[Music]
Procession is the movement of the
rotational axis of an object. It is
commonly seen in toy tops, but all
spinning objects can demonstrate
precession. In astronomy, we are most
interested in the precession of Earth.
The orbital tilt -- obliquity -- of Earth
is 23.5 degrees. Thus the axis of Earth's
rotation traces out a cone of half angle
23.5 degrees in a
period of roughly 26,000 years. This
effect is often referred to as
'precession of the equinoxes'. The
technical details behind precession are
beyond the physics covered in
introductory astronomy, but they focus on
the fact that Earth is not spherically
symmetric -- but oblate -- fatter around the
middle, and the gravitational pulls of
the Sun and Moon on this equatorial
bulge cause precession. I will
demonstrate precession with what is
known as an air gyroscope. This
particular model was manufactured by
Ealing Corporation in 1956. we will
observe precession on this four inch
steel ball and a pump will force air out
a hole beneath the ball supporting its
weight and minimizing friction.
Note that the ball has a rod attached, so
one can view the ball spinning. Part of
the interior of the ball has been
hollowed out to counterbalance the
weight of the rod.
I will turn on the air pressure and
position a weight at a predetermined
balanced position. Thus, the center of
mass of the ball rod weight is over the
center of the ball. When we spin up the
ball we see that it will spend stably
for a long while in this balanced
position.
I now move the weight outward. The ball
is no longer balanced and it is more
analogous to the oblate Earth. When I
spin up the ball we can now readily
observe the slow change of the axis of
rotation due to precession.
Now remember the Earth's axis
forms the basis for the celestial
equatorial coordinate system, thus as the
location of the vernal equinox and
celestial poles change due to precession,
the positions of stars in Right
Ascension and Declination also change. So
although right now we have a fairly
bright star in Polaris near the North
celestial pole, that will no longer be
true in about a thousand years.
Astronomical catalogues giving the
positions of objects are typically
updated every fifty or a hundred years
to the definition of the celestial
equatorial system at that time (such as
1950 or 2000) and computer programs to
process object coordinates to that
system at the current time are
ubiquitous. A telescope pointing system
that doesn't take precession into
account wouldn't be sufficiently
accurate to be very useful.
More teaching materials can be found on
the web at astro.unl.edu.
