Message-Id: <v01530500af6651bf0495@[207.90.162.211]>
Date: Tue, 1 Apr 1997 02:27:36 -0800
To: pinhole@exploratorium.edu
From: ronwong@unleashed.net
Subject: Re: Lunar eclipse
Ian Bleakney brought up a few questions regarding the lunar eclipse:
>1. The eclipse was about 92% complete, however why could we still see
>the rest of the moon in a dull burgundy color?
Our atmosphere scatters out the various colors of the visible spectrum
starting with the color violet and working down the list to red. If our
eyes were as sensitive to violet as they are to blue, the sky would appear
to us in its true color - violet. It's our lack of such sensitivity that
makes the sky appear blue to us instead of violet.
By the time the light that fell on the moon has passed through our
atmosphere and out again most of the colors of the visible spectrum have
been scatterd out leaving the deep red which is refracted onto the shadow
that falls on the moon.
>2. Are there any lunar eclipses where that color does not appear (and
>you cannot see the moon at all)?
If there is a substantial amount of cloud cover or atmospheric pollution in
that region of the earth's atmosphere through which the light would pass on
its way to the moon then the moon would appear black during a total lunar
eclipse and you'd have to look very carefully for it against the background
of the stars. I suspect that if this was up to cloud cover, the phenomena
would be extremely rare but an event like the eruption of Mt Krakatoa seems
made to order for this trick. Does anyone know if the lunar eclipses during
the following year of the eruption were noteworthy for their lack of any
color? The sunsets were supposed to have been very spectacular so a lot of
red must have been scattered out.
>3. The earth's shadow line across the moon was an arc. This arc seemed
>to suggest that the earth's shadow on the moon is much larger than the
>moon itself. Is this true? How many moons would fit into the earth's
>shadow (at the same distance?)
The arc suggests more than the fact that the earth's shadow is larger than
the moon. It suggests that the earth is spherical, an observation made by
Aristotle. The following century, Eratosthenes was able to measure the
earth's diameter to within 50 miles of the actual polar diameter!
A century later another Greek by the name of Hipparchus noted just how
large the earth's shadow was at the moon's distance. He did this by
noticing how far among the stars the moon appeared to move during a total
lunar eclipse. It amounted to about four moon diameters (3.6 to be more
exact). Using the figure of the earth's diameter arrived at by Eratosthenes
(about 8000 mi), he determined the diameter of the moon (about 2000 mi).
Since the moon is about one-half a pinkie (Linda Shore units) it must
subtend an angle of about 0.5 degrees. Hipparchus quickly figured out the
distance to the moon from this information (230,000 miles using my
figures). It involves the simple relationship between arc, angle, and
radius of a circle (arc = angle X radius. arc: moon's diameter; angle: 0.5
degrees converted into radians - the appropriate units for a proper Greek;
radius: distance from earth to moon). I've been told his results were
comparable in accuracy to Eratosthene's measurement of the earth's
diameter.
Amazing what you can come up with while watching a total lunar eclipse.
Ron Wong
Lowell High School
San Francisco, CA