Here is a pretty good summary.
All âtwilight phenomenaâ are symmetric on opposite sides of midnight, and occur in reverse order between sunset and sunrise, the authors note in âColor and Light in Natureâ (Cambridge University Press, 2001). That means thereâs no inherent, natural cause of a major optical difference between them. However, two human factors break their symmetry.
The first is in our heads. âAt sunset, our eyes are daylight adapted and may even be a bit weary from the dayâs toil,â Lynch and Livingston write. âAs the light fades, we cannot adapt as fast as the sky darkens. Some hues may be lost or perceived in a manner peculiar to sunset. At sunrise, however, the nightâs darkness has left us with very acute night vision and every faint, minor change in the skyâs color is evident.â In short, you may perceive more colors at dawn than at dusk. [Red-Green & Blue-Yellow: The Stunning Colors You Canât See]
Human activities also drive a divergence between them. âAt sunset the sky is full of pollutants and wind-borne particles,â the authors write. âDuring the night, winds die down, smog-producing urban activity eases and the atmosphere cleanses itself. The dawn is clearer than any other time of day.â
Itâs a matter of opinion whether pollution or a lack of it makes for prettier twilights. At dawn, clearer skies enable more brilliant reds and oranges to make their way through the atmosphere to your eyes, whereas thicker atmospheres at dusk tend to dull these colors, leading to more washed-out sunsets. On the other hand, more dust and smog (at sunset) can have the effect of scattering light across a greater region of the sky, creating a larger drape of colors, whereas sunrise colors tend to be more focused around the sun. Whichever you prefer, you can frequently tell a sunrise from a sunset by the fact that the latter appears more chaotic, and the former, tidier.
According to the astrophysicist Neil Degrasse Tyson, director of the Hayden Planetarium in New York, thereâs also a trick for distinguishing a sunrise from a sunset played in reverse. Because of Earthâs tilt, the sun doesnât rise or set along a vertical line, but at an angle. âWhen viewed from all latitudes north of the Tropic of Cancer (23.5 degrees north latitude), the sun always rises at an angle up and to the right, and sets and an angle down and to the right,â Tyson writes on his website. âThatâs how you can spot a faked sunrise in a movie: it moves up and to the left. Filmmakers are not typically awake in the morning hours to film an actual sunrise, so they film a sunset instead, and then time-reverse it, thinking nobody will notice.â
So if you see a rising sun move up and to the left, you know youâre in the twilight zone. Better head back to the hospital.