THE INNER PLANETS
When we recall that the Ptolemaic system of the universe was once
taught side by side with the Copernican at Harvard and at Yale, we
are impressed, not so much with the age of our universities, as with
the youth of modern astronomy and with the extraordinary vitality of
old ideas. That the Ptolemaic system in its fundamental principle
was antiquated at the start, the older Greeks having had juster
conceptions, does not lessen our wonder at its tenacity. But the fact
helps us to understand why so much fossil error holds its ground in
many astronomic text-books to-day. That stale intellectual bread is
deemed better for the digestion of the young, is one reason why it
often seems to them so dry.
[Illustration: ORBITS OF THE INNER PLANETS.]
Before entering upon the problem of the genesis and career of a world,
it is essential to have acquaintance with the data upon which our
deductions are to rest. To set forth, therefore, what is known of the
several planets of our solar system, is a necessary preliminary to any
understanding of how they came to be or whither they are tending; and
as our knowledge has been vitally affected by modern discoveries about
them, it is imperative that this exposition of the facts should be as
near as possible abreast of the research itself. I shall, therefore,
give the reader in this chapter a bird’s-eye view of the present state
of planetary astronomy, which he will find almost a different part of
speech from what it was thirty years ago. It is not so much in our
knowledge of their paths as of their persons that our acquaintance
with the planets has been improved. And this knowledge it is which has
made possible our study of their evolution as worlds.
Could we get a cosmic view of the solar system by leaving the world we
live on for some suitable vantage-point in space, two attributes of it
would impose themselves upon us—the general symmetry of the whole, and
the impressively graded proportions of its particular parts.
Round a great central globular mass, the Sun, far exceeding in size
any of his attendants, circle a series of bodies at distances from
him quite vast, compared with their dimensions. These, his principal
planets, are in their turn centres to satellite systems of like
character, but on a correspondingly reduced scale. All of them travel
substantially in one plane, a fact giving the system thus seen in its
entirety a remarkably level appearance, as of an ideal surface passing
through the centre of the Sun. Departing somewhat from this general
uniformity in their directions of motion, and also deviating more
from circularity in their paths, some much smaller bodies, a certain
distance out, dart now up now down across it at different angles and
from all the points of the compass, agreeing with the others only in
having the centre of the Sun their seemingly never attained goal of
endeavor. These bodies are the asteroids. Surrounding the whole, and
even penetrating within its orderly precincts, a third class would
be visible which might be described for size as cosmic dust, and for
display as heavenly pyrotechnics. Coming from all parts of space
indifferently they would seem to seek the Sun in almost straight lines,
bow to him in circuit, and then depart whence they came. For in such
long ellipses do they journey that these seem to be parabolas. These
visitants are the comets and their associates the meteor-streams.
Although for purposes of discrimination we have labelled the several
classes apart, an essential fact about the whole company is to be
noted: that no hard and fast line can be drawn separating the several
constituents from one another. In size the members of the one class
merge insensibly into the other. Some of the planets are hardly larger
than some of the satellites; some of the satellites than some of the
asteroids; some of the asteroids than comets and shooting-stars. In
path, too, we find every gradation from almost perfect circularity
like the orbits of Io and Europa to the very threshold of where one
step more would cease to leave the body a member of the Sun’s family
by turning its ellipse into an hyperbola. Finally, in inclination we
have every angle of departure from orthodox platitude to unconforming
uprightness. This point, that heavenly bodies, like terrestrial ones,
show all possible grades of indistinction, is kin to that specific
generalization by which Darwin revolutionized zoölogy a generation ago.
It is as fundamental to planets as to plants. For it shows that the
whole solar system is evolutionarily one.
A second point to be noticed in passing is that undue inclination and
excessive eccentricity go together. The bodies that have their paths
least circular have them, as a rule, the most atilt. And with these two
qualities goes lack of size. It is the smallest bodies that deviate
most from the general consensus of the system. With so much by way of
generic preface, the pregnancy of which will become apparent as we
proceed, we come now to particular consideration of its members in turn.
Nearest to the Sun of all the planets comes Mercury. So close is he to
that luminary, and so far within the orbit of the earth, that he is
not a very common object to the unaided eye. Copernicus is said never
to have seen him, owing, doubtless, to the mists of the Vistula. By
knowing when to look, however, he may be seen for a few days early in
the spring in the west after sunset, or before sunrise in the east in
autumn. He is then conspicuous, being about as bright as Capella, for
which star or Arcturus he is easily mistaken by one not familiar with
the constellations.
His mean distance from the Sun is thirty-six million miles, but so
eccentric is his orbit, the most so of any of the principal planets,
that he is at times half as far off again as at others. Even his
orbital behavior is the least understood of any in the solar system.
His orbit swings round at a rate which so far has defied analysis. It
may be a case of reflected perturbation, one, that is, of which the
indirect effect from another body becomes more perceptible than would
be the direct effect on the body itself. As yet it baffles geometers.
As to his person, our ignorance until lately was profound. It is only
recently that such fundamental facts about him as his size, his mass,
and his density have been reached with any approach to precision. This
was because he so closely hugs the Sun that observations upon his
full, or nearly full, disk had never been attempted. When I say that
his volume was not known to within a third of its amount, his mass not
closer than one-half, while his received density was nearly double
what we now have reason to suppose the fact, some idea of the depth
of our nescience may be imagined. This, of course, did not prevent
text-books from confidently misinstructing youth, or Nautical Almanacs
from misguiding computers with figures that thus almost achieved
immortality, so long had they passed current in spite of lacking that
perfection which is usually assigned as its warrant.
[Illustration: SULLA ROTAZIONE DI MERCURIO—DI G. V. SCHIAPARELLI.]
Schiaparelli first put astronomy on the right track. By attempting
daylight observations of the planet, not toward night, but actually
at midday, he made some remarkable discoveries, and though he did
not detect the hitherto erroneous values of the volume, the mass,
or the density, his method of observation paved the way for their
ascertainment. What he sought, and found, was evidence of markings
upon the disk by which the planet’s time of rotation might be
determined. Up to then, Schroeter’s value of about twenty-four hours
had been accepted, on very slender evidence indeed, and passed into
all the books. But when the planet came to be observed by noon, very
definite markings stood out on its face, which showed its rotation to
take place, not in twenty-four hours, but in eighty-eight days. By a
persistence equal to his able choice of observing time, he established
this beyond dispute. He proved the revolutionizing fact that Mercury’s
periods of rotation and of revolution were the same.
He detected, too, the evidence in the position of the markings of the
planet’s great libratory swing due to the eccentricity of its orbit, a
result as remarkable as a feat of observation as it was conclusive as a
proof.
If Schiaparelli had never done any other astronomical work, this study
of Mercury would have placed him as the first observer of his day. For
the observations are so difficult that the planet not only baffled all
his predecessors, but has foiled many since who are credited with being
observers of eminence.
In 1896 the study of Mercury was taken up at the Lowell Observatory
in Arizona along the same lines that had proved so successful with
Schiaparelli, but without using his observations as guide. Indeed,
his papers had not then been read there. The two conclusions were,
therefore, independent of one another. The outcome was a complete
corroboration and an extension of Schiaparelli’s work. We shall begin
with the consideration of the most fundamental point. In the clear and
steady air of Flagstaff, permitting of measurement of his disk up to
within a few degrees of the Sun, Mercury was found to be much larger
than previously thought.
Instead of a diameter of three thousand miles he proved to have one
of thirty-four hundred, making his volume nearly half as large again
as had been credited him. These measures bore intrinsic evidence
of their trustworthiness in an interesting manner, and at the same
time produced internal testimony that accounted for the smallness of
previous determinations. Measures heretofore had been made, usually if
not invariably, either when the planet transited the Sun or when it
exhibited a pronounced phase. Now in both these cases the planet looks
smaller than it is. In the first case this is due to irradiation, the
surrounding disk of the Sun encroaching both to the eye and to the
camera upon the silhouette of Mercury. And this inevitable effect had
not been allowed for in the measures. In the second case the horns of
the planet never seem to extend quite to their true position. This
was rendered evident by the Flagstaff series of measures, which began
when the planet was a half-moon and continued till it was almost
full. As it did so, the values for the diameter steadily increased,
even after irradiation was allowed for, although this against the
brilliant background of the noonday sky must have been exceeding
small, and tended in part to be diminished as the planet attained the
full, because of its consequent nearing of the Sun. The measures thus
explained themselves and vouched for their own accuracy.[3]
Then came a curious bit of unexpected proof to corroborate them. In his
“Astronomical Constants,”[4] published but a short time before, Newcomb
had detected a systematic error in the right ascensions of Mercury
which he was not able to explain. By diligent mousing that eminent
computer had discovered that Mercury was registered by observers too
far from the Sun on whichever side of him it happened to be, and
in proportion roughly not to its distance off but to the phase the
planet exhibited. When the disk was a crescent the discrepancy between
observation and theory was large, and thence decreased as the planet
passed to the full. He suspected the cause, and would have found it
had he not considered the diametral measures of the planet too well
assured to permit of doubt. As it was, he neglected a factor which
has vitiated almost all the observations made on the planets up to
within a few years, the correction for irradiation. This was the case
here. The received measures, beginning with Bradley and ending with
Todd, had almost without exception been made in transit, and, as no
regard had been paid to the contracting effect of irradiation, had been
invalidated in consequence. The new method supplied almost exactly the
amount needed to explain the right ascensions, a second of arc, and in
precise accordance with the place which the discrepancy demanded.
[3] New Observations of the Planet Mercury, _Memoirs Amer. Acad._ 1897.
Vol. XII, No. 4.
[4] “Astronomical Constants,” 1895, pp. 67, 68.
About the mass there has been, and still is, great uncertainty. This
is because it can only be found from the perturbing effect it has on
Venus, the Earth, or Encke’s comet. Modern determinations, however, are
smaller than the older ones; thus Backlund in 1894 got from the effect
on Encke’s comet only one-half the mass that Encke had, fifty-three
years before. Probably the most reliable information comes from Venus,
which Tisserand found to give for Mercury ¹/₇₁₀₀₀₀₀ of the mass of the
Sun, or ¹/₂₁ of the mass of the Earth. If we take ¹/₇₀₀₀₀₀₀ as the
nearest round number, we find the planet’s density to be 0.66 that of
the Earth.
[Illustration: MAP OF MERCURY
LOWELL OBSERVATORY 1896-97]
The same observations that disclosed at Flagstaff the planet’s size
revealed a set of markings on his face so definite as to make the
rotation period unmistakable. It takes place, as Schiaparelli found,
in eighty-eight days, or the time of the planet’s revolution round
the Sun. The markings disclosed the fact, as Schiaparelli had also
discovered, in a most interesting manner, for the ellipticity of
the planet’s orbit stood reflected in the swing of the markings
across the face of the disk, a definiteness in the proof of a really
surprising kind. What this means we shall see in a subsequent chapter
when we take up the mechanical problem of the tides. Another result
that issued from the positions of the markings was the determination
of the planet’s pole. Except for the libration above noticed, the
markings kept an invariable longitudinal position upon the illuminated
disk, showing that the planet turned always the same face to the Sun;
but latitudinally a difference was noticeable between their place
in October-November, 1896, and in February-March, 1897, the latter
being 4° farther north. Now this is just what the orbital position
should have caused, if the pole stood vertically to it. Thus a
difference of 4° from perpendicularity should have been discernible,
had it existed,—a very small amount in such a determination. We may,
therefore, conclude that the axis stands plumb to the orbit, and this
is what theory demands.
The state of things this introduces to us upon that other world is
to our ideas exceeding strange. It is not so much the slowness of
the diurnal spin, eighty-eight times as long as our own, which is
surprising, as the fact that this makes its day infinite in length.
Two antipodal hemispheres divide the planet, the one of which frizzles
under eternal sun, the other freezes amid everlasting night. The Sun
does not, indeed, stand stock-still in the sky, but nods like some
huge pendulum to and fro along a parallel of latitude. In consequence
of libration the two great domains of day and night are sundered by
a strip of debatable ground 23½° in breadth on either side, upon
which the Sun alternately rises and sets. Here there is a true day,
eighty-eight of our days in length from one sunrise to the next. But
its day and night are not apportioned alike. The eastern strip has its
daylight briefer than its starlight hours; the western has them longer.
Nor are different portions of the strips similarly circumstanced in
their sunward regard. Only the edge next perpetual day has anything
approaching an equal distribution of sunlight and shade. The farther
one just peeps at the Sun for a moment every eighty-eight days, and
then sinks back again into obscurity.
The transition from day to night is equally instantaneous and profound.
For little or no twilight here prolongs the light; since the air, if
there be any at all, is too thin to bend it to service round the edge
to illuminate the night. When the libratory Sun sets, darkness like
a mantle falls swiftly over the face of the ground. No evidence of
atmosphere has ever been perceived, and theory informs that it should
be nearly, if not wholly, absent.
In consequence of the rigid uprightness of the planet’s axis,
seasons do not exist. Their nearest simulacrum comes from the
seeming dilatation of the Sun during half the year, and its apparent
contraction during the other half. It expands so much between its
January and its July as to receive more heat in the ratio of nine to
four. A seasonless, dayless, and almost yearless planet, it is better
to look at than to look from; but its study opens our eyes to the great
diversity which even one of our nearest neighbors exhibits from what we
take as matters of course on Earth.
That what we take offhand to be purely astronomic phenomena should turn
out to be so essentially of the particular world, worldly, clarifies
vision of what these really are, and how dependent on and interwoven
with everyday life astronomy is. Or, we may consider it turned about
and realize how purely astronomic relations, such abstract mechanical
matters as rotations and revolutions, result in completely changing the
very face and character of the globe concerned. Mercury to-day stares
forever at the Sun. The markings we see have stereotyped this stare
to its inevitable result. For they seem to mark a globe sun-cracked.
At such a condition the curious crisscross of dark, irregular lines
certainly hints, accentuated and perfected as it is by a bounding curve
where the mean sunward side terminates to the enclosing them as by the
carapace of a tortoise. Though they cannot probably be actual cracks,
however much they may resemble such, yet they may well owe their
existence to that fundamental cause.
In color the planet is ghastly white; of that wan hue that suggests a
body from which all life has fled. Far whiter than Venus in point of
fact, the rosy tint with which it sparkles in the sunset glow is all
borrowed of the dying day and vanishes when the planet is looked at in
the uncompromising light of noon. Seen close together once at Flagstaff
it was possible directly to compare the two; when Mercury, although lit
by the Sun two and a half times as brilliantly as Venus, was, surface
for surface, more than twice as faint. Müller has found its intrinsic
brightness about that of our Moon, which in some respects it resembles,
though it apparently departs widely from any similarity in others. The
bleached bones of a world; that is what Mercury seems to be.
Venus comes next in order outward from the Sun. To us her incomparable
beauty is partly the result of propinquity: nearness to ourselves and
nearness to the Sun. Relatively so close is she to both that she does
not need the Sun’s withdrawal to appear, but may nearly always be seen
in the daytime in clear air if one knows where to look for her. Situate
about seven-tenths of our own distance from our common giver of light
and heat, she gets about double the amount that falls to our lot, so
that her surface is proportionately brilliantly illuminated. Being also
relatively near us, she displays a correspondingly large surface.
But though part of her lustre is due to her position, a part is her
own. Direct visual observation, as we remarked above, shows her
intrinsic brightness to be more than five times that of Mercury,
square mile to square mile of surface for the two. Now this has been
determined very carefully photometrically by Müller at Potsdam. The
result of his inquiry was to indicate that Mercury shines with 0.17 of
absolute reflection, Venus with 0.92. So high a value has seemed to
many astronomers impossible, because so far surpassing that which has
tacitly been taken as the _ne plus ultra_ of planetary brightness, that
of cloud, 0.72.
Now, one of the direct outcomes of the study of Venus at the Lowell
Observatory was an explanation of this seemingly incredible phenomenon.
When the planet came to be critically examined there under conditions
of seeing which permitted discovery, markings very faint, but
nevertheless assurable, stood presented on the planet’s face. These
markings, of which we shall have more to say in a moment, had this of
pertinency to our present point, that they kept an invariable position
to one another. They thus betrayed themselves to be surface features.
Furthermore, their dimness was as invariable an attribute of them as
their place. They were not obscured on some occasions and revealed at
others, but stayed, so far as one might judge, permanently the same.
They were thus neither clouds themselves nor subject to the caprice of
cloud. The old idea that Venus was a cloud-wrapped planet and owed her
splendor to this envelope, vanished literally into thin air.
It is precisely because she is not cloud-covered that her lustre is so
great. She “clothes herself with light as with a garment” by a physical
process of some interest. As becomes the Mother of the Loves, this is
gauze of the most attenuated character, and yet a wonderful heightener
of effect. For it consists solely of the atmosphere that compasses her
about. It is well known that a substance when comminuted reflects much
more light than when condensed into a solid state. Now an atmosphere is
itself such a comminuted affair, and, furthermore, holds in suspension
a variety of dust. This would particularly be the case with the
atmosphere of Venus, as we shall have reason to see when we consider
the conditions upon that planet made evident by study of its surface
markings. To her atmosphere, then, she owes four-fifths or more of her
brilliancy. And this stands corroborated by the low albedo of both
Mercury and the Moon, which have no atmosphere, and by the intermediate
lustre of Mars, which has some, but little.[5]
[5] _Astr. Nach._ No. 3406. Monthly Notices R. A. S., March, 1897.
The rotation time of Venus, the determination, that is, of the planet’s
day, is one of the fundamental astronomical acquisitions of recent
years. For upon it turns our whole knowledge of the planet’s physical
condition. More than this, it adds something which must be reckoned
with in the framing of any cosmogony. It is not a question of academic
accuracy merely, of a little more or a little less in actual duration,
but one which carries in its train a completely new outlook on Venus
and sheds a valuable side-light upon the history of our whole planetary
system.
Unconsciously influenced, one is inclined to think, by terrestrial
analogies, astronomers for more than a couple of centuries, ever
since the time of the first Cassini in 1666, deemed the day of Venus
to be just under twenty-four hours in length. So well attested was
its determination, and so precisely figured to the minute, that it
imposed itself upon text-books which stated it as an acquired fact
down to the last second. Nevertheless, Schiaparelli was not so sure,
and proceeded to look into the matter. He first looked for himself,
and then looked up all the old observations. His chief observational
departure was observing by day as near to noon as possible; because
then the planet was highest, to say nothing of the taking off from its
glare by the more brilliant sky. From certain dark markings around
two bright spots near the southern cusp, of one of which spots the
detection dates from the time of Schroeter, and from a long, dark
streak stretching thence well down the disk, he convinced himself that
no such period as twenty-four hours could possibly be correct, inasmuch
as whenever he looked, the markings were always there. His notes read,
“Same appearance as yesterday,” day after day, until he would really
have saved ink and penmanship had he had the phrase cut into a die and
stamped. He concluded that the rotation was at least six months long,
and was probably synchronous with the planet’s time of revolution. This
was in 1889. In 1895 he became still more sure, and showed how the
older observations were really compatible with what he had found.
In 1896 the subject was taken up at Flagstaff. Very soon it became
evident there that markings existed on the disk, most noticeable
as fingerlike streaks pointing in from the terminator, faint but
unmistakable from the identity of their successive presentation.
Schroeter’s projection near the south cusp was also clearly discernible
as well as two others, one in mid-terminator, one near the northern
cusp. Schiaparelli’s dark markings also came out, developing into a
sort of collar round the southern pole. Other spots and streaks also
were discernible, and all proved permanent in place. By watching them
assiduously it was possible to note that no change in position occurred
in them, first through an interval of five hours, then through one
of days, then of weeks. Care was taken to guard against illusion. It
thus became evident that they bore always the same relation to the
illuminated portion of the disk. This illuminated part, then, never
changed. In other words, the planet turned always the same face to
the Sun. The fact lay beyond a doubt, though of course not beyond a
doubter.[6]
[6] Monthly notices R. A. S., March, 1897.
[Illustration: VENUS. OCTOBER, 1896—MARCH, 1897—DRAWINGS BY DR.
LOWELL.]
[Illustration: VENUS. APRIL 12, 1909, 3H 26M—4H 22M—BY DR. LOWELL.]
The years that have passed since these observations were made have
brought corroboration of them. Several observers at Flagstaff have
seen and drawn them and added discoveries of their own, among whom are
especially to be mentioned, of the observatory staff: Miss Leonard, Dr.
Slipher, and Mr. E. C. Slipher.[7]
[7] Lowell Observatory Bulletin 6.
In character these markings were peculiar and distinctive. In addition
to some of more ordinary character were a set of spokelike streaks
which started from the planet’s periphery and ran inwards to a point
not very distant from the centre. The spokes started well-defined and
broad at the edge, dwindling and growing fainter as they proceeded,
requiring the best of definition for their following to their central
hub.
The peculiar symmetry thus displayed, a symmetry associated with the
planet’s sunrise and sunset line, or, strictly speaking, what would be
such did the Sun for Venus ever rise or set, would seem inexplicable,
except for that very association. When we reflect, however, upon what
this means, a very potent cause for them becomes apparent, so potent
that surprise is turned into appreciation that nothing else could well
exist. That Venus turns on her axis in the same time that she revolves
about the Sun, in consequence of which she turns always the same face
to him, must cause a state of things of which we can form but faint
conception, from any earthly analogy. One face baked for countless
æons, and still baking, backed by one chilled by everlasting night,
while both are still surrounded by air, must produce indraughts from
the cold to the hot side of tremendous power. A funnel-like rise must
take place in the centre of the illuminated hemisphere, and the partial
vacuum thus formed would be filled by air drawn from its periphery,
which, in its turn, would draw from the regions of the night side.
Such winds would sweep the surface as they entered, becoming less
superficial as they advanced, and the marks of their inrush might well
be discernible even at the distance we are off. Deltas of such inroad
would thus seam the bounding circle of light and shade.
[Illustration: I
Showing convection currents in the planet’s atmosphere.]
[Illustration: II
Showing shift in central barometric depression due to rotation of the
planet affecting the winds.
VENUS.]
Another result of the aërial circulation would be the removal of all
moisture from the sunward face, and its depositing in the form of
ice upon the night one. For the heated air would be able to carry
much water in suspension, which, on cooling, after it had reached the
dark hemisphere would unload it there. In the low temperature there
prevailing, this moisture would all be frozen, and so largely estopped
from return. This process continuing for ages would finally deplete one
side of all its water to heap it up in the form of ice upon the other.
Now it is not a little odd that a phenomenon has been observed upon
Venus which seems to display just this state of things. Many observers
have noted an ashen light on the dark side of her disk. Some have
tried to account for it as Earth shine, the same earth-reflected light
that makes dimly visible the old moon in the new moon’s arms. But the
Earth is too far away from Venus to permit of any such effect; nor is
there any other body that could thus relieve its night. But if the
night hemisphere of Venus be one vast polar sheet, we have there a
substance able to mirror the stars to a ghostlike gleam which might be
discernible even from our distant post.
[Illustration: _Venus Rotation 225 days._]
Thus when we reason upon them we see that the peculiar markings of the
planet lose their oddity, becoming the very pattern and prototype of
what we should expect to view. Interpreted, they present us the picture
of a plight more pitiable even than that of Mercury. For the nearly
perfect circularity of Venus’ orbit prevents even that slight change
from everlasting sameness which the libration of Mercury’s affords. To
Venus the Sun stands substantially stock-still in the sky,—a fact which
must prove highly reassuring to Ptolemaic astronomers there, if there
be any still surviving from her past. No day, no seasons, practically
no year, diversifies existence or records the flight of time. Monotony
eternalized,—such is Venus’ lot.
What visual observations have thus discovered of the rotation time of
Venus, with all that follows from it, the spectroscope at Flagstaff has
confirmed. At Dr. Slipher’s hands, spectrograms of the planet have told
the same tale as the markings. It was with special reference to this
point that the spectrograph there was constructed, and the first object
to which it was directed was Venus.[8]
[8] Lowell Observatory Bulletin No. 3.
The planet’s rotation time was to be investigated by means of the
motion it brought about in the line of sight. Visual observation,
telescopically, reveals motion thwart-wise by the displacement it
produces in the field of view; spectroscopic observation discloses
motion to or from the observer by the shift it causes in the spectral
lines due to a stretching or shortening of their wave-lengths.
The spectroscope is an instrument for analyzing light. Ordinary light
consists of light of various wave-lengths. By means of a prism or
grating these are dispersed into a colored ribbon or band, the longer
waves lying at the red end of the spectrum, as the ribbon is called,
the shorter at the violet. Now the spectroscope is primarily such a
prism or grating placed between the image and the observer, by means of
which a series of colored images of the object are produced. In order
that these may not overlap and so confuse one another, the light is
allowed to enter the prism only through a narrow slit placed across the
telescopic image of the object to be examined. Thus successive images
of what is contained by the slit are presented arranged according to
their wave-lengths. In practice the rays of light from the slit enter a
small telescope called the collimator, and are there rendered parallel,
in which condition they fall upon the prism. This spreads them out into
the spectrum and another small telescope focusses them, each according
to its kind, into a spectral image band which may then be viewed by the
eye or caught upon a photographic plate.
Now, if an object be coming toward the observer, emitting or reflecting
light as it does so, each wave-length of its spectrum will be shortened
in proportion to the relative speed of its approach as compared with
the speed of light, because each new wave is given out by so much
nearer the observer and in reflection the body may also meet it.
Reversely it will be lengthened if the object be receding from the
observer or he from it. This would change the color of the object were
it not that while each hue moves into the place of the next, like the
guests at Alice’s tea-party in Wonderland, some red rays pass off the
visible spectrum, but new violet rays come up from the infraviolet
and the spectrum is as complete as before. Fortunately, however, in
all spectra are gaps where individual wave-lengths are absorbed or
omitted, and these, the lines in the spectrum, tell the tale of shift.
Now if a body be rotating, one side of it will be approaching the
observer, while the opposite side is receding from him, and if the slit
be placed perpendicular to the axis about which the spin takes place,
each spectral line will appear not straight across the spectrum of the
object, but skewed, the approaching side being tilted to the violet
end, the receding side to the red.
This was to be the procedure adopted for the rotation of Venus. By
placing the slit parallel to the ecliptic, or, more properly, to the
orbit of Venus, which is practically the same thing, it found itself
along what we have reason to suppose the equator of the planet. Even a
considerable error on this point would make little difference in the
rotational result. In order that there might be no question of illusion
or personal bias, photographs instead of eye observations of the
spectrum were made. For reference and check side by side with that of
Venus were taken on either hand the spectra of iron, made by sparking a
tube containing the vapor of that metal. The vapor, of course, had no
motion with regard to the observer, and therefore its spectral lines
could have no tilt, but must represent motional verticality.
Dr. Slipher chose his time astutely. He selected the occasion when
Venus was passing through superior conjunction, or the point in her
orbit as regards us directly beyond the sun. At first sight this might
seem to be the worst as well as the most impracticable of epochs,
inasmuch as the planet is then not only at her farthest from the Earth,
but in a line with the Sun, and so drowned in his glare. But in point
of fact any tilt of the spectral lines is then, owing to phase, twice
what it is at elongation, and exceeds still more what it is when Venus
has her greatest lustre [see NOTE 3]. In his purpose he was abetted
by the Flagstaff air, which enabled the planet to be spectrographed
much nearer the sun than would otherwise have been the case. He thus
selected the best possible opportunity. To guard against any subsequent
bias on the part of the examiner of the plates, after the spectroscope
had taken a plate it was then reversed, and the process repeated on
another one, the iron being sparked as before. What had been the right
side of Venus with regard to the red end of the spectrum thus became
the left one, and _vice versa_. In this manner, when the plates came
to be measured for tilt, the measurer would have no indication from
the spectrum itself which way the lines might be expected to tilt; he
could, therefore, not be influenced either consciously or unconsciously
in his decision.
[Illustration: SPECTROGRAM OF VENUS, SHOWING ITS LONG DAY—V. M.
SLIPHER, LOWELL OBSERVATORY, 1903.]
Eight plates with their comparison ferric spectra were thus secured;
four with the spectroscope direct, four with it reversed. They
were then shuffled, their numbers hidden, and given to Dr. Slipher
to measure. The spectral lines told their own story, and without
prompting. All the plates agreed within the margin of error accordant
with their possible precision, a precision some thirty times that of
Belopolski’s experiment on the same lines,—a result not derogatory of
that investigator, but merely illustrative of superior equipment. They
showed conclusively that a rotation of anything like twenty-four hours
was out of the question. They yielded, indeed, testimony to a negative
rotation of three months, which, interpreted, means that so slow a spin
as this was beyond their power to precise.
For Dr. Slipher was at no less care to determine just what precision
was possible in the case, although a speed corresponding to a spin
of twenty-four hours on a globe the size of Venus is well known to
be spectroscopically measurable. It would mean a motion toward us
of one thousand miles an hour, or about a third of a mile a second.
The tilt occasioned by this speed is well within the spectroscope’s
ability to disclose. Not content with this, however, by two special
investigations, he proved the spectroscope’s actual limits of
performance to be far within the quantity concerned. One of them was
the determination by the same means and in like manner of the rotation
time of Mars, the length of that planet’s day, which in other ways we
know to the hundredth of a second, and which is 24ʰ 37ᵐ 23.66ˢ Now Mars
offers a test nearly twice as difficult as Venus, even supposing the
apparent disks of the two the same, because his diameter being less in
the proportion roughly of one-half, the actual speed of a particle at
his edge is less for the same time of rotation in the like proportion,
and it is only with the speed in miles, not in angular amount, that the
spectroscope is concerned. Nevertheless, when a like number of plates
were tried on him, they indicated on measurement a rotation time within
an hour of the true. This corresponds to half an hour on Venus. We see,
therefore, that had Venus’ day been anywhere in the neighborhood of
twenty-four hours, Dr. Slipher’s investigation would have disclosed it
to within thirty-one minutes.
[Illustration: SPECTROGRAM OF JUPITER, GIVING THE LENGTH OF ITS DAY BY
THE TILT OF ITS SPECTRAL LINES—V. M. SLIPHER, LOWELL OBSERVATORY.]
This result was further borne out by a similar test made by him
of Jupiter. Inasmuch as the diameter of Jupiter is twelve times
that of Venus, while the rotation time is 9ʰ 50.4ᵐ at the equator,
the precision attained on Venus should here have been about a
minute. And this is what resulted. Slipher found the rotation time
spectrographically 9ʰ 50ᵐ, or in accordance with the known facts, while
previous determinations with the spectroscope had somehow fallen short
of it.
The care at Flagstaff with which the possibility of error was sought
to be excluded in this investigation of the length of Venus’ day and
the concordant precision in the results are worthy of notice. For
it is by thus being particular and systematic that the accuracy of
the determinations made there, in other lines besides this, has been
secured.
In size, Venus of all the planets most nearly approaches the Earth.
She is 7630 miles in diameter to the Earth’s 7918. Her density, too,
is but just inferior to ours. And she stands next us in place, closest
in condition and constitution in the primal nebula. Yet in her present
state she could hardly be more diverse. This shows us how dangerous it
is to dogmatize upon what can or cannot be, and how enlightening beyond
expectation often is prolonged and systematic study of the facts.
The next planet outward is our own abode. It is one of which most of
us think we know considerable from experience and yet about which we
often reason cosmically so ill. If we knew more, we should not deem
ourselves nearly so unique. For we really differ from other members of
our system not more than they do from one another. Much that appears to
us fundamental is not so in fact. Thus many things which seem matters
of course are merely accidents of size and position. Our very day and
night upon which turn the habits of all animals and, even in a measure,
those of plants, are, as we have seen, not the possession of our
nearest of cosmic kin. Our seasons which both vegetally and vitally
mean so much are absent next door. And so the list of our globe’s
peculiar attributes might be run through to the finding of diversity
to our familiar ways at every turn. But, as we shall see later, these
differences from one planet to the next are not only not incompatible
with a certain oneness of the whole, but actually help to make the
family relationship discoverable. Analogy alone is a dangerous guide,
but analogy crossed with diversity is of all clews the most pregnant of
understanding. The very fact that we can tell them apart when we see
them together, as the Irishman remarked of two brothers he was in the
habit of confusing, points to their brotherly relation.
Proceeding still further, we come to Mars at a mean distance of one
hundred and forty-one million miles. Smaller than ourselves, his
diameter is but a little over half the Earth’s, or forty-two hundred
miles, his mass one-ninth of ours, and his density about seven-tenths
as much. Here, again, but in a different way, we find a planet unlike
ourselves, and we know more about him than of any body outside the
Earth and Moon. So much about him has been set forth elsewhere that
it is enough to mention here that no oceans diversify his surface,
no mountains relieve it, and but a thin air wraps it about,—an air
containing water-vapor, but so clear that the surface itself is almost
never veiled from view.
About the satellites Mars possesses, Deimos and Phobos, we may perhaps
say a word, as recent knowledge concerning them exemplifies the care
now taken to such ascertainment and the importance of considering
factors often overlooked. Soon after they were discovered in 1877, they
were measured photometrically, with the result of giving a diameter
of six miles to Deimos and one of seven miles to Phobos, and these
values unchallenged entered the text-books. When the satellites came
to be critically considered at Flagstaff, it was found that these
determinations were markedly in error, Phobos being very much the
larger of the two, the actual values reaching nearer ten miles for
Deimos and thirty-six for Phobos.
In getting the Flagstaff values, the size to the eye of the satellite
was corrected for the background upon which it shone; for the
background is all-important to the brilliancy of a star. In the case
of a small star near a planet, the swamping glare of the planet is
something like the inverse cube of its distance away. Furthermore, the
Flagstaff observations indicated how the previous error had crept in.
For before correction for the differing brilliancies of the field of
view, the apparent size of the satellites judged by conspicuousness
was about six to seven. The photometric values must have been taken
just as they came out, no correction apparently having been made for
the background. Now the background is a fundamental factor in all
photometric determinations, a factor somewhat too important in this
case to neglect, since it affected the result 2500 per cent.