MadSci Network: Astronomy |
Your friend's observation is correct. Since the sun rises and sets at the same angle to the horizon, one might wonder why nightfall seems to take longer. It's all in the atmosphere, and has to do with light scattering. At dawn, the sky is reasonably free of pollutants. When the sun rises in the morning, it is usually rising into a clean sky, and the sunlight is less scattered, which means the sun must be nearer the horizon before it begins to light the sky. During the day, wind and surface heating carry vehicle exhaust, dust, smoke, and water vapor into the atmosphere. At sunset, these pollutants act to scatter light, and the sun may be well below the horizon before the sky actually begins to get dark. During the night, the pollutants disperse, and the sky cleans itself for another day. Your friend's location also contributes to this effect. Southern Florida is much warmer, wetter, and more heavily populated than Northern Canada. While Canadians will notice this effect in the summer, it is much less common in the winter, when the air is cold and clean. Throughout late fall, winter, and early spring, it gets dark very fast once the sun has set, and the length of time between first light/sunrise and sunset/last light are about the same.
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