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Common misconception



From "Voyages through the universe" by Fraknoi, Morrison & Wolff:

"As it happens, Pluto is presently closer to us than is Neptune,
and it will remain so until 1999. Even thgough the orbits of
these two planets cross, there is no danger of collision because
of the high inclination of Pluto's orbit."

This selection is quite representative of many in modern astronomy
texts. The authors are under no illusions, but as this is written
it contains two nascent misconceptions. The first is a lingering
geocentrism. What the authors *mean* is that Pluto is *nearer the
Sun* than is Neptune*.

The second misconception is worse. The implication that the orbits
of the two planets cross reinforces the erroneous idea, very common
among students, that the solar system is two dimensional. Of course
as the authors clearly state, the two orbits do not cross, but the
statement is not made that way. It certainly should be. It would be
better to state that "The orbits of Pluto and neptune do not cross.
The high inclination of Pluto's orbit make the relation between it
and the orbit of Neptune very like the relation between two links
of a chain." The authors neglect mentioning the important time
dimension as well. Neptune and Pluto don't get nearer one another
than about 19 AU because of an orbital resonance. The last time
they were that close was 1897, and they "approach" one another this
close every 494 years or so.

I think that in decreasing the dimensionality of the physical
system from four to two the authors have done their readers a great
disservice. As I said, this passage from a current textbook is
quite typical. Textbook authors tend to reproduce one another's
mistakes.

Leigh

* In fact at 3:32 AM on June 28 of this year (PST) we will be
about equidistant from the two planets, and from that time until
10:16 AM on Christmas eve we will be nearer Neptune. If you
think that was an easy mental calculation you're wrong!