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From: msb@lsuc.UUCP (Mark Brader)
Newsgroups: net.astro
Subject: Re: StarDate: July 8 The Solar Corona
Message-ID: <713@lsuc.UUCP>
Date: Thu, 18-Jul-85 13:13:47 EDT
Article-I.D.: lsuc.713
Posted: Thu Jul 18 13:13:47 1985
Date-Received: Thu, 18-Jul-85 14:25:27 EDT
References: <320@utastro.UUCP>
Reply-To: msb@lsuc.UUCP (Mark Brader|LSUC|Toronto)
Organization: Law Society of Upper Canada, Toronto
Lines: 41
Summary: It was not always thus

dipper@utastro.UUCP (Debbie Byrd) writes:
> 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 reason there were no earlier scientific descriptions is that the
present visible (excited) form of the corona has only existed since about
that time, or perhaps a few decades previously.  Apparently the sun's
level of activity tends to change at intervals on the order of a century,
and the present level is exceptionally high.

Periods of high activity are characterized by the presence of sunspots
and the appearance of auroras.  (Whether there is always a (2x11)-year cycle
when there are sunspots is unknown due to the scarcity of pre-telescopic
sunspot observations.)  But even in past periods of high activity there are
no descriptions of eclipses resembling the way they appear now with the corona.
The present level, as I said, must therefore be exceptionally high.

In periods of low activity there are almost no sunspots at all.  This most
recently happened from 1645 to 1715.  (There were then almost no auroras
in Scandinavia where they are now commonplace.)  This may be why, when Schwabe
described the 11-year sunspot cycle about 1840, he was not believed at first.
When he was seen to be correct, scientists fell into believing that it had
always been that way, rather than that Schwabe had observed something new,
and the 1645-1715 "Maunder minimum" had to be rediscovered more than once
in historical records (Halley, Newton...) before IT was believed.

Anyway, the 1645-1715 period coincides with the coldest part of the "Little
Ice Age", when places like Britain had what we here call a winter.  Evidence
is that this is not a coincidence, and therefore the present global temperature
is unusually high (but may stay that way in future due to man's CO2 output).

My source for all this is the article "The Case of the Missing Sunspots",
by John A. Eddy, in Scientific American, May 1977, p.80.  Eddy was the
second rediscoverer, after Maunder (mentioned above).  Anyone know of
further developments since then?

Mark Brader