Why are sunsets red?
When the sun is low in the sky, this long journey through the atmosphere means the colors with shorter wavelengths, like blue, have already scattered or bounced off in numerous directions.
Orange sunsets (yellow and red light waves) appear when the air is clean. Sunsets that are the most spectacular occur when red wavelengths reflect off of overhead clouds.
Spectators continue to see light in the sky long after it has turned dark on the ground. Why is this? Because night doesn't "fall". It actually rises from the ground as the sun goes farther below the horizon.
Civil twilight occurs when the sun is 6° below the horizon. This is from the time that the sun drops below the horizon until artificial lights (street or home) are needed.
Astronomical twilight occurs when the sun is 18° below the horizon. This is when there is no sunlight on the western horizon and stars can be seen.
Twilight is shorter in the tropics because the sun's path is more perpendicular to the Earth's plane and it takes less time to go from 6° to 18° below the horizon at this angle.
White nights occur in extreme northern latitudes where the evening twilight merges with the morning twilight.