From: leka@mamane.ifa.hawaii.edu (K.D.Leka)
Newsgroups: sci.astro
Subject: Re: when is the dark side of the moon visible to the naked eye?
Keywords: earthshine, Galileo
Date: 7 Apr 92 23:45:04 GMT
Article-I.D.: news.1992Apr7.234504.9640
Distribution: sci
Organization: Institute for Astronomy, Hawaii

In article <CONVERSE.92Apr5210339@ubik.uchicago.edu> converse@cs.uchicago.edu (Tim Converse) writes:
>.....
>	As I walked to work tonight, the moon was visible as a very
>thin crescent.  As I looked at it, I realized that the _rest_ of it
>(the unlit portion) was visible as a disc of gray, just barely (but
>perceptibly) lighter than the surrounding sky.
>....It occurred to me
>that, since this was only a couple of hours after sunset, the darker 
>portion might be illuminated by more "earthshine" than it would be
>later at night.  The moon was low in the western sky, which I guess
>would mean that the earth might be nearly full at that point.
>
>	And a spacier historical question:  at what point in
>astronomical history was it figured out that the moon was a globe
>that was lit from different directions at different times?  Is that 
>just a post-Copernican idea?  I ask partly because it seems like it might
>be an easier conclusion to arrive at if you had the sort of experience
>that I just had.
>
>	-- Tim Converse

Well, this reminds me of a test question I asked my students just
a couple of weeks ago!

From Stillman Drake's _Discoveries and Opinions of Galileo_, and the
translation of Siderius Nuncius (Galileo Galilei - 1610AD)
		...(without permission, and with, surely, typos!)

	"When the moon is not far from the sun, just before or after
	new moon, its globe offers itself to view not only on the
	side where it is adorned with shining horns, but a certain
	faint light is also seen to mark out the periphery of the dark
	part which faces away from the sun, separating this from
Newsgroups: sci.astro
Subject: Re: when is the dark side of the moon visible to the naked eye?
Summary: 
Expires: 
Followup-To: 
Distribution: sci
Organization: Institute for Astronomy, Hawaii
Keywords: earthshine, Galileo

In article <CONVERSE.92Apr5210339@ubik.uchicago.edu> converse@cs.uchicago.edu (Tim Converse) writes:
>.....
>	As I walked to work tonight, the moon was visible as a very
>thin crescent.  As I looked at it, I realized that the _rest_ of it
>(the unlit portion) was visible as a disc of gray, just barely (but
>perceptibly) lighter than the surrounding sky.
>....It occurred to me
>that, since this was only a couple of hours after sunset, the darker 
>portion might be illuminated by more "earthshine" than it would be
>later at night.  The moon was low in the western sky, which I guess
>would mean that the earth might be nearly full at that point.
>
>	And a spacier historical question:  at what point in
>astronomical history was it figured out that the moon was a globe
>that was lit from different directions at different times?  Is that 
>just a post-Copernican idea?  I ask partly because it seems like it might
>be an easier conclusion to arrive at if you had the sort of experience
>that I just had.
>
>	-- Tim Converse

Well, this reminds me of a test question I asked my students just
a couple of weeks ago!

From Stillman Drake's _Discoveries and Opinions of Galileo_, and the
translation of Siderius Nuncius (Galileo Galilei - 1610AD)
		...(without permission, and with, surely, typos!)

	"When the moon is not far from the sun, just before or after
	new moon, its globe offers itself to view not only on the
	side where it is adorned with shining horns, but a certain
	faint light is also seen to mark out the periphery of the dark
	part which faces away from the sun, separating this from
	the darker backgroiund of the aether.  Now if we examine the
	matter more closely, we shall see that not only does the
	extremee limb of the shaded side glow with this undertain
	light, but the entire face of the moon (includeing the side
	which does not receive the glare of the sun) is whitened by a
	not inconsiderable gleam......
	.....
	Moreover, it is found that this secondary light of the moon
	(so to speak) is greater according to as the moon is closer
	to the sun.  It diminishes more and more as the moon recedes from
	that body until, after the first quarter and before the last, it is
	seen very weakly and uncertainly even when observed in the darkest
	sky.....
	....
	(He goes on to argue why it cannot be the light from the stars,
	or from Venus which is doing this illuminating)
	....
	Now since the secondary light does not inherently belong to the 
	moon, and is not received from any star or from the sun, and since
	in the whole universe there is no other body left but the earth, 
	what must we conclude?  What is to be proposed?  Surely we must
	assert that the lunar body (or any other dark and sunless orb)
	is illuminated by the earth.
	Yet what is there so remarkable about this?
	(And then one of the most famous sentences from the bood - KDL)
	The earth, in fair and grateful exchange, pays back to the moon
	ian illumination similar to that whcih is receives from her
	throughout nearly all the darkest gloom of night.....

So, yes, you were right - it's "earthshine", just like the moonshine
of the moon as seen from the earth - and Venusshine lets us see Venus -= 
it's all *reflected* light from the sun.

		-KD

p.s. I would recommend Stillman Drake's book for some fun astronomy-history
reading.  It's one of my favorites!

	the darker backgroiund of the aether.  Now if we examine the
	matter more closely, we shall see that not only does the
	extremee limb of the shaded side glow with this undertain
	light, but the entire face of the moon (includeing the side
	which does not receive the glare of the sun) is whitened by a
	not inconsiderable gleam......
	.....
	Moreover, it is found that this secondary light of the moon
	(so to speak) is greater according to as the moon is closer
	to the sun.  It diminishes more and more as the moon recedes from
	that body until, after the first quarter and before the last, it is
	seen very weakly and uncertainly even when observed in the darkest
	sky.....
	....
	(He goes on to argue why it cannot be the light from the stars,
	or from Venus which is doing this illuminating)
	....
	Now since the secondary light does not inherently belong to the 
	moon, and is not received from any star or from the sun, and since
	in the whole universe there is no other body left but the earth, 
	what must we conclude?  What is to be proposed?  Surely we must
	assert that the lunar body (or any other dark and sunless orb)
	is illuminated by the earth.
	Yet what is there so remarkable about this?
	(And then one of the most famous sentences from the bood - KDL)
	The earth, in fair and grateful exchange, pays back to the moon
	ian illumination similar to that whcih is receives from her
	throughout nearly all the darkest gloom of night.....

So, yes, you were right - it's "earthshine", just like the moonshine
of the moon as seen from the earth - and Venusshine lets us see Venus -= 
it's all *reflected* light from the sun.

		-KD

p.s. I would recommend Stillman Drake's book for some fun astronomy-history
reading.  It's one of my favorites!

^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^
Osborn's Law: "Variables won't; constants aren't; constraints don't"
K. D. Leka			Internet: leka@mamane.ifa.hawaii.edu
Institute For Astronomy		Bitnet:	kleka@susolar.
University of Hawaii
DISCLAIMER: "it's mine, all mine, and you can't have it"
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