The Essential Galileo (9 page)

But here I feel that some people may be troubled with grave doubt, and perhaps seized with a difficulty so serious as to compel them to feel uncertain about the conclusion just explained and supported by so many phenomena. For if that part of the moon's surface which reflects the sun's rays most brightly is full of innumerable sinuosities, protuberances, and cavities, why does the outer edge looking toward the west when the moon is waxing, and the other half-circumference looking toward the east when the moon is waning, and the whole circle at full moon appear not uneven, rugged, and irregular, but perfectly round and circular, as sharply defined as if marked out with a compass, and without the indentation of any protuberances or cavities? And most remarkably so, because the whole unbroken edge belongs to the brighter part of the moon's surface, which I have said to be full of protuberances and cavities; for not one of the great spots extends quite to the circumference, but all of them are seen to be together away from the edge. Of this phenomenon, which provides a handle for such serious doubts, I produce two causes, and so two solutions of the difficulty.

The first solution I offer is this. If the protuberances and cavities in the body of the moon existed only on the edge of the circle that bounds the hemisphere which we see, then the moon might, or rather would have to, show itself to us with the appearance of a toothed wheel, being bounded with an irregular and uneven circumference. But if instead of a single set of prominences arranged along the actual circumference only, there are many ranges of mountains with their cavities and ruggedness set one behind the other along the extreme edge of the moon (and that too not only in the hemisphere which we see but also in that which is turned away from us, but still near the boundary of the hemisphere), then the eye, viewing them from afar, will not at all be able to detect the differences of prominences and cavities; [70] for the intervals between the mountains situated in the same circle, or in the same chain, are hidden by the jutting forward of other prominences situated in other ranges, especially if the eye of the observer is placed in the same line with the tops of the prominences mentioned. Similarly, on the earth the summits of a number of mountains close together appear situated in one plane, if the spectator is a long way off and standing at the same elevation; and when the sea is rough, the tops of the waves seem to form one plane, although between the billows there is many a gulf and chasm, so deep that not only the hulls, but even the bulwarks, masts, and sails of stately ships are hidden among them. Therefore, within the moon as well as around her circumference, there is a manifold arrangement of prominences and cavities, and the eye, viewing them from a great distance, is placed in nearly the same plane with their summits, and so no one need think it strange that they present themselves to the visual ray which just grazes them as an unbroken line quite free from unevenness.

To this explanation may be added another, namely, that there is around the body of the moon, just as around the earth, an envelope of some substance denser than the rest of the aether, which is sufficient to receive and reflect the sun's rays, although it does not possess so much opaqueness as to be able to prevent our seeing through it— especially when it is not illuminated. That envelope, when illuminated by the sun's rays, renders the body of the moon apparently larger than it really is, and would be able to stop our sight from penetrating to the solid body of the moon, if its thickness were greater. Now, it is of greater thickness around the periphery of the moon— greater, I mean, not in actual thickness, but with reference to our sight-rays, which cut it obliquely. And so it may stop our vision, especially when it is in a state of brightness, and it may conceal the true circumference of the moon on the side towards the sun. This may be understood more clearly from the following figure:

[71] Here, the body of the moon,
ABC
, is surrounded by an enveloping atmosphere,
DEG.
An eye at
F
penetrates to the middle parts of the moon, as at
A
, through a thickness,
DA
, of the atmosphere; but towards the extreme parts a mass of atmosphere of greater depth,
EB
, shuts out its boundary from our sight. An argument in favor of this is that the illuminated portion of the moon appears of larger circumference than the rest of the globe that is in shadow. Perhaps some will also think that this same cause provides a very reasonable explanation why the greater spots on the moon are not seen to reach the edge of the circumference on any side, although it might be expected that some would be found near the edge as well as elsewhere; it seems credible that there are spots there, but that they cannot be seen because they are hidden by a mass of atmosphere too thick and too bright for the sight to penetrate.

I think it has been sufficiently made clear, from the description of the phenomena given above, that the brighter part of the moon's surface is dotted everywhere with protuberances and cavities. It only remains for me to speak about their size, and to show that the disparities of the earth's surface are far smaller than those of the moon's— smaller, I mean, absolutely, so to speak, and not only smaller in proportion to the size of the globes on which they are. And this is plainly shown thus.

I often observed in various positions of the moon with reference to the sun that some summits within the portion of the moon in shadow appeared illuminated, although at some distance from the boundary of the light. Then by comparing their distance with the complete diameter of the moon, I learned that it sometimes exceeded one-twentieth of the diameter. Suppose the distance to be exactly one-twentieth of the diameter, and let the following diagram represent the moon's globe:

Here,
CAF
is a great circle,
E
its center, and
CF
a diameter, which consequently bears to the diameter of the earth the ratio 2:7; and since the diameter of the earth, according to the most exact observations, contains 7,000 Italian miles,
CF
will be 2,000,
CE
1,000, and one-twentieth of the whole
CF
will be 100 miles. Also let
CF
be a diameter of the great circle that divides the bright part of the moon from the dark part, [72] for owing to the very great distance of the sun from the moon this circle does not differ sensibly from a great one; let the distance of
A
from point
C
be one-twentieth of that diameter; let the radius
EA
be drawn, and let it be extended to cut the tangent line
GCD
(which represents a light ray) at point
D
. Then the arc
CA
, or the straight line
CD
, will be 100 of such units, and
CE
is 1,000. But the sum of the squares of
CD
and
CE
is 1,010,000, and the square of
ED
is equal to this; thus, the whole
ED
will be more than 1,004, and
AD
will be more than four of such units since
CE
was 1,000. Therefore, the height of
AD
on the moon, which represents a summit reaching up to the sun's ray
GCD
and separated from the extremity
C
by the distance
CD
, is more than four Italian miles. But on the earth there are no mountains that reach to the perpendicular height of even one mile. We are therefore left to conclude that it is clear that the prominences on the moon are loftier than those on the earth.

I wish in this place to assign the cause of another lunar phenomenon well worthy of notice. This phenomenon was observed by me not lately, but many years ago, and it has been pointed out to some of my intimate friends and pupils, explained, and assigned to its true cause. Yet since the observation of it is rendered easier and more vivid by the help of a spyglass, I have considered that it would not be unsuitably introduced in this place. I wish to introduce it chiefly in order that the connection and resemblance between the moon and the earth may appear more plainly.

When the moon, both before and after conjunction, is found not far from the sun, not only does its globe show itself to our sight on the side where it is furnished with shining horns, but a slight and faint circumference is also seen to mark out the circle of the dark part (namely, the part that is turned away from the sun), and to separate it from the darker background of the sky. Now, if we examine the matter more closely, we shall see that not only is the extreme edge of the part in shadow shining with a faint brightness, but that the entire face of the moon (namely, the side that does not feel the sun's glare) is whitened with a not-inconsiderable light. At first glance only a fine circumference appears shining, on account of the darker part of the sky adjacent to it; whereas, on the contrary, the rest of the surface appears dark, on account of the contiguity of the shining horns, which obscures our vision. But if one looks at the moon from such a position that by the interposition of a distant roof, chimney, or some other object the shining horns are hidden and the rest of the lunar globe is left exposed to one's view, [73] then one will find that this part of the moon (although deprived of sunlight) also gleams with considerable light, and particularly so if the gloom of the night has already deepened from the absence of the sun; for with a darker background the same light appears brighter. Moreover, it is found that this secondary brightness of the moon, as I may call it, is greater in proportion as the moon is less distant from the sun, and that it abates more and more in proportion to the moon's distance from that body; thus, after the first quarter and before the end of the second it is found to be weak and very faint, even if it is observed in a darker sky; whereas, at an angular distance of sixty degrees or less, even during twilight, it is wonderfully bright, so bright indeed that, with the help of a good spyglass, the great spots may be distinguished in it.

This strange brightness has presented no small perplexity to philosophical minds; and some have mentioned one thing, some another, as the cause to be alleged for it. Some have said that it is the inherent and natural glow of the moon; some that it is imparted to that body by the planet Venus, or, as others maintain, by all the stars; while some have said that it comes from the sun, whose rays find a way through the solid mass of the moon. But statements of this kind are disproved without much difficulty and convincingly demonstrated to be false.

In fact, if this kind of light were the moon's own, or were contributed by the stars, the moon would retain it, particularly in eclipses, and would show it then, when she is in an unusually dark sky. But this is contrary to experience since the brightness that is seen on the moon in eclipses is far less intense, being somewhat reddish and almost copper-colored, whereas this is brighter and whiter. Moreover, the brightness seen during an eclipse is changeable and shifting, for it wanders over the face of the moon in such a way that the part near the circumference of the circle of shadow cast by the earth is bright, but the rest of the moon is always seen to be dark. From this circumstance we understand without hesitation that this brightness is due to the proximity of the sun's rays coming into contact with some denser region that surrounds the moon as an envelope; owing to this contact a sort of dawn-light is diffused over the neighboring regions of the moon, just as twilight spreads in the morning and evening on the earth. But I will treat more fully of this matter in my book
On the System of the World
.

On the other hand, to assert that this sort of light is imparted to the moon by the planet Venus is so childish as to be undeserving of an answer. For who is so ignorant as not to understand that at conjunction and within an angular distance of sixty degrees it is quite impossible for the part of the moon turned away from the sun to be seen by the planet Venus?

However, that this light is derived from the sun penetrating with its light the solid mass of the moon, and rendering it luminous, is equally untenable. For then this light would never lessen, since a hemisphere of the moon is always illuminated by the sun, except at the moment of a lunar eclipse; but in reality it quickly decreases while the moon [74] is drawing near to the end of her first quarter, and when she has passed her first quarter it becomes quite dull.