ASTRONOMICAL MESSAGE
Which contains and explains recent observations
made with the aid of a new spyglass
concerning the surface of the moon,
the Milky Way, nebulous stars, and
innumerable fixed stars,
as well as four planets never before seen, and
now named
The Medicean Stars
Great indeed are the things which in this brief
treatise I propose for observation and consideration by all students of
nature. I say great, because of the excellence of the subject itself, the
entirely unexpected and novel character of these things, and finally
because of the instrument by means of which they have been revealed to our
senses.
Surely
it is a great thing to increase the numerous host of fixed stars
previously visible to the unaided vision, adding countless more which have
never before been seen, exposing these plainly to the eye in numbers ten
times exceeding the old and familiar stars.
It
is a very beautiful thing, and most gratifying to the sight, to behold the
body of the moon, distant from us almost sixty earthly radii, as if it
were no further away than two such measures–so that its diameter appears
almost thirty times larger, its surface nearly nine hundred times, and its
volume twenty-seven thousand times as large as when viewed with the naked
eye. In this way one may learn with all the certainty of sense evidence
that the moon is not robed in a smooth and polished surface but is in fact
rough and uneven, covered everywhere, just like the earth’s surface,
with huge prominences, deep valleys, and chasms.
Again,
it seems to me a matter of no small importance to have ended the dispute
about the Milky Way by making its nature manifest to the very senses as
well as to the intellect. Similarly it will be a pleasant and elegant
thing to demonstrate that the nature of those stars which astronomers have
previously called “nebulous” is far different from what has been
believed hitherto. But what surpasses all wonders by far, and what
particularly moves us to seek the attention of all astronomers and
philosophers, is the discovery of four wandering stars not known or
observed by any man before us. Like Venus and Mercury, which have their
own periods about the sun, these have theirs about a certain star that is
conspicuous among those already known, which they sometimes precede and
sometimes follow, without ever departing from it beyond certain limits.
All these facts were discovered and observed by me not many days ago with
the aid of a spyglass which I devised, after first being illuminated by
divine grace. Perhaps other things, still more remarkable, will in time be
discovered by me or by other observers with the aid of such an instrument,
the form and construction of which I shall first briefly explain, as well
as the occasion of its having been devised. Afterwards I shall relate the
story of the observations I have made.
About
ten months ago a report reached my ears that a certain Fleming had
constructed a spyglass by means of which visible objects, though very
distant from the eye of the observer, were distinctly seen as if nearby.
Of this truly remarkable effect several experiences were related, to which
some persons gave credence while others denied them. A few days later the
report was confirmed to me in a letter from a noble Frenchman at Paris,
Jacques Badovere, which caused me to apply myself wholeheartedly to
inquire into the means by which I might arrive at the invention of a
similar instrument. This I did shortly afterwards, my basis being the
theory of refraction. First I prepared a tube of lead, at the ends of
which I fitted two glass lenses, both plane on one side while on the other
side one was spherically convex and the other concave. Then placing my eye
near the concave lens I perceived objects satisfactorily large and near,
for they appeared three times closer and nine times larger than when seen
with the naked eye alone. Next, I constructed another one, more accurate,
which represented objects as enlarged more than sixty times. Finally,
sparing neither labor nor expense, I succeeded in constructing for myself
so excellent an instrument that objects seen by means of it appeared
nearly one thousand times larger and over thirty times closer than when
regarded with our natural vision.
It
would be superfluous to enumerate the number and importance of the
advantages of such an instrument at sea as well as on land. But forsaking
terrestrial observations, I turned to celestial ones, and first I saw the
moon from as near at hand as if it were scarcely two terrestrial radii
away. After that I observed often with wondering delight both the planets
and the fixed stars, and since I saw these latter to be very crowded, I
began to seek (and eventually found) a method by which I might measure
their distances apart. Here it is appropriate to convey certain cautions
to all who intend to undertake observations of this sort, for in the first
place it is necessary to prepare quite a perfect telescope, which will
show all objects bright, distinct, and free from any haziness, while
magnifying them at least four hundred times and thus showing them twenty
times closer. Unless the instrument is of this kind it will be vain to
attempt to observe all the things which I have seen in the heavens, and
which will presently be set forth. Now in order to determine without much
trouble the magnifying power of an instrument, trace on paper the contour
of two circles or two squares of which one is four hundred times as large
as the other, as it will be when the diameter of one is twenty times that
of the other. Then, with both these figures attached to the same wall,
observe them simultaneously from a distance, looking at the smaller one
through the telescope and at the larger one with the other eye unaided.
This may be done without inconvenience while holding both eyes open at the
same time; the two figures will appear to be of the same size if the
instrument magnifies objects in the desired proportion.
Such
an instrument having been prepared, we seek a method of measuring
distances apart. This we shall accomplish by the following contrivance.
Let
ABCD be the tube and E be the eye of the observer. Then if there were no
lenses in the tube, the rays would reach the object FG along the straight
lines ECF and EDG. But when the lenses have been inserted, the rays go
along the refracted lines ECH and EDI; thus they are brought closer
together, and those which were previously directed freely to the object FG
now include only the portion of it HI. The ratio of the distance EH to the
line HI then being found, one may by means of a table of sines determine
the size of the angle formed at the eye by the object HI, which we shall
find to be but a few minutes of arc. Now, if to the lens CD we fit thin
plates, some pierced with larger and some with smaller apertures, putting
now one plate and now another over the lens as required, we may form at
pleasure different angles subtending more or fewer minutes of arc, and by
this means we may easily measure the intervals between stars which are but
a few minutes apart, with no greater error than one or two minutes. And
for the present let it suffice that we have touched lightly on these
matters and scarcely more than mentioned them, as on some other occasion
we shall explain the entire theory of this instrument.
Now
let us review the observations made during the past two months, once more
inviting the attention of all who are eager for true philosophy to the
first steps of such important contemplations. Let us speak first of that
surface of the moon which faces us. For greater clarity I distinguish t
two parts of this surface, a lighter and a darker; the lighter part seems
to surround and to pervade the whole hemisphere, while the darker part
discolors the moon’s surface like a kind of cloud, and makes it appear
covered with spots. Now those spots which are fairly dark and rather large
are plain to everyone and have been seen throughout the ages; these I
shall call the “large” or “ancient” spots, distinguishing them
from others that are smaller in size but so numerous as to occur all over
the lunar surface, and especially the lighter part. The latter spots had
never been seen by anyone before me. From observations of these spots
repeated many times I have been led to the opinion and conviction that the
surface of the moon is not smooth, uniform, and precisely spherical as a
great number of philosophers believe it (and the other heavenly bodies) to
be, but is uneven, rough, and full of cavities and prominences, being not
unlike the face of the earth, relieved by chains of mountains and deep
valleys. The things I have seen by which I was enabled to draw this
conclusion are as follows. On the fourth or fifth day after new moon, when
the moon is seen with brilliant horns, the boundary which divides the dark
part from the light does not extend uniformly in an oval line as would
happen on a perfectly spherical solid, but traces out an uneven, rough,
and very wavy line as shown in the figure below. Indeed, many luminous
excrescences extend beyond the boundary into the darker portion, while on
the other hand some dark patches invade the illuminated part. Moreover a
great quantity of small blackish spots, entirely separated from the dark
region, are scattered almost all over the area illuminated by the sun with
the exception only of that part which is occupied by the large and ancient
spots. Let us note, however, that the said small spots always agree in
having their blackened parts directed toward the sun, while on the side
opposite the sun they are crowned with bright contours, like shining
summits. There is a similar sight on earth about sunrise, when we behold
the valleys not yet flooded with light though the mountains surrounding
them are already ablaze with glowing splendor on the side opposite the
sun. And just as the shadows in the hollows on earth diminish in size as
the sun rises higher, so these spots on the moon lose their blackness as
the illuminated region grows larger and larger.
Again,
not only are the boundaries of shadow and light in the moon seen to be
uneven and wavy, but still more astonishingly many bright points appear
within the darkened portion of the moon, completely divided and separated
from the illuminated part and at a considerable distance from it. After a
time these gradually increase in size and brightness, and an hour or two
later they become joined with the rest of the lighted part which has now
increased in size. Meanwhile more and more peaks shoot up as if sprouting
now here, now there, lighting up within the shadowed portion; these become
larger, and finally they too are united with that same luminous surface
which extends ever further. An illustration of this is to be seen in the
figure above. And on the earth, before the rising of the sun, are not the
highest peaks of the mountains illuminated by the sun’s rays while the
plains remain in shadow? Does not the light go on spreading while the
larger central parts of those mountains are becoming illuminated? And when
the sun has finally risen, does not the illumination of plains and hills
finally become one? But on the moon the variety of elevations and
depressions appears to surpass in every way the roughness of the
terrestrial surface, as we shall demonstrate further on.
At
present I cannot pass over in silence something worthy of consideration
which I observed when the moon was approaching first quarter, as shown in
the previous figure. Into the luminous part there extended a great dark
gulf in the neighborhood of the lower cusp. When I had observed it for a
long time and had seen it completely dark, a bright peak began to emerge,
a little below its center, after about two hours. Gradually growing, this
presented itself in a triangular shape, remaining completely detached and
separated from the lighted surface. Around it three other small points
soon began to shine, and finally, when the moon was about to set, this
triangular shape (which had meanwhile become more widely extended) joined
with the rest of the illuminated region and suddenly burst into the gulf
of shadow like a vast promontory of light, surrounded still by the three
bright peaks already mentioned. Beyond the ends of the cusps, both above
and below, certain bright points emerged which were quite detached from
the remaining lighted part, as may be seen depicted in the same figure.
There were also a great number of dark spots in both the horns, especially
in the lower one; those nearest the boundary of light and shadow appeared
larger and darker, while those more distant from the boundary were not so
dark and distinct. But in all cases, as we have mentioned earlier, the
blackish portion of each spot is turned toward the source of the sun’s
radiance, while a bright rim surrounds the spot on the side away from the
sun in the direction of the shadowy region of the moon. This part of the
moon’s surface, where it is spotted as the tail of a peacock is
sprinkled with azure eyes, resembles those glass vases which have been
plunged while still hot into cold water and have thus acquired a crackled
and wavy surface, from which they receive their common name of
“ice-cups.”
As
to the large Iunar spots, these are not seen to be broken in the above
manner and full of cavities and prominences; rather, they are even and
uniform, and brighter patches crop up only here and there. Hence if anyone
wished to revive the old Pythagorean opinion that the moon is like another
earth, its brighter part might very fitly represent the surface of the
land and its darker region that of the water. I have never doubted that if
our globe were seen from afar when ~flooded with sunlight, the land
regions would appear brighter and the watery regions darker. The large
spots in the moon are also seen to be less elevated than the brighter
tracts, for whether the moon is waxing or waning there are always seen,
here and there along its boundary of light and shadow, certain ridges of
brighter hue around the large spots (and we have attended to this in
preparing the diagrams); the edges of these spots are not only lower, but
also more uniform, being uninterrupted by peaks or ruggedness.
Near
the large spots the brighter part stands out particularly in such a way
that before first quarter and toward last quarter, in the vicinity of a
certain spot in the upper (or northern) region of the moon, some vast
prominences arise both above and below as shown in the figures reproduced
below. Before last quarter this same spot is seen to be walled about with
certain blacker contours which, like the loftiest mountaintops, appear
darker on the side away from the sun and brighter on that which faces the
sun. (This is the opposite of what happens in the cavities, for there the
part away from the sun appears brilliant, while that which is turned
toward the sun is dark and in shadow.) After a time, when the lighted
portion of the moon’s surface has diminished in size and when all (or
nearly all) the said spot is covered with shadow, the brighter ridges of
the mountains gradually emerge from the shade. This double aspect of the
spot is illustrated in the ensuing figures.
There
is another thing which I must not omit, for I beheld it not without a
certain wonder; this is that almost in the center of the moon there is a
cavity larger than all the rest, and perfectly round in shape. I have
observed it near both first and last quarters, and have tried to represent
it as correctly as possible in the second of the above figures. As to
light and shade, it offers the same appearance as would a region like
Bohemia if that were enclosed on all sides by very lofty mountains
arranged exactly in a circle. Indeed, this area on the moon is surrounded
by such enormous peaks that the bounding edge adjacent to the dark portion
of the moon is seen to be bathed in sunlight before the boundary of light
and shadow reaches halfway across the same space. As in other spots, its
shaded portion faces the sun while its lighted part is toward the dark
side of the moon; and for a third time I draw attention to this as a very
cogent proof of the ruggedness and unevenness that pervades all the bright
region of the moon. Of these spots, moreover, those are always darkest
which touch the boundary line between light and shadow, while those
farther off appear both smaller and less dark, so that when the moon
ultimately becomes full (at opposition to the sun), the shade of the
cavities is distinguished from the light of the places in relief by a
subdued and very tenuous separation.
The
things we have reviewed are to be seen in the brighter region of the moon.
In the large spots, no such contrast of depressions and prominences is
perceived as that which we are compelled to recognize in the brighter
parts by the changes of aspect that occur under varying illumination by
the sun’s rays throughout the multiplicity of positions from which the
latter reach the moon. In the large spots there exist some holes rather
darker than the rest, as we have shown in the illustrations. Yet these
present always the same appearance, and their darkness is neither
intensified nor diminished, although with some minute difference they
appear sometimes a little more shaded and sometimes a little lighter
according as the rays of the sun fall on them more or less obliquely.
Moreover, they join with the neighboring regions of the spots in a gentle
linkage, the boundaries mixing and mingling. It is quite different with
the spots which occupy the brighter surface of the moon; these, like
precipitous crags having rough and jagged peaks, stand out starkly in
sharp contrasts of light and shade. And inside the large spots there are
observed certain other zones that are brighter, some of them very bright
indeed. Still, both these and the darker parts present always the same
appearance; there is no change either of shape or of light and shadow;
hence one may affirm beyond any doubt that they owe their appearance to
some real dissimilarity of parts. They cannot be attributed merely to
irregularity of shape, wherein shadows move in consequence of varied
illuminations from the sun, as indeed is the case with the other, smalIer,
spots which occupy the brighter part of the moon and which change, grow,
shrink, or disappear from one day to the next, as owing their origin only
to shadows of prominences.
But
here I foresee that many persons will be assailed by uncertainty and drawn
into a grave difficulty, feeling constrained to doubt a conclusion already
explained and confirmed by many phenomena. If that part of the lunar
surface which reflects sunlight more brightly is full of chasms (that is,
of countless prominences and hollows), why is it that the western edge of
the waxing moon, the eastern edge of the waning moon, and the entire
periphery of the full moon are not seen to be uneven, rough, and wavy? On
the contrary they look as precisely round as if they were drawn with a
compass; and yet the whole periphery consists of that brighter lunar
substance which we have declared to be filled with heights and chasms. In
fact not a single one of the great spots extends to the extreme periphery
of the moon, but all are grouped together at a distance from the edge.
Now
let me explain the twofold reason for this troublesome fact, and in turn
give a double solution to the difficulty. In the first place, if the
protuberances and cavities in the lunar body existed only along the
extreme edge of the circular periphery bounding the visible hemisphere,
the moon might (indeed, would necessarily) look to us almost like a
toothed wheel, terminated by a warty or wavy edge. Imagine, however, that
there is not a single series of prominences arranged only along the very
circumference but a great many ranges of mountains together with their
valleys and canyons disposed in ranks near the edge of the moon, and not
only in the hemisphere visible to us but every where near the boundary
line of the two hemispheres. Then an eye viewing them from afar will not
be able to detect the separation of prominences by cavities, because the
intervals between the mountains located in a given circle or a given chain
will be hidden by the interposition of other heights situated in yet other
ranges. This will be especially true if the eye of the observer is placed
in the same straight line with the summits of these elevations. Thus on
earth the summits of several mountains close together appear to be
situated in one plane if the spectator is a long way off and is placed at
an equal elevation. Similarly in a rough sea the tops of the waves seem to
lie in one plane, though between one high crest and another there are many
gulfs and chasms of such depth as not only to hide the hulls but even the
bulwarks, masts, and rigging of stately ships. Now since there are many
chains of mountains and chasms on the moon in addition to those around its
periphery, and since the eye, regarding these from a great distance, lies
nearly in the plane of their summits, no one need wonder that they appear
as arranged in a regular and unbroken line.
To
the above explanation another may be added; namely, that there exists
around the body of the moon, just as around the earth, a globe of some
substance denser than the rest of the aether. This may serve to receive
and reflect the sun’s radiations without being sufficiently opaque to
prevent our seeing through it, especially when it is not illuminated. Such
a globe, lighted by the sun’s rays, makes the body of the moon appear
larger than it really is, and if it were thicker it would be able to
prevent our seeing the actual body of the moon. And it actually is thicker
near the circumference of the moon; I do not mean in an absolute sense,
but relatively to the rays of our vision, which cut it obliquely there.
Thus it may obstruct our vision, especially when it is lighted, and cloak
the lunar periphery that is exposed to the sun. This may be more clearly
understood from the figure below, in which the body of the moon, ABC, is
surrounded by the vaporous globe DEG.
The eyesight from F reaches the moon in the central
region, at A for example, through a lesser thickness of the vapors DA,
while toward the extreme edges a deeper stratum of vapors, EB, limits and
shuts out our sight. One indication of this is that the illuminated
portion of the moon appears to be larger in circumference than the rest of
the orb, which lies in shadow. And perhaps this same cause will appeal to
some as reasonably explaining why the larger spots on the moon are nowhere
seen to reach the very edge, probable though it is that some should occur
there. Possibly they are invisible by being hidden under a thicker and
more luminous mass of vapors.
That
the lighter surface of the moon is everywhere dotted with protuberances
and gaps has, I think, been made sufficiently clear from the appearances
already explained. It remains for me to speak of their dimensions, and to
show that the earth’s irregularities are far less than those of the
moon. I mean that they are absolutely less, and not merely in relation to
the sizes of the respective globes. This is plainly demonstrated as
follows.
I
had often observed, in various situations of the moon with respect to the
sun, that some summits within the shadowy portion appeared lighted, though
lying some distance from the boundary of the light. By comparing this
separation to the whole diameter of the moon, I found that it sometimes
exceeded one-twentieth of the diameter. Accordingly, let CAF be a great
circle of the lunar body, E its center, and CF a diameter, which is to the
diameter of the earth as two is to seven.
Since
according to very precise observations the diameter of the earth is seven
thousand miles, CF will be two thousand, CE one thousand, and
one-twentieth of CF will be one hundred miles. Now let CF be the diameter
of the great circle which divides the light part of the moon from the dark
part (for because of the very great distance of the sun from
the moon, this does not differ appreciably from a
great circle), and let A be distant from C by one-twentieth of this. Draw
the radius EA, which, when produced, cuts the tangent line GCD
(representing the illuminating ray) in the point D. Then the arc CA, or
rather the straight line CD, will consist of one hundred units whereof CE
contains one thousand, and the sum of the squares of DC and CE will be
1,010,000. This is equal to the square of DE; hence ED will exceed 1,004,
and AD will be more than four of those units of which CE contains one
thousand. Therefore the altitude AD on the moon, which represents a summit
reaching up to the solar ray GCD and standing at the distance CD from C,
exceeds four miles. But on the earth we have no mountains which reach to a
perpendicular height of even one mile. Hence it is quite clear that the
prominences on the moon are loftier than those on the earth.
Here
I wish to assign the cause of another lunar phenomenon well worthy of
notice. I observed this not just recently, but many years ago, and pointed
it out to some of my friends and pupils, explaining it to them and giving
its true cause. Yet since it is rendered more evident and easier to
observe with the aid of the telescope, I think it not unsuitable for
introduction in this place, especially as it shows more clearly the
connection between the moon and the earth.
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 background of the aether. Now if we examine the matter
more closely, we shall see that not only does the extreme limb of the
shaded side glow with this uncertain light, but the entire face of the
moon (including the side which does not receive the glare of the sun) is
whitened by a not inconsiderable gleam. At first glance only a thin
luminous circumference appears, contrasting with the darker sky
coterminous with it; the rest of the surface appears darker from its
contact with the shining horns which distract our vision. But if we place
ourselves so as to interpose a roof or chimney or some other object at a
considerable distance from the eye, the shining horns may be hidden while
the rest of the lunar globe remains exposed to view. It is then found that
this region of the moon, though deprived of sunlight, also shines not a
little. The effect is heightened if the gloom of night has already
deepened through departure of the sun, for in a darker field a given light
appears brighter.
Moreover,
it is found that this secondary light of the moon (so to speak) is greater
according 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. But when the moon is within sixty degrees of the sun it
shines remarkably, even in twilight; so brightly indeed that with the aid
of a good telescope one may distinguish the large spots. This remarkable
gleam has afforded no small perplexity to philosophers, and in order to
assign a cause for it some have offered one idea and some another. Some
would say it is an inherent and natural light of the moon’s own; others,
that it is imparted by Venus; others yet, by all the stars together; and
still others derive it from the sun, whose rays they would have permeate
the thick solidity of the moon. But statements of this sort are refuted
and their falsity evinced with little difficulty. For if this kind of
light were the moon’s own, or were contributed by the stars, the moon
would retain it and would display it particularly during eclipses, when it
is left in an unusually dark sky. This is contradicted by experience, for
the brightness which is seen on the moon during eclipses is much fainter
and is ruddy, almost coppercolored, while this is brighter and whitish.
Moreover the other light is variable and movable, for it covers the face
of the moon in such a way that the place near the edge of the earth’s
shadow is always seen to be brighter than the rest of the moon; this
undoubtedly results from contact of the tangent solar rays with some
denser zone which girds the moon about. By this contact a sort of twilight
is diffused over the neighboring regions of the moon, just as on earth a
sort of crepuscular light is spread both mourning and evening; but with
this I shall deal more fully in my book on the system of the world.
To
assert that the moon’s secondary light is imparted by Venus is so
childish as to deserve no reply. Who is so ignorant as not to understand
that from new moon to a separation of sixty degrees between moon and sun,
no part of the moon which is averted from the sun can possibly be seen
from Venus? And it is likewise unthinkable that this light should depend
upon the sun’s rays penetrating the thick solid mass of the moon, for
then this light would never dwindle, inasmuch as one hemisphere of the
moon is al ways illuminated except during lunar eclipses. And the light
does diminish as the moon approaches first quarter, becoming completely
obscured after that is passed.
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? The earth, in fair and grateful exchange, pays
back to the moon an illumination similar to that which it receives from
her throughout nearly all the darkest gloom of night.
Let
us explain this matter more fully. At conjunction the moon occupies a
position between the sun and the earth; it is then illuminated by the
sun’s rays on the side which is turned away from the earth. The other
hemisphere, which faces the earth, is covered with darkness; hence the
moon does not illuminate the surface of the earth at all. Next, departing
gradually from the sun, the moon comes to be lighted partly upon the side
it turns toward us, and its whitish horns, still very thin, illuminate the
earth with a faint light. The sun’s illumination of the moon increasing
now as the moon approaches first quarter, a reflection of that light to
the earth also increases. Soon the splendor on the moon extends into a
semicircle, and our nights grow brighter; at length the entire visible
face of the moon is irradiated by the sun’s resplendent rays, and at
full moon the whole surface of the earth shines in a flood of moonlight.
Now the moon, waning, sends us her beams more weakly, and the earth is
less strongly lighted; at length the moon returns to conjunction with the
sun, and black night covers the earth.
In
this monthly period, then, the moonlight gives us alternations of brighter
and fainter illumination; and the benefit is repaid by the earth in equal
measure. For while the moon is between us and the sun (at new moon), there
lies before it the entire surface of that hemisphere of the earth which is
exposed to the sun and illuminated by vivid rays. The moon receives the
light which this reflects, and thus the nearer hemisphere of the
moon–that is, the one deprived of sunlight–appears by virtue of this
illumination to be not a little luminous. When the moon is ninety degrees
away from the sun it sees but half the earth illuminated (the western
half), for the other (the eastern half) is enveloped in night. Hence the
moon itself is illuminated less brightly from the earth, and as a result
its secondary light appears fainter to us. When the moon is in opposition
to the sun, it faces a hemisphere of the earth that is steeped in the
gloom of night, and if this position occurs in the plane of the ecliptic
the moon will receive no light at all, being deprived of both the solar
and the terrestrial rays. In its various other positions with respect to
the earth and sun, the moon receives more or less light according as it
faces a greater or smaller portion of the illuminated hemisphere of the
earth. And between these two globes a relation is maintained such that
whenever the earth is most brightly lighted by the moon, the moon is least
lighted by the earth, and vice versa.
Let
these few remarks suffice us here concerning this matter, which will be
more fully treated in our System of
the world. In that book, by a multitude of arguments and experiences,
the solar reflection from the earth will be shown to be quite
real–against those who argue that the earth must be excluded from the
dancing whirl of stars for the specific reason that it is devoid of motion
and of light. We shall prove the earth to be a wandering body surpassing
the moon in splendor, and not the sink of all dull refuse of the universe;
this we shall support by an infinitude of arguments drawn from nature.
Thus
far we have spoken of our observations concerning the body of the moon.
Let us now set forth briefly what has thus far been observed regarding the
fixed stars. And first of all, the following fact deserves consideration:
The stars, whether fixed or wandering, appear not to be enlarged by the
telescope in the same proportion as that in which it magnifies other
objects, and even the moon itself. In the stars this enlargement seems to
be so much less that a telescope which is sufficiently powerful to magnify
other objects a hundredfold is scarcely able to enlarge the stars four or
five times. The reason for this is as follows.
When
stars are viewed by means of unaided natural vision, they present
themselves to us not as of their simple (and, so to speak, their physical)
size, but as irradiated by a certain fulgor and as fringed with sparkling
rays, especially when the night is far advanced. From this they appear
larger than they would if stripped of those adventitious hairs of light,
for the angle at the eye is determined not by the primary body of the star
but by the brightness which extends so widely about it. This appears quite
clearly from the fact that when stars first emerge from twilight at sunset
they look very small, even if they are of the first magnitude; Venus
itself, when visible in broad daylight, is so small as scarcely to appear
equal to a star of the sixth magnitude. Things fall out differently with
other objects, and even with the moon itself; these, whether seen in
daylight or the deepest night, appear always of the same bulk. Therefore
the stars are seen crowned among shadows, while daylight is able to remove
their headgear; and not daylight alone, but any thin cloud that interposes
itself between a star and the eye of the observer. The same effect is
produced by black veils or colored glasses, through the interposition of
which obstacles the stars are abandoned by their surrounding brilliance. A
telescope similarly accomplishes the same result. It removes from the
stars their adventitious and accidental rays, and then it enlarges their
simple globes (if indeed the stars are naturally globular) so that they
seem to be magnified in a lesser ratio than other objects. In fact a star
of the fifth or sixth magnitude when seen through a telescope presents
itself as one of the first magnitude.
Deserving
of notice also is the difference between the appearances of the planets
and of the fixed stars. The planets show their globes perfectly round and
definitely bounded, looking like little moons, spherical and flooded all
over with light; the fixed stars are never seen to be bounded by a
circular periphery, but have rather the aspect of blazes whose rays
vibrate about them and scintillate a great deal. Viewed with a telescope
they appear of a shape similar to that which they present to the naked
eye, but sufficiently enlarged so that a star of the fifth or sixth
magnitude seems to equal the Dog Star, largest of all the fixed stars.
Now, in addition to stars of the sixth magnitude, a host of other stars
are perceived through the telescope which escape the naked eye; these are
so numerous as almost to surpass belief. One may, in fact, see more of
them than all the stars included among the first six magnitudes. The
largest of these, which we may call stars of the seventh magnitude, or the
first magnitude of invisible stars, appear through the telescope as larger
and brighter than stars of the second magnitude when the latter are viewed
with the naked eye. In order to give one or two proofs of their almost
inconceivable number, I have adjoined pictures of two constellations. With
these as samples, you may judge of all the others.
In
the first I had intended to depict the entire constellation of Orion, but
I was overwhelmed, by the vast quantity of stars and by limitations of
time, so I have deferred this to another occasion. There are more than
five hundred new stars distributed among the old ones within limits of one
or two degrees of arc. Hence to the three stars in the Belt of Orion and
the six in the Sword which were previously known have, I have added eighty
adjacent stars discovered recently, preserving the intervals between them
as exactly as I could. To distinguish the known or ancient stars, I have
depicted them larger and have outlined them doubly; the other (invisible)
stars I have drawn smaller and without the extra line. I have also
preserved differences of magnitude as well as possible.
The Belt and
Sword of Orion
In
the second example I have depicted the six stars of Taurus known as the
Pleiades (I say six, inasmuch as the seventh is hardly ever visible) which
lie within very narrow limits in the sky. Near them are more than forty
others, invisible, no one of
which is much more than half a degree away from the original six. I have
shown thirty-six of these in the diagram; as in the case of Orion I have
preserved their intervals and magnitudes, as well as the distinction
between old stars and new.
The Pleiades
Third,
I have observed the nature and the material of the Milky Way. With the aid
of the telescope this has been scrutinized so directly and with such
ocular certainty that all the disputes which have vexed philosophers
through so many ages have been resolved, and we are at last freed from
wordy debates about it. The galaxy is, in fact, nothing but a congeries of
innumerable stars grouped together in clusters. Upon whatever part of it
the telescope is directed, a vast crowd of stars is immediately presented
to view. Many of them are rather large ;and quite bright, while the number
of smaller ones is quite beyond calculation.
But
it is not only in the Milky Way that whitish clouds are seen; several
patches of similar aspect shine with faint light here and there throughout
the aether, and if the telescope is turned upon any of these it confronts
us with a tight mass of stars. And what is even more remarkable, the stars
which have been called “nebulous” by every astronomer up to this time
turn out to be groups of very small stars ar ranged in a wonderful manner.
Although each star separately escapes our sight on account of its
smallness or the immense distance from us, the mingling of their rays
gives rise to that gleam which was formerly believed to be some denser
part of the aether that was capable of reflecting rays from stars or from
the sun. I have observed some of these constellations and have decided to
depict two of them.
In
the first you have the nebula called the Head of Orion, in which I have
counted twenty-one stars. The second contains the nebula called Praesepe,
which is not a single star but a mass of more than forty starlets. Of
these I have shown thirty-six, in addition to the Aselli, arranged in the
order shown.
Nebula of
Orion
Nebula of Praesepe
We
have now briefly recounted the observations made thus far with regard to
the moon, the fixed stars, and the Milky Way. There remains the matter
which in my opinion deserves to be considered the most important of
all—the disclosure of four planets
never seen from the creation of the world up to our own time, together
with the occasion of my having discovered and studied them, their
arrangements, and the observations made of their movements and alterations
during the past two months. I invite all astronomers to apply themselves
to examine them and determine their periodic times, something which has so
far been quite impossible to complete, owing to the shortness of the time.
Once more, however, warning is given that it will be necessary to have a
very accurate telescope such as we have described at the beginning of this
discourse.
On
the seventh day of January in this present year 1610, at the first hour of
night, when I was viewing the heavenly bodies with a telescope, Jupiter
presented itself to me; and because I had prepared a very excellent
instrument for myself, I perceived (as I had not before, on account of the
weakness of my previous instrument) that beside the planet there were
three starlets, small indeed, but very bright. Though I believed them to
be among the host of fixed stars, they aroused my curiosity somewhat by
appearing to lie in an exact straight line parallel to the ecliptic, and
by their being more splendid than others of their size. Their arrangement
with respect to Jupiter and each other was the following:
East
West
that is, there were two stars on the eastern side
and one to the west. The most easterly star and the western one appeared
larger than the other. I paid no attention to the distances between them
and Jupiter, for at the outset I thought them to be fixed stars, as I have
said. But returning to the same investigation on January eighth–led by
what, I do not know–I found a very different arrangement. The three
starlets were now all to the west of Jupiter, closer together, and at
equal intervals from one another as shown in the following sketch:
East
West
At
this time, though I did not yet turn my attention to the way the stars had
come together, I began to concern myself with the question how Jupiter
could be east of all these stars when on the previous day it had been west
of two of them. I commenced to wonder whether Jupiter was not moving
eastward at that time, contrary to the computations of the astronomers,
and had got in front of them by that motion. Hence it was with great
interest that I awaited the next night. But I was disappointed in my
hopes, for the sky was then covered with clouds everywhere.
On
the tenth of January, however, the stars appeared in this position with
respect to Jupiter:
East
West
that is, there were but two of them, both easterly,
the third (as I supposed) being hidden behind Jupiter. As at first, they
were in the same straight line with Jupiter and were arranged precisely in
the line of the zodiac. Noticing this, and knowing that there was no way
in which such alterations could be attributed to Jupiter’s motion, yet
being certain that these were still the same stars I had observed (in fact
no other was to be found along the line of the zodiac for a long way on
either side of Jupiter), my perplexity was now transformed into amazement.
I was sure that the apparent changes belonged not to Jupiter but to the
observed stars, and I resolved to pursue this investigation with greater
care and attention.
And
thus, on the eleventh of January, I saw the following disposition:
East
West
There were two stars, both to the east, the central
one being three times as far from Jupiter as from the one farther east.
The latter star was nearly double the size of the former, whereas on the
night before they had appeared approximately equal.
I
had now decided beyond all question that there existed in the heavens
three stars wandering about Jupiter as do Venus and Mercury about the sun,
and this became plainer than daylight from observations on similar
occasions which followed. Nor were there just three such stars; four
wanderers complete their revolutions about Jupiter, and of their
alterations as observed more precisely later on we shall give a
description here. Also I measured the distances between them by means of
the telescope, using the method explained before. Moreover I recorded the
times of the observations, especially when more than one was made during
the same night—for the revolutions of these planets are so speedily
completed that it is usually possible to take even their hourly
variations.
Thus
on the twelfth of January at the first hour of night I saw the stars
arranged in this way:
East
West
The
most easterly star was larger than the western one, though both were
easily visible and quite bright. Each was about two minutes of arc distant
from Jupiter. The third star was invisible at first, but commenced to
appear after two hours; it almost touched Jupiter on the east, and was
quite small. All were on the same straight line directed along the
ecliptic.
On
the thirteenth of January four stars were seen by me for the first time,
in this situation relative to Jupiter:
East
West
Three were westerly and one was to the east; they
formed a straight line except that the middle western star departed
slightly toward the north. The eastern star was two minutes of arc away
from Jupiter, and the intervals of the rest from one another and from
Jupiter were about one minute. All the stars appeared to be of the same
magnitude, and though small were very bright, much brighter than fixed
stars of the same size. . . .
On
the twenty-sixth of February, midway in the first hour of night, there
were only two stars:
East
West
One was to the east, ten minutes from Jupiter; the
other to the west, six minutes away. The eastern one was somewhat smaller
than the western. But at the fifth hour three stars were seen:
East
West
In addition to the two already noticed, a third was
discovered to the west near Jupiter; it had at first been hidden behind
Jupiter and was now one minute away. The eastern one appeared farther away
than before, being eleven minutes from Jupiter.
This
night for the first time I wanted to observe the progress of Jupiter and
its accompanying planets along the line of the zodiac in relation to some
fixed star, and such a star was seen to the east, eleven minutes distant
from the easterly starlet and a little removed toward the south, in the
following manner:
East
West
On
the twenty-seventh of February, four minutes after the first hour, the
stars appeared in this configuration:
East
West
The most easterly was ten minutes from Jupiter; the
next, thirty seconds; the next to the west was two minutes thirty seconds
from Jupiter, and the most westerly was one minute from that. Those
nearest Jupiter appeared very small, while the end ones were plainly
visible, especially the westernmost. They marked out an exactly straight
line along the course of the ecliptic. The progress of these planets
toward the east is seen quite clearly by reference to the fixed star
mentioned, since Jupiter and its accompanying planets were closer to it,
as may be seen in the figure above. At the fifth hour, the eastern star
closer to Jupiter was one minute away.
At
the first hour on February twenty-eighth, t two stars only were seen; one
easterly, distant nine minutes from Jupiter, and one to the west, two
minutes away. They were easily visible and on the same straight line. The
fixed star, perpendicular to this line, now fell under the eastern planet
as in this figure:
East
West
At
the fifth hour a third star, two minutes east of Jupiter, was seen in this
position:
East
West
On
the first of March, forty minutes after sunset, four stars all to the east
were seen, of which the nearest to Jupiter was two minutes away, the next
was one minute from this, the third two seconds from that and brighter
than any of the others; from this in turn the most easterly was four
minutes distant, and it was smaller than the rest. They marked out almost
a straight line, but the third one counting from Jupiter was a little to
the north. The fixed star formed an equilateral triangle with Jupiter and
the most easterly star, as in this figure:
East
West
On
March second, half an hour after sunset, there were three planets, two to
the east and one to the west, in this configuration:
East
West
The most easterly was seven minutes from Jupiter
and thirty seconds from its neighbor; the western one was two minutes away
from Jupiter. The end stars were very bright and were larger than that in
the middle, which appeared very small. The most easterly star appeared a
little elevated toward the north from the straight line through the other
planets and Jupiter. The fixed star previously mentioned was eight minutes
from the western planet along the line drawn from it perpendicularly to
the straight line through all the planets, as shown above.
I
have reported these relations of Jupiter and its companions with the fixed
star so that anyone may comprehend that the progress of those planets,
both in longitude and latitude, agrees exactly with the movements derived
from planetary tables.
Such
are the observations concerning the four Medicean planets recently first
discovered by me, and although from these data their periods have not yet
been reconstructed in numerical form, it is legitimate at least to put in
evidence some facts worthy of note. Above all, since they sometimes follow
and sometimes precede Jupiter by the same intervals, and they remain
within very limited distances either to east or west of Jupiter,
accompanying that planet in both its retrograde and direct movements in a
constant manner, no one can doubt that they complete their revolutions
about Jupiter and at the same time effect all together a twelve-year
period about the center of the universe. That they also revolve in unequal
circles is manifestly deduced from the fact that at the greatest
elongation from Jupiter it is never possible to see two of these planets
in conjunction, whereas in the vicinity of Jupiter they are found united
two, three, and sometimes all four together. It is also observed that the
revolutions are swifter in those planets which describe smaller circles
about Jupiter, since the stars closest to Jupiter are usually seen to the
east when on the previous day they appeared to the west, and vice versa,
while the planet which traces the largest orbit appears upon accurate
observation of its returns to have a semimonthly period.
Here
we have a fine and elegant argument for quieting the doubts of those who,
while accepting with tranquil mind the revolutions of the planets about
the sun in the Copernican system, are mightily disturbed to have the moon
alone revolve about the earth and accompany it in an annual rotation about
the sun. Some have believed that this structure of the universe should be
rejected as impossible. But now we have not just one planet rotating about
another while both run through a great orbit around the sun; our own eyes
show us four stars which wander around Jupiter as does the moon around the
earth, while all together trace out a grand revolution about the sun in
the space of twelve years.
And
finally we should not omit the reason for which the Medicean stars appear
sometimes to be twice as large as at other times, though their orbits
about Jupiter are very restricted. We certainly cannot seek the cause in
terrestrial vapors, as Jupiter and its neighboring fixed stars are not
seen to change size in the least while this increase and diminution are
taking place. It is quite unthinkable that the cause of variation should
be their change of distance from the earth at perigee and apogee, since a
small circular rotation could by no means produce this effect, and an oval
motion (which in this case would have to be nearly straight) seems
unthinkable and quite inconsistent with the appearances. But I shall
gladly explain what occurs to me on this matter, offering it freely to the
judgment and criticism of thoughtful men. It is known that the
interposition of terrestrial vapors makes the sun and moon appear large,
while the fixed stars and planets are made to appear smaller. Thus the two
great luminaries are seen larger when close to the horizon, while the
stars appear smaller and for the most part hardly visible. Hence the stars
appear very feeble by day and in twilight, though the moon does not, as we
have said. Now from what has been said above, and even more from what we
shall say at greater length in our System, it follows that not only the
earth but also the moon is surrounded by an envelope of vapors, and we may
apply precisely the same judgment to the rest of the planets. Hence it
does not appear entirely impossible to assume that around Jupiter also
there exists an envelope denser than the rest of the aether, about which
the Medicean planets revolve as does the moon about the elemental sphere.
Through the interposition of this envelope they appear larger when they
are in perigee by the removal, or at least the attenuation, of this
envelope.
Time
prevents my proceeding further, but the gentle reader may expect more
soon.
finis
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