Re: astronomy

[Leigh Palmer <palmer@sfu.ca>]

> Like many of you I have been pressed into teaching astronomy without having
> been formally taught in this area.

I didn't have to be pressed; I accepted eagerly. The course is so
well supported by competitive textbooks and resource packages that
one is never at a loss for choice*. The books are a bit cheaper than
comparable physics texts, and they do stray a bit more adventurously
away from a single strict stereotype than the physics texts do.

> However, I do have a Ph.D. in geology
> so I think I know a little bit about at least one planet.

Mine is in experimental low temperature physics, and I don't treat
the chapter on Earth in complete detail. Too many of my students
will know more than I do; they won't be getting value for money, and
I'd hate to have them learn of the many lacunae in my knowledge.

> Two questions at
> the end of the Chaisson/McMillan chapter on the earth suggest possible
> misconceptions and since this list seems to thrive on such issues I though
> I would mention them.  I don't have an answer key (is there one?) so I'm not
> sure what the authors have in mind.  I'm only guessing.  Here goes.

Chaisson and McMillan is one of my current choices (the other being
Kaufmann and Comins) for the course I teach. The authors have been
very helpful, answering questions put to them directly by email. I
have on rare occasions been able to correct their errors, too. When
he was alive Kaufmann also was very helpful. I don't know GR, and he
gave me brilliantly clear answers to my emailed questions, even
sending me on one occasion just the right printed matter to help me
over a conceptual block.

> P 169 Review & Discussion

I take this heading to mean that there are no "correct" answers
expected.

> #5. How would our knowledge of Earth's interior change if our planet were
> geologically dead, like the moon?  I imagine that the idea is that we would
> not know as much because without earthquakes we would be short on seismic
> sources.  However, man made sources are generally preferred to earth quakes.
> One never knows exactly when or where an earthquake took place.  For info
> on the deep interior we clearly need a big source, but nuclear weapons tests
> have proved to be ideal.

I would think that to be a good response. What is your question?

> #10. How did radioactive decay heat the Earth early in its history?  When did
> this heating end?  Perhaps this is supposed to be a trick question?
> Considering the fact that the junk that floated around in space for an unknown
> length of time before we claimed it and that there are many isotopes with
> half lives on the order of billions of years, this question seems pecular.
> There have been many competing models proposed for the thermal history of the
> earth and as far as I know the jury is still out on most of them.

If you think that the degree of uncertainty about the mechanisms
operating in the formation of the Earth and Moon are too great to
permit their discussion in a course for nonscience students then I
think you should raise objections to almost every other chapter in
the introductory astronomy course. I try to keep my students aware
of the nature of science itself, especially its capability to adapt
to the acquisition of new information. Last week's flap over
1997 XF11 was an excellent opportunity to illustrate the process.
If any of you out there have lost interest in this affair just
because you now know the asteroid won't hit any time soon I urge
you to look at the excellent science that led to this conclusion
presented as it evolved over a couple of days' time. See
http://ssd.jpl.nasa.gov/ca_97xf11.html for the details.

Surely if the "jury is still out" on the thermal history of the
Earth, then one must characterize the current state of cosmology as
being one in which it is still uncertain that a crime has occurred!
Would you leave cosmology out of the course in recognition of that
fact?

You know a lot about the Earth; I envy you your knowledge and your
ability to answer your students' questions authoritatively in an
area where I fear my own knowledge is too weak to expose to such
scrutiny. On the other hand, I don't touch the last chapter (Life
in the Universe - Are We Alone?) at all, and I know a lot more
relevant biology than my colleagues here. There are so many more
interesting things I can and do talk about, including cosmology,
where I can help my students. One of the best things about teaching
this course is that it is prerequisite to nothing, not even to my
third year astrophysics course (for physics students). I can choose
my topics pretty much as I please. Who could ask for a better
teaching assignment?

> #14. If the Moon had oceans like the Earth's, what would the tidal effect be
> like there?  How many high and low tides are there during a "day?"  How would
> the variations in height compare to those on the Earth?  I think Jim Green
> wants to answer this one.

I'm quite sure that C & M knew the answers to expect to this one.
What is the misconception you feel underlies the question? Among
the misconceptions that abound among the Great Unwashed is the idea
that one will see "Earthrise" from a vantage point on the Moon. It
is possible, of course, with incredibly slow motion and small
amplitude due to libration in longitude, but the movie made from an
Apollo orbiter which shows Earth "rising" over the lunar horizon
has received such wide circulation that I think this question is,
perhaps, very necessary to correct misapprehensions.

> I do think these guys do a nice job on Mercury's 3:2 spin-orbit resonance
> or "tidal lock" to the sun. (three rotations for 2 revolutions)

Needless to say, I spend a lot more time on tidal effects than the
book does.

Leigh

* I just counted up 14 elementary astronomy textbooks I've been
sent in about the last year, none of them requested, and not
counting duplicates (of which there are three, I think).