PURE AND REFLECTED LIGIIT 
17 
spring sometimes show wreaths and scarves of 
yellow or red upon the clouds after sunset ; but 
as a general rule these are not the seasons for 
bright displays. 
The coming of the dawn and the passing of 
the sunset doubtless occupy the same length of 
time, but to us the latter often seems of shorter 
duration. At the equator there is compara- 
tively no glow on the sky after the sun disap- 
pears. Almost immediately upon the vanish- 
ing of the disk from view there is darkness. 
Along the coast of Norway one may see the 
after-glow upon the sky far into the night ; 
and farther up the coast the sun itself may be 
seen at midnight. The shape of the globe and 
the inclination of its axis account for both these 
appearances. In the temperate zones we have 
something between the two extremes. The 
sun for some time after its disappearance from 
view keeps throwing light from below the 
horizon upon the upper sky, and thus produces 
the effect we call twilight. It used to be reck- 
oned that when the sun had fallen eighteen or 
nineteen degrees below the horizon the twilight 
ceased eritirely ; but according to astronomers 
it ceases whenever a star of the sixth magnitude 
can be seen in the sky directly overhead. The 
Twilight, 
