On Earth, since we have an atmosphere that diffuses sunlight, we have twilight, the time in between day and night. Twilight happens when sunlight passes through the atmosphere, diffusing, or scattering and absorbing various wavelengths of light. Most of the objects in our solar system have no atmospheres and go directly from day to night with no twilight.

When the sun is above the horizon, the rays of light have to pass through 13 times more atmosphere than when the sun is high overhead. The shorter wavelengths are blue and get scattered out, leaving only the longer wavelengths. The setting sun looks orange without the blue wavelengths. Rainbows seen before sunset have little or no blue color in them.

Sunset is when the sun’s disk transits or crosses below the horizon. It takes 2 minutes at the equator and 3 minutes in other places due to the angled path. Twilight begins after the sun sinks below the horizon. There are 3 distinct times of twilight: civil, nautical, and astronomical.

Civil twilight begins at sunset and officially ends when the sun has moved 6° below the horizon. Colors are very intense as the wavelengths go through a variety of low air thicknesses near the skyline. In most cities it is the official time when streetlights must be turned on.

Nautical twilight lasts longer, until the sun has dropped to 12° below the horizon. It’s named nautical because a mariner can’t distinguish between the sea and the sky. Colors are gone. Satellites and bright stars appear, as well as the zodiacal light.

Astronomical twilight is the final stage of twilight, until the suns drops to 18° below the horizon. Now the faintest stars can be seen. Full darkness occurs after the sun falls below 18° and twilight has ended.
The length of time twilight lasts varies by season and by latitude. Civil twilight can last only a half an hour or linger throughout the night at higher latitudes. The tropics have the shortest twilight with only 24 minutes. Overall, twilight occurs slowly. Ambient illumination has become 500,000 times dimmer than daylight but varies locally due to light pollution.