Have you ever noticed how many people profess to "enjoy long walks on the beach at sunset?" And they don't all have personal ads out there, either. At any rate, I seldom see anyone walking at sunset, on or off the beach.

But my dogs and I are in the habit of walking at sunset. Coming back from the grocery store the other day, the western sky started putting on quite a show. Why hadn't I brought my camera?! That's such an easy thought to have, when looking at pink-reds and the darkening blue sky. But how would my photograph be any different than the millions you could find on Flickr, travel blogs, etc.? Still, something seemed special about this sunset, and it wasn't just pretty colors.

The shades of red changed every second. Maybe that's it: it was the precious transience, the evanescence. Birders must feel the same thing when a rare bird alights nearby, and then disappears before they have a chance to completely study it. Or maybe it was something else.

Does anyone understand what makes them pick up a certain book? It's always been a mystery to me. Recently I revisited "The Sleepwalkers," by Arthur Koestler, one of the under-rated authors of the twentieth century. The book retraces the brilliant beginnings, the detours, and the dramatic denouement of astronomy from the ancient Pythagoreans to Galileo. Most of the book is concerned with Copernicus, Tycho Brahe, Johannes Kepler and Galileo.

I only vaguely remembered the specialness of this book. Normally a great change in the world involves many people and circumstances over years. But in this case there was one special moment: Tycho was walking to dinner one evening and noticed a new star, a super nova that was bright enough for some people to see in the day. He had the exact instruments to study it, and in doing so, produced a conundrum that the old Ptolemaic astronomical system could not explain.

Thus irrupted a change in the mindset of European Christendom so fundamental that it was as if a whole new animal species had come into the world.

After reading a book my brain can stay in a muddle for quite awhile. It wants to get unified again. It needs a clarifying image. Perhaps that's what was so special about the transience of that colorful sunset. Its speed made it easy to imagine the motion of the sun. Oops, I meant the earth.

The history of astronomy seemed like more than details and facts now. It didn't matter that I couldn't sort out the entire book at once. On a personal level this particular sunset and Koestler's book helped me appreciate one of the most fundamental miracles in the sorry history of our species. I felt like a part of the cosmic drama.