I’ve been asked and given permission to publish this week a series of chapters from the new A Faith and Culture Devotional: Daily Readings on Art, Science, and Life
The Copernican Principle
By Guillermo Gonzalez, PhD, associate professor of physics at Grove City College
and a recognized expert on the astrophysical requirements for habitability. Gonzalez
is cofounder of the Galactic Habitable Zone and coauthor of The Privileged Planet:
How Our Place in the Cosmos Is Designed for Discovery; www.privilegedplanet.com.
In a memorable passage from the 1994 book Pale Blue Dot, the late astronomer
Carl Sagan reflects on an image of Earth taken by Voyager 1 from four
billion miles away:
Earth seems to be sitting in a beam of light, as if there were some special
significance to this small world. But it’s just an accident of geometry and optics.
. . . Our posturings, our imagined self-importance, the delusion that we
have some privileged position in the universe, are challenged by this point of
pale light. Our planet is a lonely speck in the great enveloping cosmic dark.1
The idea that we are insignificant in the cosmic scheme, fashionable among
modern scientists, is known as the Copernican Principle, named after astronomer
Nicolaus Copernicus (1473 – 1543). According to the popular story,
Copernicus demoted us from our place of importance by showing that our
universe was not Earth-centered, but rather sun-centered, Earth revolving
around the sun like all the other planets. Or so the story goes. That story has
a single, decisive problem: it’s false.
The real story is more subtle. The pre-Copernican cosmology envisioned
by Aristotle, Ptolemy, and other ancients was a set of nested, concentric
spheres that encircled our Earth. The “center” of the universe was considered
no place of honor any more than we think of the center of the Earth as being
somehow exalted. It was seen as the corruptible, base, and heavy portion of
the cosmos. Things were thought to fall to Earth because of their heaviness.
The Earth in pre-Copernican cosmology was the “bottom” of the universe
rather than its “center.”
When Christian theology was added to the mix in the Middle Ages, the
bottom of the universe became, quite literally, hell. Dante’s Divine Comedy
immortalized this vision, taking the reader from the Earth’s surface through
the nine circles of hell, which mirror the nine celestial spheres above.
The “official story” gives the false impression that Copernicus relegated us
to an insignificant backwater and scientifically established the unimportance
of our “pale blue dot.” But far from demoting the status of Earth, Copernicus,
Galileo, and Kepler saw the new scheme as exalting it. They thought the
Earth’s new position removed it from its place of dishonor.
Let’s fast-forward four centuries and see whether recent astronomical
discoveries confirm these early astronomers’ convictions about Earth’s
If you were a cosmic chef, your recipe for “cooking” up a habitable planet
would need many ingredients. In order to maintain a stable, moderate climate
and produce an atmosphere that would sustain sentient, intelligent life, you
would need a rocky planet large enough to hold on to a substantial atmosphere
and oceans of water. You would need a large moon to stabilize the tilt
of the planet’s rotation axis. You would need the planet to have a nearly circular
orbit around a main sequence star similar to our sun. In order to avoid
excessive asteroid and comet impacts, you would need to give that planet the
right kind of planetary neighbors within its star system and put that system
far from the center, edges, and spiral arms of a galaxy like the Milky Way.
And, you would need to “cook” it during a narrow window of time in the history
of the universe.
The probability of having all these ingredients come together is small.
Earth-like planets are rare. While this fact contradicts the Copernican
Principle, rarity alone does not make the Earth and its inhabitants truly
Consider what it takes for scientific discovery. Read any book on the history
of scientific discovery, and you’ll find magnificent tales of human ingenuity,
persistence, and dumb luck. What you probably won’t see is any discussion
of the conditions necessary for such feats. A discovery requires a person to
do the discovering and a set of circumstances that makes it possible. Without
both, nothing gets discovered.
Although scientists don’t often discuss it, the degree to which we can
“measure” the wider universe from our Earthly home is surprising. Few have
considered what science would have been like in, say, a different planetary
Think of the following features of our Earthly home: the transparency of
its atmosphere in the visual region of the spectrum, a large moon (just large
enough to perfectly cover the sun during a total solar eclipse), and its particular
location in the Milky Way Galaxy. Without each of these assets, we would
have a very hard time learning about the universe. For example, scientists
were able to test Einstein’s General Theory of Relativity by observing light
“bending” around the sun during a total eclipse.
It is not idle speculation to ask how our view of the universe would be
impaired if, for example, our home world were perpetually covered by thick
clouds. After all, our Solar System contains several examples of such worlds:
Venus, Jupiter, Saturn, and Saturn’s moon, Titan. These would be crummy
places to do astronomy.
The central argument, the central wonder of our research is this: If a planet
satisfies all the requirements for habitability, it also satisfies the requirements
for making a wide range of important scientific discoveries. In other words,
the best places for observers are also the best places for observing. It is the
connection between life and discovery that makes our home a truly “Privileged
For reflection and discussion
? What new scientific discoveries have seemed to challenge your faith?
? How have you responded? How might you use the Internet or resources
in this book to answer any questions?
? If we are not at the center of the Universe, but rather on a terrifically
positioned observation platform, what does that suggest about God? And