The aurora borealis is one of the most beautiful spectacles in the sky.
The colours and shape change every instant; sometimes a fan-like cluster
of rays, at other times long golden draperies gliding one over the
other. Blue, green, yellow, red, and white combine to give a glorious
display of colour. The theory of its origin is still, in part, obscure,
but there can be no doubt that the aurora is related to the magnetic
phenomena of the earth and therefore is connected with the electrical
influence of the sun.]
It is from the study of sun-spots that we have learned that the sun's
surface does not appear to rotate all at the same speed. The
"equatorial" regions are rotating quicker than regions farther north or
south. A point forty-five degrees from the equator seems to take about
two and a half days longer to complete one rotation than a point on the
equator. This, of course, confirms our belief that the sun cannot be a
solid body.
What is its composition? We know that there are present, in a gaseous
state, such well-known elements as sodium, iron, copper, zinc, and
magnesium; indeed, we know that there is practically every element in
the sun that we know to be in the earth. How do we know?
It is from the photosphere, as has been said, that we have won most of
our knowledge of the sun. The instrument used for this purpose is the
spectroscope; and before proceeding to deal further with the sun and the
source of its energy it will be better to describe this instrument.