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From: Scheuer.Wbst@Xerox.ARPA
Newsgroups: net.physics
Subject: Re: Clouds
Message-ID: <12310@sri-arpa.UUCP>
Date: Fri, 13-Apr-84 14:42:00 EST
Article-I.D.: sri-arpa.12310
Posted: Fri Apr 13 14:42:00 1984
Date-Received: Sat, 21-Apr-84 01:32:33 EST
Lines: 30

Excuse the delay in responding but the Physics mail off the ARPANET was
delayed a month at Xerox.

I have seen the green flash.  However, the explanation given by Minnaert
is WRONG.

The green flash is only observable when there is little turbulence in
the atmosphere and the western horizon is sharp.  As the sun sets, there
is a point at which the photosphere is completely obscured by the
horizon (including the effect of light bending by the atmosphere) and
the chromosphere (a 2000 km layer  @ 4500 degrees K between the
photosphere @ 6000 degrees K and the corona @ 1,000,000 degrees K) is
still visible.  At this instant the green flash can be observed.  It is
due to the Calcium K line which is prevelant in the chromosphere.  The
motion of the earth quickly moves the chromosphere below the horizon,
limiting the duration of the flash.  Don't blink.  You will miss it.

By the way, the procedure of blocking out the photosphere is used in the
coronagraph to study the corona during the day at all major solar
observatories.

My view of the green flash occurred while I was working at Sacramento
Peak Observatory in Sunspot, N.M., which is at 9500 feet just outside of
Alamagordo and where the western horizon is formed by the mountains
surrounding Las Cruces (70 miles away). The conditions there are
near-perfect.

Mark Scheuer
Xerox Corporation
Webster, New York 14580