Of the scene in New York, The New York Times of Nov. 28, 1883, reported: ''Soon after 5 o'clock the western horizon suddenly flamed into a brilliant scarlet, which crimsoned sky and clouds. People in the streets were startled at the unwonted sight and gathered in little groups on all the corners to gaze into the west. Many thought that a great fire was in progress.'' Two days later, Munch's hometown paper carried this account: ''A strong light was seen yesterday and today around 5 o'clock to the west of the city. People believed it was a fire: but it was actually a red refraction in the hazy atmosphere after sunset.''

Munch wasn't even alone on his twilight perambulation around Christiania. He wrote in his journal: ''I was walking along the road with two friends -- then the Sun set -- all at once the sky became blood red -- and I felt overcome with melancholy. I stood still and leaned against the railing, dead tired -- clouds like blood and tongues of fire hung above the blue-black fjord and the city.''

Where Munch was alone, though, was in his response. ''My friends went on,'' the journal entry continues, ''and I stood alone, trembling with anxiety. I felt a great, unending scream piercing through nature.''

That's mystery No. 1: why him? Of the three friends walking along the Ljabrochausseen, of the hundreds or thousands strolling the streets of Christiania at that hour, of the millions who must have witnessed similar sunsets around the world during those months, why did one Edvard Munch see in the sky the sound of a scream? What was it about this sight that would haunt him in particular for the better part of a decade? A friend wrote about discussing art with Munch in the winter of 1891-92: ''For a long time he had wanted to paint the memory of a sunset. Red as blood. No, it was coagulated blood. But no one else would perceive it the same way he did. They would think only about clouds.''

In 1893, Munch succeeded in capturing not only the coloring of the sky on that unforgettable (for him) evening, but the emotion accompanying it. And he succeeded in a way the sky itself did not. It's his sky that has become iconic, immortal. It's his individual emotional response that the two friends who kept walking along a road in Christiania, or the masses who went back to their business in Manhattan, presumably didn't feel -- but, judging from the response of generations to come, would have felt if only they'd seen his red instead. And that's mystery No. 2.

The answers to the other questions -- why the sky was red, what planet rose over what house, how Leonardo got that effect -- satisfy our curiosity about the literal truth. Maybe they even satisfy some desire to demystify art. But they also serve a subtler, perhaps equally unconscious, purpose. They enhance the questions we can't answer, the twin mysteries that arise out of the individual response of one artist in one place and the universal response of all audiences everywhere: Why the ''Mona Lisa'' smile to begin with? And why will we continue to seek it forever -- even when we know its not there?