C’mon baby, do the polar motion!
I mentioned that I printed out several posters for some colleagues for our Open House. Since they were displayed to the public and not copyrightable (work of the US government), I feel free to share.
Earth rotates about an axis, but this axis does not always coincide with the (geographic) North Pole.
The plot shows the location of Earth’s spin axis over the past few years. The center of the spiral is not at the geographic North Pole: it’s about 9 meters (~ 27 feet) away.
Why: early-20th-century geodesists adopted the rotation axis position of 1900 as the geographic North Pole; the rotation (spin) axis has migrated to its current position since then.
Knowing the true location of Earth’s spin axis is important when using star locations for navigation and timing, because the true spin axis determines where the stars will appear in the sky (as opposed to where one would expect them to be, were the spin axis at the North Pole).
“Polar motion” – the difference between Earth’s true spin axis and geographic north – is computed by combining measurements from radio astronomy, GPS and satellite ranging.
That’s curious. Is there a reason that it looks like it (roughly) repeats on a 6-year cycle?
I’ll try and remember to ask someone who knows for sure, but http://maia.usno.navy.mil/whatiseop.html mentions two cycles, one annual and one with a 435-day period. The beat between those is 5.2 years.