Re: Retrograde Motion - Virtual Particles

[Jerry Lisantti <lisantti@beta.centenary.edu>]

One of the exercises we have our students in the intro astronomy lab do 
is to make two plots which help them to understand retrograde motion.

The first is to give them a star chart of a particular region in the sky
and then have them plot the position of Mars for particular dates using 
right ascension and declination.  We have picked a period of one earth year
in which Mars exhibits retrograde motion.  This gives them a general idea
of retrograde motion.

The next exercise is a little more complicated and some students who have
problems graphing somtimes struggle a little but overall it works.  Here
we have them tape together two large pieces of computer paper.  The sun 
is represented near the center of the paper.  Then using a compass students 
make a circle
of radius 14.1 cm (141 million miles Sun-Mars distance).  They do the 
same for the earth's orbit 9.3 cm.  With a horizontal baseline representing
0 degrees they then plot the position of the earth in its orbit over the
period of one year using about 16 data points.  The same is done for Mars 
and lines are drawn from the
same date on earth through the same date on Mars out to the edge of the 
paper.  These lines will overlap during retrograde motion and you can see
against a backdrop of distant stars the retrograde motion.  One can even
measure some angles with this diagram and compare it to the starchart.

These exercises help to give the students some understanding of retrograde
motion plus it is a good lab for getting them to be careful in their 
graphing.

On Tue, 25 Jun 1996, Jim Green wrote:

> David, et al., thank you for your review of planetary retrograde motion.
> (and for the Gauge particle stuff which I am still mulling.)
>
> I am in intellectual agreement with what you say about the planets, but
> somehow I have a mental block visualizing the action.  Maybe I will unblock
> tonight at something like 3am -- sitting bolt upright with some sort of
> epiphany:
>
> Here is the problem:  I see that as my fast car passes a slower car on the
> interstate, the slower car seems to reverse direction just as I pass.  BUT
> that car never seems to reverse again to go forward.
>
> But Mars, etc *do*  -- somebody help me out here!!!!   Please!
>
> Maybe Tycho was right.
>
> Jim Green
>
>
>
> At 11:11 PM 6/25/96 -0400, you wrote:
> > Retrograde Motion
> >
> > All the planets (other than Earth) exhibit retrograde motion during a portion
> > of their synodic period.  The superior planets (those orbiting the Sun outside
> > the Earth's orbit) display retrograde motion near opposition when the Earth
> > passes between them and the Sun.  The inferior planets (those orbiting the
> > Sun inside the Earth's orbit) display retrograde motion near inferior
> > conjunction when they pass between the Sun and the Earth.  (This motion is
> > difficult to see as one has to look into the glare of the Sun when looking at
> > an inferior planet whose phase goes from waning crescent to new to waxing
> > crescent as the planet passes from being an evening star to being a morning
> > star.)
> >
> > th a
> > group velocity slower than c for the massive gauge boson case.  It is the
> > nonzero mass of the Weak gauge bosons which gives the Weak interaction its
> > very short range of operation.  The other interactions have an "infinite"
> > range of operation (1/r^2 force counts as infinite here).  The Strong force
> > is only noticed strongly inside hadrons and slightly in nuclei (but strong
> > enough to bind the nucleons into the nucleus) not because it is a short ranged
> > force, but because its potential energy *grows* with distance and this causes
> > confinement of its sources (colored quarks) into color-neutral particles (such
> > as the mesons and the baryons) and the strong interaction between these color-
> > neutral composite particles does fall off rapidly with distance.
> >
> > David Bowman
> > Georgetown College
> > dbowman@gtc.georgetown.ky.us