Galilei, Galileo, The systems of the world, 1661

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1the wall hath it? The Sun ſhineth on that wall; from thence it

is reverberated upon the wall of the Hall, from thence it's
cted upon that chamber, ſo that it falls on it at the third reflection:
and I am very certain, that there is in that place more light, than
if the Moons light had directly faln upon it.
The third
ction of a Wall
minates more than
the firſt of the
Moon.
SIMPL. But this I cannot believe; for the illumination of the
Moon, eſpecially when it is at the full, is very great.
SAGR. It ſeemeth great by reaſon of the circumjacent dark

places; but abſolutely it is not much, and is leſs than that of the
twilight half an hour after the Sun is ſet; which is manifeſt,
cauſe you ſee not the ſhadows of the bodies illuminated by the
Moon till then, to begin to be diſtinguiſhed on the Earth.

ther, again, that third reflection upon that chamber, illuminates
more than the firſt of the Moon, may be known by going thether,
and reading a Book, and afterwards ſtanding there in the night
by the Moons light, which will ſhew by which of them lights one
may read more or leſs plainly, but I believe without further tryal,
that one ſhould ſee leſs diſtinctly by this later.
The light of the
Moon weaker than
that of the
light.
SALV. Now, Simplicius, (if haply you be ſatisfied) you may
conceive, as you your ſelf know very well, that the Earth doth
ſhine no leſs than the Moon; and the only remembring you of ſome
things, which you knew of your ſelf, and learn'd not of me, hath
aſſured you thereof: for I taught you not that the Moon ſhews
lighter by night than by day, but you underſtood it of your ſelf;
as alſo you could tell me that a little Cloud appeareth as lucid as
the Moon: you knew alſo, that the illumination of the Earth
not be ſeen by night; and in a word, you knew all this, without
knowing that you knew it.
So that you have no reaſon to be
pulous of granting, that the dark part of the Earth may illuminate
the dark part of the Moon, with no leſs a light than that
with the Moon illuminates the obſcurities of the night, yea rather
ſo much the greater, inaſmuch as the Earth is forty times bigger
than the Moon.
SIMPL. I muſt confeſs that I did believe, that that ſecondary
light had been the natural light of the Moon.
SALV. And this alſo you know of your ſelf, and perceive not
that you know it.
Tell me, do not you know without teaching,
that the Moon ſhews it ſelf more bright by night than by day, in

reſpect of the obſcurity of the ſpace of the ambient?
and
quently, do you not know in genere, that every bright body ſhews
the clearer, by how much the ambient is obſcurer?
Luminous bodies
appear the brighter
in an obſcurer
bient.
SIMPL. This I know very well.
SALV. When the Moon is horned, and that ſecondary light
ſeemeth to you very bright, is it not ever nigh the Sun, and
ſequently, in the light of the crepuſculum, (twilight?)

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