MadSci Network: Astronomy

Re: Why does it appear that the sunrise is shorter than the sunset

Date: Sun Dec 30 13:00:36 2001
Posted By: Denni Windrim, Director of Education, Sylvan Learning Centre
Area of science: Astronomy
ID: 1009160936.As

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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