; land animals, 400 million; anatomically modern humans, 200 thousand. In all that time, not one of those countless children of Earth ventured to another world, not until Neil Armstrong set foot upon a dusty alien landscape. Our form of life could now be said to belong to worlds beyond that which had borne us. And I was always awestruck to think that we lived in the 0.02% of human history, in the 0.0000009% of Earth history, when the pioneer of this new era walked amongst us.

And now we don't.