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Physical Science
3.1 The Solar System
Presearch SS1: Models of the Solar System
“Therefore the solid body of the Earth is reasonably considered as being the largest
and as such remains unmoved. For if it had some common movement, the same as other
weights, it would clearly leave them all behind, and the animals and other weights would
be left hanging in the air, and the Earth would very quickly fall out of the heavens.
Merely to conceive such things makes them appear ridiculous.”
- Ptolemy, ca. 150 C.E.
the Almegest
“Whatever motion appears in the firmament arises not from any motion of
the firmament, but from Earth’s motion. What appears to us as motions of the sun arise
not from its motion but from the motion of the Earth, with which we revolve around
the sun like any other planet.”
- Nicolaus Copernicus, 1543
de Revolutionibus
Theme:
Structure & Function
Objective:
1) Distinguish between geocentric and heliocentric models of the
Solar System
Primary Questions:
1) Upon what evidence or reasoning was the geocentric model
founded?
2) Upon what evidence or reasoning was the heliocentric model
founded?
Illustration:
____ The Ptolemaic Model
Compose an illustration of the Ptolemaic model of the solar system.
Include labels and annotation as necessary.
____ The Copernican Model
Compose an illustration of the Copernican model of the solar system.
Include labels and annotation as necessary.
Reading:
Read each adaptation actively: margin notes, highlights, etc.
Compose two-column notes on loose-leaf, and attach to the reading.
____ “Raising (and Lowering) the Roof,” by Timothy Ferris
(attached)
____ “The Ptolemaic Model,” by Joel Davis
(attached)
____ “Copernicus and the Sun-centered Cosmos,” by Joel Davis
(attached)
____ “Welcome to the Solar System,” by Bill Bryson
(attached)
Vocabulary:
____ Add the following to your Academic Vocabulary. Define each as it
applies to the subject under study.
geocentric
heliocentric
epicycle
retrograde motion
Who’s Who:
____ Identify each of the following, and briefly describe their contributions
to the subject under study:
Ptolemy
Nicolaus Copernicus
Raising (and Lowering) the Roof
adapted from
Coming of Age in the Milky Way, by Timothy Ferris, 1988
The Earth-centered universes of the ancients were small by today’s standards.
Ptolemy’s appears to have been the most generous. Certainly he thought it grand, and
he liked to remark, with an astronomer’s fondness for wielding big numbers, that in his
universe the Earth was but “a point” relative to the heavens. And, indeed, it was
enormous by the standards of a day when celestial objects were assumed to be small
and to lie close at hand. Nevertheless, the Ptolemaic universe is estimated to have
measured only some fifty million miles in radius, meaning that it could easily fit inside
what we now know to be the dimensions of the Earth’s orbit around the sun.
The diminutive scale of early models of the cosmos resulted from the assumption
that the Earth sits, immobile, in the center of the universe. If the Earth does not move,
then the stars do: the starry sphere must rotate on its axis once per day in order to bring
the stars trooping overhead on schedule, and the larger the sphere, the faster it must
rotate. Were such a cosmos very large, the speed necessary for the celestial sphere
would become unreasonably high. The stars of Ptolemy’s universe already were obliged
to hustle along at better than a million miles per hour, and were the celestial sphere
imagined to be a hundred times larger it would have to be turning faster than the speed
of light. One did not have to be an Einstein, or even to know the velocity of light, to
intuit that that was too fast—a point that began to worry cosmologists by the sixteenth
century. All geocentric, immobile-Earth cosmologies tended to inhibit appreciation of
the true dimensions of space.
To set the Earth in motion would be to expand the universe, a step that seemed
both radical and counterintuitive. The Earth does not feel as if it is spinning, nor does
the observational evidence suggest any such thing: were the Earth turning on its axis,
Athens and all its citizens would be hurtling eastward at a thousand miles per hour. If
so, the Greeks reasoned, gale-force easterlies ought constantly to sweep the world, and
broad-jumpers in the Olympics would land in the stands far to the west of their
jumping-off points. As no such effects were observed, most of the Greeks concluded
that the Earth does not move.
If one goes further and imagines that the Earth not only spins on its axis but
orbits the sun as well, then one’s estimations of the dimensions of the cosmos must be
enlarged even more. The reason for this is that if the Earth orbits the sun, then it must
alternately approach and withdraw from one side of the sphere of the stars—just as,
say, a child riding a carousel first approaches and then recedes from the gold ring. If the
stellar sphere were small, the differing distance would show up as an annual change in
the apparent brightness of stars along the zodiac; in summer, for instance, when the
Earth is on the side of its orbit closer to the star Spica, its proximity would make Spica
look brighter than it does in winter, when the Earth is on the far side of its orbit. As no
such phenomenon is observed, the stars must be very far away, if indeed the Earth
orbits the sun.
The astonishing thing, then, given their limited understanding of physics and
astronomy, is not that the ancient thought of the universe in geometric terms, but that
they did not all think of it that way. The great exception was Aristarchus, the Greek
philosopher whose heliocentric cosmology predated that of Copernicus by some
seventeen hundred years.
Aristarchus came from Samos, a wooded island, where three centuries earlier
Pythagoras had first proclaimed that “all is number.” Aristarchus was a skilled
geometer who had a taste for the third dimension, and he drew, in his mind’s eye, vast
geometrical figures that stretched not only across the sky but out into the depths of
space as well. While still a young man, he published a book suggesting that the sun was
nineteen times the size and distance of the moon; his conclusions were quantitatively
erroneous (the sun is actually four hundred times larger and farther away than the moon)
but his methods were sound.
It may have been this work that first led Aristarchus to contemplate a suncentered cosmos: having concluded that the sun was larger than the Earth, he would
have found that for a giant sun to orbit a smaller Earth was intuitively as absurd as to
imagine that a hammer thrower could swing a hammer a hundred times his own
weight. The evolution of Aristarchus’ theory cannot be verified, however, for his book
proposing the heliocentric theory has been lost. We know of it only from a paper
written in about 212 B.C.E. by Archimedes, the geometer.
Archimedes’ paper was titled “The Sand Reckoner,” and its purpose was to
demonstrate that a system of mathematical notation he had developed was effective in
dealing with large numbers. To make the demonstration vivid, Archimedes wanted to
show that he could calculate even such a huge figure as the number of grains of sand it
would take to fill the universe. The paper was intended as but an entertainment or a
piece of popular science writing. What makes it vitally important today is that
Archimedes, wanting to make the numbers as large as possible, based his calculations
on the dimensions of the most colossal universe he had ever heard of—the universe
according to the novel theory of Aristarchus of Samos.
Archimedes, a man of strong opinions, had a distaste for loose talk of “infinity,”
and he begins “The Sand Reckoner” by assuring that the number of grains of sand on
the beaches of the world, though very large, is not infinite, but can, instead, be both
estimated and expressed:
“I will try to show you, by means of geometrical proofs, which you will be able to follow,
that, of the numbers named by me, some exceed not only the number of the mass of sand equal to
the Earth filled up in the way described, but also that of a mass equal in magnitude to the
universe.”
Continuing in this vein, Archimedes adds that he will calculate how many grains of
sand will be required to fill, not the relatively cramped universe envisioned by the
traditional cosmologies, but the much larger universe depicted in the new theory by
Aristarchus:
“Aristarchus of Samos brought out a book consisting of certain hypotheses, in which it
appears, as a consequence of the assumptions made, that the universe is many times greater in
size than that now so called. His hypotheses are that the fixed stars and the sun remain
unmoved, that the Earth revolves about the sun in the circumference of a circle, the sun lying in
the middle of the orbit, and that the sphere for the fixed stars, situated about the same center as
the sun, is so great that the circle in which he supposes the Earth to revolve bears such a
proportion to the distance of the fixed stars as the center of the sphere bears to its surface”.
To plug hard numbers into Aristarchus’ model, Archimedes takes Aristarchus to
mean that the ratio of the size of the Earth to the size of the universe is comparable to
that of the orbit of the Earth compared to the sphere of stars. Now he can calculate.
Incorporating contemporary estimates of astronomical distances, Archimedes derives a
distance to the sphere of stars of, in modern terminology, about six trillion miles, or one
light-year.
This was a stupendous result for its day—a heliocentric universe with a radius
more than a hundred thousand times larger that that of the Ptolemaic model, proposed
four centuries before Ptolemy was born! Although we know today that one light-year is
but a quarter of the distance to the nearest star, and less than one ten-billionth the
radius of the observable universe, Aristarchus’ model nonetheless represented a
tremendous increase in the scale that the human mind had yet assigned to the cosmos.
Had the world listened, we today would speak of an Aristarchian rather than a
Copernican revolution in science, and cosmology might have been spared a millennium
of delusion. Instead, the work of Aristarchus was all but forgotten. Seleucus, the
Babylonian, championed the Aristarchian system a century later, but appears to have
been lonely in his enthusiasm for it. Then came the paper triumph of Ptolemy’s
shrunken geocentric universe, and the world stood still.
Ferris, Timothy. Coming of Age in the Milky Way. New York, NY: HarperCollins, 1988.
The Ptolemaic Model
adapted from
Journey to the Center of Our Galaxy, by Joel Davis
Little is known about the life of Claudius Ptolemy. We do not even know the
year he was born or died. From his published astronomical observations, however, we
know that he was working sometime around 125 C.E. We also know that Ptolemy lived
and worked in Alexandria, and had access to the centuries of astronomical records and
writings that were stored there. His great achievement was a 13-volume treatise entitled
Almagest. In it was a catalog of more than 1,000 stars, considerable mathematical
information, and his detailed creation of a geocentric cosmology. The aptly named
“Ptolemaic System” became the reigning astronomical model of the Western world for
more than fourteen hundred years.
Ptolemy’s model was based upon one developed earlier by Aristotle. In that
model, Earth was a sphere at the center of the universe. Around it, revolving in a
uniform manner, were other spheres carrying the sun and planets. The Earth itself did
not move. Ptolemy began modifying this system in order to make it fit more closely the
observable movements of the planets. In particular, he adopted geometric devices
developed by Hipparchus—the epicycle and deferent—to explain retrograde motion (that
is, the apparent “backward” motion of the planets against the stars). Then Ptolemy did
something truly revolutionary: to explain additional motions, he invented a geometric
device of his own called the equant, which offset the center of planetary orbits some
distance away from the center of the solar system. In doing so, Ptolemy violated one of
the primary aspects of Aristotle’s cosmology, that is, the assumption that the centers of
motion for all celestial bodies shared the same point at the center of the universe. And
he did so for a very good reason. It was the only way he could make his system conform
reasonably well to increasingly accurate observations of the planets’ actual motions in the sky.
And it was a very important step for Ptolemy to take.
His action, in fact, lies at the heart of science as we know it today, for the driving
force of science is not so much to confirm an explanation or theory but to find ways to
tear it apart. The best way to do that is to test a theory against actual observations of the
real world. If a theory or explanation does not conform with reality, it must be either
modified or thrown out. Aristotle’s assumption was that all celestial motions must be
uniform and circular, but this did not match reality. So Ptolemy modified it. Eventually,
many Aristotelian views would be tossed into the trashcan of history, but for the time
being Ptolemy’s move was a start in the right direction.
Figure 1: The Ptolemaic Model of planetary motion.
Planet revolves counterclockwise on epicycle, while epicycle’s centerpoint revolves
counterclockwise along deferent. The combined motions are illustrated at right: planet’s motion
moves through points 1 – 7, but from Earth appears to be moving backward (retrograde) from
points 3 – 5. Rate of motion along deferent also varies with distance from equant: slower motions
occur nearer the equant.
When finished, Ptolemy’s model of the cosmos was fairly simple. However, it
did treat some of the planets differently than others. Mercury and Venus somehow had
to be forced to stay near the sun, as they do in real life, so Ptolemy fiddled with their
geometries to make them do so. He also had to modify the orbits of Mars, Jupiter and
Saturn to explain the fact that they go retrograde only when they are at opposition: that
is, when they are at a point in the sky opposite the sun.
Another interesting aspect of Ptolemy’s cosmology was that he considered the
heavenly spheres to be real physical things. They were made, he thought from a
substance called quintessence. The word literally means “fifth essence,” distinguishing it
from the four classical essences of earth, air, fire and water—the four fundamental
elements from which, Aristotle theorized, the cosmos is made
Ptolemy also took a stab at estimating the size of the universe. Hipparchus and
others before him had set the Earth-moon distance at about 50 Earth radii, and Ptolemy
worked from that figure. He assumed that the celestial spheres were all nested tightly
together, and made allowances for the room needed for the planets to move along their
various orbits. He ended up placing the sphere of the fixed stars about 20,000 Earth
radii distant— not much more than the distance we know today between the sun and
the planet Venus. Ptolemy’s universe was very small, indeed, but it was the universe
the Western world embraced as its own for more than 14 centuries.
Finally, it’s important for us to realize why the Ptolemaic model was accepted as
truth. It wasn’t because people in Ptolemy’s era were stupid or unobservant. They were
as intelligent and aware of the world they lived in as we are today—perhaps more so.
Rather, the Ptolemaic system was accepted because it worked, and it worked well. It
allowed astronomers to predict the positions of the planets with the accuracy they
demanded. It agreed with—and more or less supported—the Greek system of
philosophy and science. And, as Christianity rose from an obscure sect to the dominant
religious system of the West, its leaders and teachers were able to use the Ptolemaic
model to confirm and support a vision of the cosmos that placed Earth at the center of
all creation. The model would not be overthrown until the Reformation, when the
Christian church itself was irreparably torn apart.
Davis Joel. Journey to the Center of Our Galaxy. Chicago Il: Contemporary Books, 1991.
Copernicus and the Sun-Centered Cosmos
adapted from
Journey to the Center of Our Galaxy, by Joel Davis
It was the year 1543, and an obscure Polish cleric was about to gain a measure of
immortality by shaking the foundations of Western cosmology. His name was Nicholas
Copernicus.
Copernicus studied law and medicine in Italy where he came across the works of
Aristotle, Pythagoras, and Plato. He was particularly fascinated with the philosophical
model called Neoplatonism, which was based on some of Plato’s teachings. Among other
things, Neoplatonism stressed the uniqueness of the sun, which was considered to be
the source of all knowledge
But Copernicus began to consider a model of the cosmos different from the
geocentric model Ptolemy had proposed centuries earlier: a heliocentric model with the
sun at its center. Not that he was bothered by any obvious shortcomings of the
Ptolemaic model. Indeed, at that time there seemed to be very few; the Ptolemaic model
worked very well as a predictive tool, which is what we expect of a good scientific
theory. In fact, the cosmological model that Copernicus was to develop was, at first, no
better at predicting planetary positions than that proposed by Ptolemy. Rather, what
inspired Copernicus to think about the cosmos in a new way was the same kind of
thinking that inspired many other scientists. It was his natural sense of harmony, order
and beauty. It simply seemed to Copernicus that a sun-centered cosmos was more
elegant than one with Earth at the center.
Copernicus first presented his ideas in a short unpublished manuscript entitled
Commentariolis, written in 1512 when he was about 40 years old. Some 17 years later, he
wrote another summary of his thesis and circulated it among some friends. His
heliocentric theory was essentially the same as that presented by Aristarchus some 1800
years earlier. In it, Copernicus asserted that the sun was at the center of the solar system
and that the planets—including Earth—moved around it. The Earth took one year to
make its way around the sun, and it also rotated on its own axis once per day.
Copernicus believed that the orbits of the planets were perfect circles, and that
Ptolemy’s clever and inventive geometry was in violation of that cosmic ideal. The
problem with the Ptolemaic model, he wrote, was that it was not sufficiently pleasing to
the mind.
Copernicus continued working on his theory, slowly writing a more complete
version which he titled de Revolutionibus, or “On the Revolutions.” Sometime around
1542 or 1543, Copernicus’ privately distributed work came to the attention of others,
and he was urged by a friend to allow the manuscript to be printed. Not long before the
book was published in April, 1543, Copernicus suffered a serious stroke. A copy was
delivered to him, but it is likely he died without having read it.
Copernicus’ heliocentric model of the cosmos started with two assumptions:
1) Planets move in circular paths at constant speeds.
2) The closer a planet is to the sun the faster it moves: that is to say,
Mercury moves faster than Venus, which in turn moves faster than
Earth, and so on.
These assumptions are no different from those of Ptolemy, with the exception of
the sun replacing the Earth at the center. Copernicus then went on to present five other
ideas:
1) All the heavenly spheres revolve around the sun, not the Earth and the
sun is at the center of the universe
2) The distance from the Earth to the starry sphere is much greater than
the distance from the sun to the Earth, and is immensely greater than
anyone had previously assumed.
`
3) The heavens appear to move around the Earth in their daily pattern due
to the rotation of the spherical Earth upon its axis.
4) The apparent motion of the sun through the zodiac is caused by the
Earth’s yearly motion around the sun.
5) The retrograde motions of the planets in the sky are illusions, and are
actually caused by their own motions around the sun combined with
the motion of the Earth.
This last point could use a little elaboration. Suppose the Earth is a car driving
smoothly and steadily along a road. You are a passenger in the car. An “outer” planet,
say Jupiter, is another car traveling along a parallel road, beyond which is a range of
mountains silhouetted against the afternoon sky. Your car is somewhat behind the
Jupiter car, but you are moving much faster. You start catching up with the Jupiter car,
and eventually pass it. When you are behind it, the Jupiter car will appear to be moving
forward against the mountains beyond. Briefly, however, while you are passing it, the
Jupiter car will appear to be moving backward against the distant mountains. Once you
have passed it, the Jupiter car will again appear to be moving forward. This is the
illusion of retrograde motion. Copernicus didn’t know anything about automobiles, but
he most definitely knew about retrograde motion.
Figure 1: Retrograde motion, as seen from Earth.
Copernicus was able to calculate the orbital periods of the planets with respect to
the stars, as seen from the sun. From this he found that the order of the planets fit their
orbital periods: Mercury, with the shortest period, was closest to the sun, followed by
Venus, Earth, Mars, Jupiter ,and Saturn. He also calculated the relative distances from
the sun to each planet using the distance from the sun to the Earth as the unit of
distance (a measure that is now called an astronomical unit, or A.U.) Interestingly, his
calculated distances were quite close to the actual ones.
Copernicus’s model was far from perfect. Though he threw out Ptolemy’s
geometry, he still had to use his own geometric devices to explain the nagging
variations in the planets’ orbits. But the real impact of the Copernican heliocentric
model was not in its greater predictive power. Rather, it lay in its appeal to a harmony
of cosmic design and to a “symmetry of its parts,” as Copernicus wrote. And its appeal
lay in its firm grounding in observation. Some 1800 years earlier Aristarchus had come
up with much the same cosmological vision. But for Aristarchus a heliocentric cosmos
was an interesting idea. It was not meant to be taken as something real. By contrast,
Copernicus’ heliocentric model was always intended to be a model for the reality.
Davis, Joel. Journey to the Center of Our Galaxy. Chicago, Il: Contemporary Books, 1991.
Welcome to the Solar System
adapted from
A Short History of Nearly Everything, by Bill Bryson, 2003
Astronomers these days can do the most amazing things. If someone
struck a match on the Moon, they could spot the flare. From the tiniest throbs
and wobbles of distant stars they can find the size and character and even
potential habitability of planets much too remote to be seen—planets so distant
that it would take us half a million years in a spaceship to get there. With their
radio telescopes they can capture wisps of radiation so preposterously faint
that the total amount of energy collected from outside the solar system by all of
them together since collecting began (in 1951) is “less than the energy of a
single snowflake striking the ground,” in the words of Carl Sagan.
In short, there isn’t a great deal that goes on in the universe that
astronomers can’t find when they have a mind to. Which is why it is all the
more remarkable to reflect that until 1978 no one had ever noticed that Pluto
has a moon. In the summer of that year, a young astronomer named James
Christy at the U.S. Naval Observatory in Flagstaff, Arizona, was making a
routine examination of photographic images of Pluto when he saw that there
was something there—something blurry and uncertain, but definitely other
than Pluto. Consulting a colleague named Robert Harrington, he concluded
that what he was looking at was a moon. And it wasn’t just any moon. Relative
to the planet, it was the biggest moon in the solar system.
Now, a natural question is why it took so long for anyone to find a moon in
our solar system. The answer is that is partly a matter of where astronomers
point their instruments, and partly a matter of what their instruments are
designed to detect, and partly it’s just Pluto. Mostly it’s where they point their
instruments. In the words of the astronomer Clark Chapman: “Most people
think that astronomers get out at night in observatories and scan the skies.
That’s not true. Almost all the telescopes we have in the world are designed to
peer at very tiny pieces of the sky way off in the distance to see a quasar or
hunt for black holes or look at a distant galaxy. The only real network of
telescopes that scans the skies has been designed and built by the military.”
We have been spoiled by artists’ renderings into imagining a clarity of
resolution that doesn’t exist in actual astronomy. Pluto in Christy’s photograph
is faint and fuzzy—a piece of cosmic lint—and its moon is not the romantically
backlit, crisply defined companion orb you would get in a National Geographic
painting, but rather just a tiny and extremely indistinct hint of additional
fuzziness. Such was the fuzziness, in fact, that it took seven years for anyone
to spot the moon again and thus confirm its existence.
As for Pluto itself, nobody is quite sure how big it is, or what it is made of,
what kind of atmosphere it has, or even what it really is. A lot of astronomers
believe it isn’t a planet at all.
It is certainly true that Pluto doesn’t act much like the other planets. Not
only is it runty and obscure, but it is so variable in its motions that no one can
tell you exactly where Pluto will be a century from now. Whereas the other
planets orbit more or less on the same plane, Pluto’s orbital path is tipped (as it
were) at an angle of seventeen degrees, like the brim of a hat tilted rakishly on
someone’s head. Its orbit is so irregular that for substantial periods on each of
its lonely circuits around the Sun it is closer to us than Neptune is. For most of
the 1980s and 1990s, Neptune was in fact the solar system’s most far-flung
planet. Only on February 11, 1999, did Pluto return to the outside lane, there
to remain for the next 228 years.
So if Pluto is a planet, it certainly is an odd one. It is very tiny: just onequarter of one percent as massive as Earth. If you set it down on top of the
United States, it would cover not quite half the lower forty-eight states. This
alone makes it unique; it means that our planetary system consists of four
rocky inner planets, four gassy outer giants, and a tiny, solitary iceball.
Moreover, there is every reason to suppose that we may soon begin to find
other even larger icy spheres in the same portion of space. Then we will have
problems. After Christy spotted Pluto’s moon, astronomers began to regard that
same section of the cosmos more attentively and as of early December 2002
had found over six hundred additional Trans-Neptunian Objects, or Plutinos as
they are alternatively called. One, dubbed Varuna, is nearly as big as Pluto’s
moon. Astronomers now think there may be billions of these objects. The
difficulty is that many of them are awfully dark. Typically, they have an albido,
or reflectiveness, of just 4 percent, about the same as a lump of charcoal—and
of course these lumps of charcoal are about four billion miles away.
And how far is that exactly? It’s almost beyond imagining. Let’s imagine,
for purposes of entertainment, that we are about to go on a journey by rocket
ship. We won’t go terribly far—just to the edge of our own solar system—but we
need to get a fix on how big a place space is and what a small part of it we
occupy.
Now the first thing you are likely to realize is that space is extremely well
named and rather dismayingly uneventful.
Our solar system may be the
liveliest thing for trillions of miles, but all the visible stuff in it—the Sun, the
planets and their moons, the billion or so tumbling rocks of the asteroid belt,
comets, and other miscellaneous drifting detritus—fills less than a trillionth of
the available space. You also quickly realize that none of the maps you have
ever seen of the solar system were remotely drawn to scale. Most schoolroom
charts show the planets coming one after the other at neighborly intervals—the
outer giants actually cast shadows over each other in many illustrations—but
this is a necessary deceit to get them all on the same piece of paper. Neptune
in reality isn’t just a little bit beyond Jupiter, it’s way beyond Jupiter—five
times farther from Jupiter than Jupiter is from us, so far out that it receives
only 3 percent as much sunlight as Jupiter.
Such are the distances, in fact, that it isn’t possible, in any practical
terms, to draw the solar system to scale. Even if you added lots of fold-out
pages to your textbooks or used a really long sheet of poster paper, you
wouldn’t come close. On a diagram of the solar system to scale, with Earth
reduced to about the diameter of a pea, Jupiter would be over a thousand feet
away, and Pluto would be a mile and a half distant. On the same scale,
Proxima Centauri, our nearest star, would be almost ten thousand miles away.
Even if you shrank down everything so that Jupiter was as small as the period
at the end of this sentence, and Pluto was no bigger than a molecule, Pluto
would still be over thirty-five feet away.
So the solar system is really quite enormous. By the time we reach Pluto,
we have come so far that the Sun—our dear, warm, skin-tanning, life-giving
Sun—has shrunk to the size of a pinhead. It is little more than a bright star. In
such a lonely void you can begin to understand how even the most significant
objects—Pluto’s moon for example—have escaped attention. In this respect,
Pluto has hardly been alone. Until the Voyager expeditions, Neptune was
thought to have two moons; Voyager found six more. When I was a boy, the
solar system was thought to contain thirty moons. The total now is “at least
ninety,” about a third of which have been found in just the last ten years.
The point to remember, of course, is that when considering the universe
at large we don’t actually know what is in our own solar system.
Now the other thing you will notice as we speed past Pluto is that we are
speeding past Pluto. If you check your itinerary, you will see that this is a trip
to the edge of our solar system, and I’m afraid we’re not there yet. Pluto may be
the last object marked on schoolroom charts, but the system doesn’t end there.
In fact, it isn’t even close to ending there. We won’t get to the solar system’s
edge until we have passed throughout the Oort cloud, a vast celestial realm of
drifting comets, and we won’t reach the Oort cloud for another—I’m sorry to
say—ten thousand years. Far from marking the edge of the solar system, as
those schoolroom maps so boldly imply, Pluto is barely one-fifty-thousandth of
the way.
Of course we have no prospect of such a journey. A trip of 240,000 miles
to the moon still represents a very big undertaking for us. A manned mission to
Mars, called for by the first President Bush in a moment of passing giddiness,
was quietly dropped when someone worked out that it would cost $450 billion
dollars and probably result in the deaths of all the crew (their DNA torn to
tatters by high-energy solar particles from which they could not be shielded.)
Based on what we know now and can reasonably imagine, there is
absolutely no prospect that any human being will ever visit the edge of our own
solar system—ever. It is just too far.
But let’s pretend again that we have made it to the Oort cloud. The fist
thing you might notice is how very peaceful it is out here. We’re a long way
from anywhere now—so far from our own Sun that it’s not even the brightest
star in the sky. It is a remarkable thought that that distant tiny twinkle has
enough gravity to hold all these comets in orbit. It’s not a very strong bond, so
the comets drift in a stately manner, moving at only about 220 miles an hour.
From time to time some of these lonely comets are nudged out of their normal
orbits by some slight gravitational perturbation—a passing star perhaps.
Sometimes they are ejected into the emptiness of space, never to be seen again,
but sometimes they fall into a long orbit around the Sun. About three of four of
these a year, known as long-period comets, pass through the inner solar
system. Just occasionally these stray visitors smack into something solid, like
Earth. That’s why we’ve come out here now—because the comet we have come
to see has just begun a long fall toward the center of the solar system. It is
going to take a long time to get there—three or four million years at least—and
it is headed for, of all places, Manson, Iowa.
So that’s your solar system. And what else is out there, beyond the solar
system? Well, nothing and a great deal, depending on how you look at it.
In the short term, it’s nothing. The most perfect vacuum ever created by
humans is not as empty as the emptiness of interstellar space. And there is a
great deal of this nothingness until you get to the next bit of something. Our
nearest neighbor in the cosmos, Proxima Centauri, which is part of a three-star
cluster known as Alpha Centauri, is 4.3 light-years away, a sissy step in
galactic terms, but that is still a hundred million times farther than a trip to
the Moon. To reach it by spaceship would take at least twenty-five thousand
years, and even if you made the trip you still wouldn’t be anywhere except at a
lonely clutch of stars in the middle of a vast nowhere. To reach the next
landmark of consequence, Sirius, would involve another 4.6 light-years of
travel. And so it would go if you tried to star-hop your way across the cosmos.
Just reaching the center of our own galaxy would take far longer than we have
existed as beings.
Space, let me repeat, is enormous. The average distance between stars out
there is twenty million million miles. Even at speeds approaching those of light,
these are fantastically challenging distances for any traveling individual.
So even if we are not really alone, in all practical terms we are. Carl Sagan
calculated the number of probable planets in the universe at 10 billion
trillion—a number vastly beyond imagining. But what is equally beyond
imagining is the amount of space through which they are lightly scattered. “If
we were randomly inserted into the universe,” Sagan wrote, “ the chances that
you would be on or near a planet would be less than one in a billion trillion
trillion.” (That’s a one followed by thirty-three zeros.) “Worlds are precious.”
Which is why perhaps it is good news that in February 1999 the
International Astronomical Union ruled officially that Pluto is a planet. The
universe is a big and lonely pace. We can do with all the neighbors we can get.
Bryson, Bill. A Short History of Nearly Everything. New York, NY:
Broadway Books, 2003.