WHAT IS THE SUN? 
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and again as they disappear on the western edge. 
This is just what has been observed. Again, the 
cavity would appear with its dark edge toward us, as 
the shadow would be on that side. This shadow 
would gradually disappear as the spot reaches the 
center of the sun’s disk, where it would be uniform 
on all sides. The reverse would be true as it moves 
toward the western limb. 
WHAT IS THE SUN? 
The spectroscope tells us that it is composed of 
the same elements as the earth; that these elements 
are in a bright glowing or in a burning condition. 
But if the sun be a glowing solid, the spots could not 
be explained. They are not cavities in rigid matter, 
for they gradually, and sometimes quickly, change 
their shapes. Observers have noticed that occasion¬ 
ally several spots have run together into one con¬ 
tinuous spot, having the long axis parallel to the 
equator. Furthermore, the spots seldom continue 
more than a month, and many for a less time. 
When the spots are near the center of the sun 
they appear with a dark center. Between this dark 
spot and the rest of the surface is a band that seems 
to be half dark. The dark part is called the umbra, 
and the partially dark the penumbra. The penumbra 
varies in width, as would be the case if we viewed 
the cavity from different positions. 
If the sun were a glowing solid, would cavities in 
the surface appear as dark spots ? 
The cavity theory, then, has led to the idea that 
