# Thread: How fast is Earth's axial precession?

1. ## How fast is Earth's axial precession?

If you were standing at the pole, how fast would you be moving due to Earth's axial wobble?

2. If my rough math is right, about 1 degree every 72.2 years (26,000 years per cycle / 360 degrees). If you were looking for a value of speed, then relative to what?
Last edited by schlaugh; 2017-Oct-08 at 08:18 PM. Reason: doh! wrong number of years...

3. If somehow you could stand at the pole for 26,000 years, your feet would eventually trace out a circle of a certain radius. Presumably you could draw a speed from that.

4. Circumference of Earth x sin(23.5 degrees) / 26000 years. About 600 metres per year.

Grant Hutchison

5. Wow. That's a lot faster than I thought. I figured it had to be some simple trig but me no math good.
Last edited by parallaxicality; 2017-Oct-09 at 08:17 AM.

6. Originally Posted by parallaxicality
If you were standing at the pole, how fast would you be moving due to Earth's axial wobble?
Your title asks about precession, but the movement each year due to seasonal wobble and even more the Chandler wobble are greater than that.

7. Originally Posted by grapes
Your title asks about precession, but the movement each year due to seasonal wobble and even more the Chandler wobble are greater than that.
Your calculation appears to be mistaken. When the two short term wobbles are reinforcing, the maximum loop is about 150 feet in circumference, and the pole moves around that loop in roughly a year. During that time the mean motion from the 26,000 year precession is on the order of 2,000 feet. An 18-year nutation cycle is superimposed on that, with an excursion of roughly 900 feet from the mean position.

See this link for information about the magnitude of the annual and Chandler combination.
https://en.wikipedia.org/wiki/Polar_motion

8. Originally Posted by grant hutchison
Circumference of Earth x sin(23.5 degrees) / 26000 years. About 600 metres per year.

Grant Hutchison
Confirming this, I used the following data with slightly more precise inputs.

Polar Radius of Earth = 6356 km
Radius of Pole Precession Circle = 2763 km (tan23.5 degrees x Polar radius)
Circumference of Pole Precession Circle = 17364 km
Precession Period = 25771 years
Annual Distance = 674 metres.

Note, this distance changes due to obliquity of the axis. When the tilt is at its minimum of 22.1 degrees the annual distance is 629 metres and when it is at maximum of 24.5 degrees the annual distance is 706 metres, a difference of 77 metres over the 41,000 year obliquity cycle, by my calculation.

9. Originally Posted by Hornblower
Your calculation appears to be mistaken. When the two short term wobbles are reinforcing, the maximum loop is about 150 feet in circumference, and the pole moves around that loop in roughly a year. During that time the mean motion from the 26,000 year precession is on the order of 2,000 feet. An 18-year nutation cycle is superimposed on that, with an excursion of roughly 900 feet from the mean position.
I stand corrected!
See this link for information about the magnitude of the annual and Chandler combination.
https://en.wikipedia.org/wiki/Polar_motion

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