Volume Volume2\Astronomy
Entry# 902. Explanation of the lumen cinereum in the moon.
Contents: Vol. 1 | Vol. 2
Explanation of the lumen cinereum in the moon.
OF THE MOON.
No solid body is less heavy than the atmosphere.
Footnote: 1. On the margin are the words _tola romantina,
tola--ferro stagnato_ (tinned iron); _romantina_ is some special
kind of sheet-iron no longer known by that name.
Having proved that the part of the moon that shines consists of
water, which mirrors the body of the sun and reflects the radiance
it receives from it; and that, if these waters were devoid of waves,
it would appear small, but of a radiance almost like the sun; -- 5
It must now be shown whether the moon is a heavy or a light body:
for, if it were a heavy body--admitting that at every grade of
distance from the earth greater levity must prevail, so that water
is lighter than the earth, and air than water, and fire than air and
so on successively--it would seem that if the moon had density as it
really has, it would have weight, and having weight, that it could
not be sustained in the space where it is, and consequently that it
would fall towards the centre of the universe and become united to
the earth; or if not the moon itself, at least its waters would fall
away and be lost from it, and descend towards the centre, leaving
the moon without any and so devoid of lustre. But as this does not
happen, as might in reason be expected, it is a manifest sign that
the moon is surrounded by its own elements: that is to say water,
air and fire; and thus is, of itself and by itself, suspended in
that part of space, as our earth with its element is in this part of
space; and that heavy bodies act in the midst of its elements just
as other heavy bodies do in ours Footnote 15: This passage would
certainly seem to establish Leonardo's claim to be regarded as the
original discoverer of the cause of the ashy colour of the new moon
(_lumen cinereum_). His observations however, having hitherto
remained unknown to astronomers, Moestlin and Kepler have been
credited with the discoveries which they made independently a
century later.
Some disconnected notes treat of the same subject in MS. C. A. 239b;
718b and 719b; "_Perche la luna cinta della parte alluminata dal
sole in ponente, tra maggior splendore in mezzo a tal cerchio, che
quando essa eclissava il sole. Questo accade perche nell' eclissare
il sole ella ombrava il nostro oceano, il qual caso non accade
essendo in ponente, quando il sole alluma esso oceano_." The editors
of the "_Saggio_" who first published this passage (page 12) add
another short one about the seasons in the moon which I confess not
to have seen in the original manuscript: "_La luna ha ogni mese un
verno e una state, e ha maggiori freddi e maggiori caldi, e i suoi
equinozii son piu freddi de' nostri._"
When the eye is in the East and sees the moon in the West near to
the setting sun, it sees it with its shaded portion surrounded by
luminous portions; and the lateral and upper portion of this light
is derived from the sun, and the lower portion from the ocean in the
West, which receives the solar rays and reflects them on the lower
waters of the moon, and indeed affords the part of the moon that is
in shadow as much radiance as the moon gives the earth at midnight.
Therefore it is not totally dark, and hence some have believed that
the moon must in parts have a light of its own besides that which is
given it by the sun; and this light is due, as has been said, to the
above- mentioned cause,--that our seas are illuminated by the sun.
Again, it might be said that the circle of radiance shown by the
moon when it and the sun are both in the West is wholly borrowed
from the sun, when it, and the sun, and the eye are situated as is
shown above.
Footnote 23. 24: The larger of the two diagrams reproduced above
stands between these two lines, and the smaller one is sketched in
the margin. At the spot marked _A_ Leonardo wrote _corpo solare_
(solar body) in the larger diagram and _Sole_ (sun) in the smaller
one. At _C luna_ (moon) is written and at _B terra_ (the earth).
Some might say that the air surrounding the moon as an element,
catches the light of the sun as our atmosphere does, and that it is
this which completes the luminous circle on the body of the moon.
Some have thought that the moon has a light of its own, but this
opinion is false, because they have founded it on that dim light
seen between the hornes of the new moon, which looks dark where it
is close to the bright part, while against the darkness of the
background it looks so light that many have taken it to be a ring of
new radiance completing the circle where the tips of the horns
illuminated by the sun cease to shine Footnote 34: See Pl. CVIII,
No. 5. . And this difference of background arises from the fact that
the portion of that background which is conterminous with the bright
part of the moon, by comparison with that brightness looks darker
than it is; while at the upper part, where a portion of the luminous
circle is to be seen of uniform width, the result is that the moon,
being brighter there than the medium or background on which it is
seen by comparison with that darkness it looks more luminous at that
edge than it is. And that brightness at such a time itself is
derived from our ocean and other inland-seas. These are, at that
time, illuminated by the sun which is already setting in such a way
as that the sea then fulfils the same function to the dark side of
the moon as the moon at its fifteenth day does to us when the sun is
set. And the small amount of light which the dark side of the moon
receives bears the same proportion to the light of that side which
is illuminated, as that... Footnote 42: Here the text breaks off;
lines 43-52 are written on the margin. .
If you want to see how much brighter the shaded portion of the moon
is than the background on which it is seen, conceal the luminous
portion of the moon with your hand or with some other more distant
object.
On the spots in the moon (903-907).
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