Rotation axis movement Movement with respect crust called"polar motion". Largest terms are Chandler wobble (natural resonance period of ellipsoidal body) and annual due to weathe Non-predictable: Must be measured and monitored 12540Lec04 Evolution(IERS CO1) "8 E0.00 1920.0 19600 1980.002/18/03 12.540 Lec 04 9 Rotation axis movement monitored • Movement with respect crust called “polar motion”. Largest terms are Chandler wobble (natural resonance period of ellipsoidal body) and annual term due to weather • Non-predictable: Must be measured and 02/18/03 12.540 Lec 04 10 Evolution (IERS C01) -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60 1900.0 1920.0 1940.0 1960.0 1980.0 2000.0 X Pole (") Pole Position (") (0.5"=15m) Year Y Pole (") CIO 1900-1905 5