Reason for Procession of the Equinox?

I'm not a complete stranger to celestial mechanics, being familiar and
comfortable with such concepts as nutation and cyclical changes in the
obliquity of the Ecliptic. However, I've never really understood the
precise *cause* of the Earth's precession, and why its period is so
long. My Google searches have not been particularly productive, and
some explanations I've found seem rather farfetched, even bizarre.

I have other questions about some peculiar little-known effects of
precession, but they should wait until I have a better understanding of
precession itself. Any help is much appreciated.

Starry Skies,
Rich

PS:

I'm aware that sci.astro is the more appropriate usenet group for this
type of question. Unfortunately, like many of us, I don't spend much
time there.

Richard DeLuca wrote:
I'm not a complete stranger to celestial mechanics, being familiar and
comfortable with such concepts as nutation and cyclical changes in the
obliquity of the Ecliptic. However, I've never really understood the
precise *cause* of the Earth's precession, and why its period is so
long.


Obliquity-InducedPrecession
http://scienceworld.wolfram.com/phys...recession.html
Precession rate is given by equation (3)

Precession of the Equinoxes
http://scienceworld.wolfram.com/phys...Equinoxes.html

John Steinberg wrote in news:291120042329163870%
lid:

Richard DeLuca wrote:

I'm not a complete stranger to celestial mechanics, being familiar and
comfortable with such concepts as nutation and cyclical changes in the
obliquity of the Ecliptic. However, I've never really understood the
precise *cause* of the Earth's precession...

*The way I understand it, it's due to the equatorial bulge of the earth
and centrifugal force of the earth's rotation. The rotation creates an
oblate earth which coupled with the moon and Sun's gravitational pull,
makes the Earth precess.

Or to put it more simply... Weebles wobble but they don't fall down. g

"Richard DeLuca" wrote:
I'm not a complete stranger to celestial mechanics, being familiar and
comfortable with such concepts as nutation and cyclical changes in the
obliquity of the Ecliptic. However, I've never really understood the
precise *cause* of the Earth's precession, and why its period is so
long. My Google searches have not been particularly productive, and
some explanations I've found seem rather farfetched, even bizarre.
I have other questions about some peculiar little-known effects of
precession, but they should wait until I have a better understanding of
precession itself. Any help is much appreciated.

-----BEGIN PGP SIGNED MESSAGE-----

Whether or not you consider the fall of Autumn & Evening from the
Garden of Eden (circa 200,000 BC) "farfetched" and "bizarre" etc.,
it is nonetheless the actual *cause* of the significant obliquity
of the Earth's rotational axis to the Sun and resulting precession.

Now, I don't expect the predominantly-atheistic orthodox academia
to acknowledge this, much less to comprehend the truth of it. But
that doesn't change the fact that precession was and is caused by
the fallen man. When man fell, the whole Earth fell with him. See?
But of course you don't. Some decades from now, your descendants
will be singing an entirely different tune. Meanwhile, you'll just
write Judeo-Xians off ad hoc as "kooks" for want of comprehension.
Nothing personal, it's just the way it is, for now...

In Vigilance,
Daniel Joseph Min

*Min's Google-Archived Home Page On The WWW:
http://groups.google.com/groups?selm...ymous.pos ter

-----BEGIN PGP SIGNATURE-----
iQA/AwUBQav8YJljD7YrHM/nEQKigQCgu82uNIPabD8zdekC1CQUquahA+QAoM9m
y+RgdrgsClVfWVkSeor01JDw
=A3DU
-----END PGP SIGNATURE-----

In article ,
John Steinberg wrote:



*The way I understand it, it's due to the equatorial bulge of the earth
and centrifugal force of the earth's rotation. The rotation creates an
oblate earth which coupled with the moon and Sun's gravitational pull,
makes the Earth precess.


and why its period is so long.

~26,000 years? Isn't that the wait for a new A-P?


Thanks John,

I had forgotten that the earth is an oblate spheroid. BTW, happy
belated birthday!

Rich

In article I2Tqd.111678$V41.24948@attbi_s52,
Sam Wormley wrote:


Obliquity-InducedPrecession
http://scienceworld.wolfram.com/phys...recession.html
Precession rate is given by equation (3)

Precession of the Equinoxes
http://scienceworld.wolfram.com/phys...Equinoxes.html


Thank you, Sam. That's exactly what I wanted.

Another question:

The obliquity of the ecliptic changes over long periods of time (from
about 24.5? down to about 22?) and results in small changes in the
extreme positions of sunrise and sunset from a given point on the earth.
Does the cycle of precession do the same? Any formulae relating to this?

Starry Skies,
Rich

In article ,
John Steinberg wrote:

Daniel Joseph Min wrote:

Whether or not you consider the fall of Autumn & Evening from the
Garden of Eden (circa 200,000 BC) "farfetched" and "bizarre" etc.,
it is nonetheless the actual *cause* of the significant obliquity
of the Earth's rotational axis to the Sun and resulting precession.

Th comedy stylings of DJ Min! Show him some love, people!

He'll be here all week, folks!

Try the baby back ribs, they're marvelous.


Hey,

Min is in my killfile, so if it weren't for you I wouldn't have seen
this. Believe it or not, the internet is chock FULL of explanations
like the above..............

Rich

Richard DeLuca wrote:

I'm not a complete stranger to celestial mechanics, being familiar and
comfortable with such concepts as nutation and cyclical changes in the
obliquity of the Ecliptic.

How can you possibly be "familiar" with nutation but _not_ precession?
Nutation is short-period, low-amplitude, precessional motion. The
driving forces are the same. To be "comfortable" with second-order
effects but ignorant of the first-order causes ... well, it's like
knowing all about Zeeman Effect, but ignorant of orbital's, quantum
mechanics, etc. You make no sense.

However, I've never really understood the
precise *cause* of the Earth's precession, and why its period is so
long. My Google searches have not been particularly productive, and
some explanations I've found seem rather farfetched, even bizarre.

All the explanations posted so far are exactly what you found during
your claimed googling. One example:

www.google.com: cause of precession of equinoxes

http://en.wikipedia.org/wiki/Precession

You claim to have forgotten that the Earth is oblate, yet the above
link is right up there at the top of the google search and contains
the "forgotten" information. How could you have missed this,
especially since Wikipedia is the next place you go if google is less
than enlightening? (Sometimes it's the first place to go.)

Richard DeLuca wrote in message ...
In article ,
John Steinberg wrote:
snip

Richard asked "What causes precession?"

The classical explanation given in Robin M. Green's _Spherical
Astronomy_ (Cambridge Press 1985), is to first visualize the motion of
a an off-kilter, spinning top, such as one you played with as a small
child:

http://www-istp.gsfc.nasa.gov/starga...s/Sprecess.gif
from - http://www-istp.gsfc.nasa.gov/stargaze/Sprecess.htm

When the spinning top is not vertical, it wobbles around a central
axis. The angle of tilt is the result of the vector between
centrifugal force of the top tending to push the spinning top upright,
counteracting the downward pull of gravity caused by the Earth.

Translate this mental picture to Earth-Sun system. The Earth is
spinning top. The Sun takes the place of the Earth in our childhood
spinning top model -

http://csep10.phys.utk.edu/astr161/l...precession.gif
from - http://csep10.phys.utk.edu/astr161/l...recession.html

The end result is the "wobbling" of the Earth around a central axis
with approximately at 26,000 year cycle:

http://astro.wsu.edu/worthey/astro/h...precession.gif
from - http://astro.wsu.edu/worthey/astro/h...recession.html

Unlike our spinning top model that contained only one large
gravitational attractor, the Earth has two large nearby gravitational
sources trying it pull it "downwards" at the same time. The first is
the Sun; its gravitational contribution to the Earth's wobble is
called precession. The second is the Moon; its gravitational
contribution to Earth's wobble is called nutation. Nutation follows
approximately a nineteen year cycle.

Nutation alone looks like this:

http://www.pietro.org/Astro_Util_Sta...n/Image283.gif
from - http://www.pietro.org/Astro_Util_Sta...Visualized.htm

- Canopus

Richard DeLuca wrote:
I'm not a complete stranger to celestial mechanics, being familiar and
comfortable with such concepts as nutation and cyclical changes in the
obliquity of the Ecliptic. However, I've never really understood the
precise *cause* of the Earth's precession, and why its period is so
long. My Google searches have not been particularly productive, and
some explanations I've found seem rather farfetched, even bizarre.

I have other questions about some peculiar little-known effects of
precession, but they should wait until I have a better understanding of
precession itself. Any help is much appreciated.

Hey, this is one of my upcoming Astronomical Games essays. Here's a
summary.

As others have pointed out, precession occurs because of the gravitational
pull of the Moon on the Earth's equatorial bulge. I've never been
satisfied with that explanation myself, because it's just too brief. For
instance, let me open with a couple of questions.

1. Precession is an example of symmetry breaking. Why does the
Earth's axis precess in the direction that it does, and not the
other?

2. The precession of the Earth's axis has been likened to that of
a spinning top. Anyone who's played with a top knows that it
precesses (although they usually don't know the name for it).
But observe: If you spin a top clockwise, as seen from above,
the top's axis also precesses clockwise.

Now consider the Earth's axis. The Earth rotates on that axis
from west to east--that is, counter-clockwise as seen from above
the north pole. But the path of the celestial pole in the skies
as seen from the Earth is counter-clockwise, meaning that as
seen from above the north pole, the axis precesses *clockwise*,
or the direction opposite from the spin. Why should they be
different, if the precession is like that of a top?

To answer these questions, consider a simple top, which consists of a
flat disc impaled by the spin axis. If you lean the top over, so that
it's not standing straight up, and then let go, it falls over in the
direction that it was leaning; that's simple gravity. But if you spin
it first, it precesses. Why?

The common answer is that it now has angular momentum. You'll excuse
me if I find that a non-answer. Let's suppose the top is rotating
clockwise as seen from above, and that we have it leaning to the left.
If it weren't for gravity, then the lowest point of the top, at the
left, would be moving away from you, and the highest point of the top,
at the right, would be moving toward you.

But gravity adds a tipping force to the top. The leftmost point of the
top therefore moves away from you *and a little downward*, and the
rightmost point moves toward you *and a little upward*. But that's just
the same as a top that is tilted to the left *and a little away from
you*. That is a slight clockwise motion of the top's axis.

What's more, if you now shift your way around the top so that the new
axis is pointing directly to your left, gravity will now act in the same
way to tip the axis again to the left *and a little bit away from you*.
This continues to happen no matter how far around the top you revolve
to maintain the same perspective on the top. In other words, the axis
of the top precesses.

Note that the top precesses clockwise, as seen from the top. This is
now seen not simply as a consequence of the spin of the top, but the way
that spin interacts with the direction of gravity's pull.

Now, over to the Earth and the Moon. The Moon revolves around the Earth
in more or less the ecliptic plane. (We'll ignore the inclination of
the orbit for the time being.) If the Earth were perfectly spherical,
there would be no "handle" for the Moon to pull on, and no precession.

But the Earth does bulge, as a result of its relatively fast rotation.
The Moon pulls differently on different parts of that bulge. That part
of the bulge which lies above the ecliptic plane, the Moon pulls down,
toward the plane. That part of it which lies below the plane, the Moon
pulls up, again toward the plane. The net force is to tip the Earth's
"upward," so that it is perpendicular to the ecliptic. Observe that
this tipping is opposite to the tip that gravity exerts on the top; there
gravity tended to pull the axis down so the top falls. And this makes
all the difference in the world! (Ahem.)

Now, when it's northern summer, say, the northern axis is tilted toward
the Sun. If you view the Sun and Earth so that the Sun is on the left,
the axis is also tilted leftward, and the leftmost portion of the bulge
is rotating toward you. The rightmost portion of the bulge is rotating
away from you. But the tipping force exerted by the Moon means that
that leftmost part (which is below the ecliptic) is also moving just a
tiny bit upward, and the rightmost part (which is above the ecliptic)
just a tiny bit downward. That's the same as saying the axis is now
tilted to the left *plus a little bit away from you*. And that's the
clockwise precession of the axis. Note that it goes opposite from the
Earth's spin, because the Moon's pull does not tend to tip the axis over;
it tends to tip it back up, so to speak.

The Moon's pull is so weak, and the Earth's bulge so small (it amounts to
only about a third of a percent of the Earth's average diameter) that the
precession is very slow indeed--about 26,000 years, as measured directly.
Amazingly, this precession was first measured, by comparing star maps of
different ages, by Hipparchus around the second century B.C.!

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt