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From: dipper@utastro.UUCP (Debbie Byrd)
Newsgroups: net.astro
Subject: StarDate: July 8 The Solar Corona
Message-ID: <320@utastro.UUCP>
Date: Mon, 8-Jul-85 02:00:24 EDT
Article-I.D.: utastro.320
Posted: Mon Jul  8 02:00:24 1985
Date-Received: Sat, 13-Jul-85 15:21:55 EDT
Organization: U. Texas, Astronomy, Austin, TX
Lines: 36

The word corona means "crown" -- and the sun's corona really looks like
a crown.  More -- after this.

July 8 The Solar Corona

On today's date in the year 1842, there was a total eclipse of the
sun.  It became famous for some of the earliest scientific descriptions
of the sun's corona.

The solar corona is just the tenuous outermost atmosphere of the sun.
The corona is visible to us only during a total eclipse, when the black
disk of the moon blots out the main body of the sun -- and the corona
appears as a lovely encircling shell of wispy white light.

Some early astronomers believed that the solar corona was only an
optical illusion.  But the first photographs of a total eclipse proved
that the corona is real.

In 1869, scientists used a device called a spectroscope to split the
white light of the corona into its component rainbow of colors.
Instead of all colors being present, only a few puzzling sharp bright
discrete colors were found.  Fifty years later this evidence was
understood to mean that the sun's outer atmosphere has a much higher
temperature than the photosphere, which is the visible surface of the
sun.

In fact, the temperature of the corona is about two million degrees
centigrade, compared to only about six thousand degrees for the
photosphere.  In other words, as you go up from the visible surface of
the sun to the sun's outer atmosphere, things get hotter -- not colder
as you might expect.  Some combination of magnetic effects and shock
waves appears to be transmitting energy from beneath the sun's surface
up into the tenuous corona.

Script by Deborah Byrd and Harlan Smith.
(c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin