Don: The dawn's early light, or the twilight's last gleaming.

Ya?l: Don?

D: Yes Ya?l?

Y: We've been in this art museum for five hours now, and you've spent the whole time staring at that one photograph of a sunset.

D: Are you sure?

Y: Of course I'm sure! You haven't budged!

D: But are you sure it's a sunset. It might be a sunrise.

Y: Ahhh...

D: I wonder if we can tell, just by looking at it.

Y: Actually, although there's no way to tell with certainty, there are a couple of clues you can look for.

D: Really?

Y: Sure. Let's think about what happens to the atmosphere during the day. It warms up, which means it can hold more moisture, and traffic and other activities churn a certain amount of dust and pollutants into it. All this extra stuff in the air can make the sunset much redder than sunrise. It can also make the light more diffuse. Artists sometimes remark that the light at sunset is softer and warmer than the sharp light at sunrise.

D: Is that so?

Y: Yes, although rain or other weather can disrupt this pattern. Another clue is the ground, which cools down overnight. This can cause dew to condense out of the air, and mist to form in the air near the ground. If there's any rising mist or dew in your photo, chances are it's sunrise.

D: OK then, that means this picture's...Hey! They turned off the lights!

Y: The museum's closed, Don. You should have asked me earlier, instead of standing there for five hours!