Bad Astronomy: Misconceptions and Misuses Revealed, from ...

WHEN THE UNIVERSE THROWS YOU A CURVE 143
saying, “From my fixed location on the surface of the spherical
Earth, the horizon moved below the apparent position of the Sun
at 6:30 this morning.” But it is easier to say.
This Earth-centered model was fine-tuned by the Greek astronomer
Ptolemy around a.d. 150 or so. People used it to predict
planet positions, but the planets stubbornly refused to follow the
model. The model was “refined”—that is, made more complicated—but
it never quite made the grade.
Eventually, a series of discoveries over the centuries removed
the Earth from the center of the universe. First, Nicolaus Copernicus
presented a model of the solar system in which the Earth went
around the Sun, rather than vice-versa. His model wasn’t really all
that much better than Ptolemy’s model at figuring out where the
planets would be. But then Johannes Kepler came along a few centuries
later and tweaked the model, discovering that the planets
orbit in ellipses instead of circles, and things were a lot better.
So with Copernicus’s model it looked like the Sun was the center
of the universe. That’s not as good as having the Earth there,
but it’s not too bad.
Then around the turn of the twentieth century, Jacobus Kapteyn
tried to figure out how big the universe was. He did this in a
simple way: he counted stars. He assumed that the universe had
some sort of shape, and that it was evenly distributed with stars. If
you saw more stars in one direction, then the universe stretched
farther that way.
He found an amazing thing: the Sun really was the center of
the universe! When he mapped out the stars, the universe was
blobby, like an amoeba, but it seemed to be fairly well centered on
the Sun. Maybe the ancients were right after all.
Or not. What Kapteyn didn’t realize is that space is filled with
gas and dust, which obscures our view. Imagine standing in the
middle of a vast, smoke-filled room, like an airplane hangar. You
can only see, say, 20 meters in any direction because smoke blocks
your vision. You have no idea what shape the room is; it might be
a circle, or a square, or a pentagram. You don’t even know how
big it is! It could have walls just a meter beyond your vision, or it
could stretch halfway to the Moon. You can’t tell just by looking.