Magellan's demise premature?

Ted Davis (
Tue, 16 Dec 1997 10:01:22 -0500

Alas, I hear Magelllan has died. Please forgive the following (final?)

Owen Gingerich can be reached at I suggest a
preliminary inquiry before sending him anything substantive. Another
possible contact on this issue is Bruce Stephenson, newly appointed as
historian at the Adler Planetarium in Chicago: 1300 S. Lake Shore Drive,
Chicago, 60605. I regret I have no e-mail for him. He's a very good Kepler
scholar who probably knows this stuff as well as anyone.

Glenn wrote: "One of the sources I consulted stated that the moon's
actual diameter had to increase and decrease every month by about 100 per
cent so that the apparent size could remain the same. The ptolemaic distance

varied that much. This apparently was known for a long time prior to
Magellan and didn't cause a crisis. Not all counter evidence causes crises.

Why doesn't the counter evidence for the young earth cause a crisis among
the young-earth creationists?"

This is about right: Ptolemy had quite an apparatus to save the moon's
motion; for a short, accurate description, I recommend A. Pannekoek, A
History of Astronomy (London, 1961), pp. 153-6. On his hypothesis, the
moon's distance from the earth would change by a factor of 33/17 during a
month. There are two ways to respond to this conclusion, which was of
course known not to be true even in Ptolemy's day. One is to point out, as
I would, that this counts very strongly for the usual interpretation of
ancient/medieval mathematical astronomy: namely, that the epicycles and
deferents of astronomers were almost always assumed NOT to be true. An
instrumentalist interpretation of nature was the norm. So one should not be
bothered; the whole point is to "save" the phenomena (planetary
observations) by circles in uniform motion -- an elegant research program,
in my opinion, not unlike modern quantum mechanics -- but not truly to
describe the heavens. After all, no one has ever traversed the heavens
actually to know, first hand, how they are composed. Certainty -- "science"
in the Platonic sense, as opposed to mere "opinion" -- lies elsewhere,
perhaps in natural philosophy and its doctrine of celestial perfection, but
not in mathematical astronomy.

As I stated, that is my explanation. But Pannekoek, following Ptolemy's
account in Book V of the Planetary Syntaxis (Almagest), paints a less clear
picture. I haven't time to study this carefully enough to get it exactly
right. Suffice it to say that I still think the former paragraph would be
the view of most astronomers down to Copernicus (who was almost certainly a
realist, not an instrumentalist), whether or not Ptolemy himself held it.
If he did not hold it, then he certainly had an obvious anomaly on his

Why doesn't "clear" evidence produce abandonment of the YEC position? For
the same reasons that it almost never produces abandonment of any paradigm:
scientists, like other people, are conservative creatures who don't like to
change theories until they are convinced the one their riding on is dead in
the water. This is hardly surprising. I'm not talking here about little
theories, like how the moon moves or how to account for starlight that seems
to have originated before the earth could have been here (on YEC theories);
I'm talking about the whole shebang, the entire theory of YEC or geocentric
astronomy. For a Ptolemaic astronomer to admit the earth's motion was
literally to admit the incredible: how possibly can we be spinning at
hundreds of miles per hour around the earth's axis, and at tens of thousands
of miles per hour through space, and we not be able to tell from ordinary
physical and astronomical observations? As Galileo said, he admired those
[Copernicans] who, through the force of reason, were able to fly in the face
of evidence. [One of many reasons why Galileo cannot accurately be
described as an "empiricist," in a simple sense.] For a YEC to admit that
the earth is "old" is to admit that God doesn't love us -- since this
implies (at least) animal death before the fall, and (surely) God wouldn't
have made a world like that if he really is a loving God. Don't confuse us
with fancy theories, when the evidence from our belief system is

Ted Davis