"the97fan" wrote in message
news:A29Bb.30610$Bk1.17269@fed1read05...
Alright, it was my fault. She said each planet's orbit has two foci. One
is
the sun. She wants to know any new theories on what the other focal point
is. My fault. I need some opinions from some astronomers/professors from
around the world. I'd figure this newsgroup could help me out. When
replying, please post exactly what you do in this field. Thanks alot. All
replies appreciated.

There's actually not a lot to say about the empty focus of an elliptic
orbit. I suspect your teacher may be having a bit of fun with the class.
However, the following may be of interest and may giver her a surprise (I
know it surprised me when I read it).

The result applies to objects that are in a 1:1 spin-orbit resonance around
their primary. This includes all (most???) of the major moons of the solar
system. For the purposes of this discussion, lets consider the Earth-Moon
system.

The Moon is locked into a 1:1 spin-orbit resonance around the Earth. This
means that its rotational period is equal to its orbital period. It is often
said that this causes the Moon to always present (sort-of/kind-of) the same
face to the Earth. Strictly speaking this is not entirely true. In the
course of one orbit the face of the Moon wobbles back and forth. We actually
see about 59% of the Moons surface during the course of one orbit. This
wobble is called "libration". You can get a good sense of it from this site
http://antwrp.gsfc.nasa.gov/apod/ap991108.html

The classic explanation of libration is that for short periods of time (on
the order of an orbital period) the rotation rate is constant but the
orbital speed is not (the orbital speed will be greatest at perigee and
least at apogee) and this leads to the wobble. Up until about a year ago
this is the only explanation of libration that I had ever heard but I found
a result in a book that I bought that leads to a very elegant explanation of
libration (the underlying reasons are of course the same, this is just a
different description of it).

(I'm getting to the interesting bit ... finally)

The book is ...

Solar System Dynamics, C. D. Murray, S. F. Dermott, Cambridge University
Press, 1999

In it, the authors derive a result that is summarized as follows ...

"... a synchronously rotating satellite rotates with one face pointing
toward the empty focus of its orbit."

This means that the Moon would *actually* keep the *exact* same face toward
the empty focus of its orbit. If we could image the Moon from the vantage
point of the empty focus during the course of an orbit, there would be *no*
libration (there would still be a slight bobbing up and down but that is due
to something else). Since it keeps the exact same face pointed toward its
empty focus during the course of an orbit it can't possibly keep the same
face pointed toward the full focus (the Earth) hence the wobble from our
perspective.

I honestly don't know if this is a "new" result (it was new to me) and I
would be interested to hear from anyone who had heard of this before from
another source.

(Note: I'm guessing you are in high school. The material in this book is
well beyond high school level but hopefully just a reference will do. If you
would like to try and contact one of the authors, you can find his e-mail
address here ...
http://ssdbook.maths.qmw.ac.uk/ )

Good Luck