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toothdust
2008-Sep-17, 04:26 AM
Reason I ask this is that I had a geology professor (who was interested in astronomy on the side) teaching us that Earth's precession operated in two cycles; one 19,000 years and the other 23,000 years. This came up one day when we were talking about Mayan astronomy and how accurate they were. Every "authority" website I visited (NASA, JPL, etc.) was saying it was right around 26,000 years. She emailed me some obscure site that detailed this precession of 19k and 23k years.

What gives?

Marty1468
2008-Sep-17, 04:34 AM
Can you explain further about the Earths precession rate for those of us who would like to understand? :-)

01101001
2008-Sep-17, 04:38 AM
Like NASA Earth Observatory feature: Paleoclimatology: Explaining the Evidence (http://earthobservatory.nasa.gov/Study/Paleoclimatology_Evidence/)?


Finally, the Earth wobbles on its axis as it spins. Like the handle of a toy top that wobbles toward you and away from you as the toy winds down, the “handle” of the Earth, the axis, wobbles toward and away from the Sun over the span of 19,000 to 23,000 years (precession).

Marty1468
2008-Sep-17, 05:12 AM
Thanks 01101001 (should i call you digital for short lol). I remember now as i have heard of it before.

toothdust
2008-Sep-17, 06:19 AM
"Like a wobbling top, the orientation of the Earth's axis is slowly but continuously changing, tracing out a conical shape in a cycle of approximately 25,765 years" (Wikipedia)

http://www-spof.gsfc.nasa.gov/stargaze/Sprecess.htm

"The spinning Earth moves like that, too, though the time scale is much slower--each spin lasts a day, and each gyration around the cone takes 26 000 years."

"Through each 26 000-year cycle, the direction in the sky to which the axis points goes around a big circle..."

"Because of the precession of the equinoxes, the position among the stars of the celestial pole--the pivot around which the celestial sphere seems to rotate--traces a circle every 26,000 years of so."

Can you see the contradiction? I didn't see dates on any of this stuff. Is the 19K/23K precession rates relatively new findings?

frankuitaalst
2008-Sep-17, 09:37 AM
Indeed , not very clear ...
Again the good (old) Wikipedia may give some clarification .
http://en.wikipedia.org/wiki/Precession_(astronomy)

01101001
2008-Sep-17, 12:41 PM
http://en.wikipedia.org/wiki/Precession_(astronomy)

Link slightly damaged (by automatic URL-tagging software):

http://en.wikipedia.org/wiki/Precession_(astronomy)


Can you see the contradiction?

Not really. I see small slips, in some of what you cite, of failure to note that the values are estimates, and estimates of a dynamic statistic.

Are you after the exact current value? To what precision do you need it? What if it has never been measured that accurately -- and can't be?

StupendousMan
2008-Sep-17, 01:19 PM
The value of the luni-solar precession rate has been measured by astronomers for many centuries. The period of this precession has been determined to lie between 25,000 and 26,000 years for at least the last 70 years (in a quick check of the literature), with a representative recent value being 25,720 years -- see

http://cdsads.u-strasbg.fr/full/1996A%26A...308.1001W

The uncertainty in recent values appears to be around 0.006 percent, given the size of revisions made in recent years.

If geologists are quoting periods of 19,000 and 23,000 years, then they are probably talking about some different phenomenon. There are complications to the description of precession -- one may describe only the precession due to the Sun, or due to the Moon, or due to both combined, and there are small effects due to redistribution of mass within the Earth and due to other planets. Perhaps these different figures merely derive from different definitions of the "precession of the equinoxes."

a1call
2008-Sep-17, 01:39 PM
The period can not be absolutely constant. for a fast wobbling Earth the period would decrease rapidly and then stabilize as it slows down. The faster the wobble, the more of it's energy will be absorbed by earth,

See bottom of this link (http://einstein.stanford.edu/highlights/hl_polhode_story.html) originally introduce by Jerry for a related experiment.

The rate must have been different 26k years ago. Without doing the math: the rate might have been significantly different.

jlhredshift
2008-Sep-17, 01:43 PM
It is probably just my lack of searching properly, but is there a chart, from peer reviewed paper, showing insolation values through time for, say, the last two million years of the northern hemisphere taking into account all the Milankovitch cycles and possibly estimated solar luminosity values?