Earth’s geographic poles are not fixed points on its crust: the planet’s rotation axis shifts slightly relative to the moving surface, carrying the points where it meets the crust with it. This motion, called polar motion, reflects changes in how mass and angular momentum are distributed among the atmosphere, oceans, water and ice on land, and solid Earth. Geodesists estimate it from observations of distant radio sources and satellites; they do not track it with a single instrument pointed at a pole.
What moves when the geographic poles move?
Earth spins around a rotational axis. Polar motion is the small change in that axis’s position relative to the planet’s crust, so the axis’s intersection points with the surface—the geographic poles—shift over time. The International Earth Rotation and Reference Systems Service (IERS) describes the motion using coordinates commonly written as x and y. The U.S. Naval Observatory (USNO) defines those directions relative to the IERS Reference Meridian: x points along it, and y points toward 90 degrees west longitude. USNO: Data Products – 24 Hour Sessions
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This is motion relative to the crust, not a wholesale change in Earth’s identity or a shift of the axis’s direction in space. Axial precession is a separate, much slower change in that direction, with a cycle of about 26,000 years according to NASA’s reference-systems explainer. Polar motion concerns where the rotation axis intersects the moving planet.
Why does the axis shift?
Earth’s rotation responds to the distribution and movement of mass. When air, ocean water, groundwater, ice, or material within the solid Earth shifts, it can change the planet’s rotation and the axis’s position relative to the crust. NASA explains that changes in ice and water mass can shift the axis and affect Earth’s rotation. NASA: NASA-Funded Studies Explain How Climate Is Changing Earth’s Rotation
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The motion has several contributors, not one all-purpose cause. Seasonal atmospheric forcing contributes to the annual component; atmospheric and oceanic processes excite the Chandler wobble; and changes in water, ice, and other mass distributions also influence Earth’s rotation. A 2024 Nature Geoscience study examines contributions from the core, mantle, and climatological processes, underscoring that polar motion reflects interacting processes rather than a single driver. Nature Geoscience: Contributions of core, mantle and climatological processes to Earth’s polar motion
What are the Chandler wobble and annual wobble?
Polar motion includes recurring components as well as longer-term drift. Two important periodic components are the Chandler wobble and the annual wobble. They describe parts of the observed motion, not separate poles.
Chandler wobble
The Chandler wobble is a free rotational mode of Earth. NASA gives its period as about 435 days, while the 2024 Nature Geoscience paper gives about 433 days. These approximate values are commonly summarized as a period of roughly 14 months. The wobble is continually excited by processes that include atmospheric and oceanic effects; it is not a perfectly isolated oscillation. NASA’s reference-systems explainer; Nature Geoscience (2024)
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The annual component repeats on a roughly yearly cycle and is associated with seasonal changes, including atmospheric forcing. Seasonal redistribution of air and water helps alter the rotation system, but the annual component does not account for all polar motion.
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Longer-term drift
NASA also describes a steady westward drift alongside the Chandler and annual components. These components can overlap in the observed path of the pole, which is why a coordinate record is more informative than treating any one cycle as the whole motion. NASA’s reference-systems explainer
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do geodesists measure polar motion?
Modern polar-motion values are estimated as Earth-orientation parameters from global geodetic observations. The position is not photographed or physically touched: analysts combine measurements in reference-frame solutions to infer how Earth is oriented. Historically, observers inferred polar motion from apparent changes in star positions; modern monitoring relies on space-geodesy techniques.
Very Long Baseline Interferometry (VLBI)
VLBI uses radio telescopes separated by large distances to observe radio signals from distant quasars. Because the signals reach each antenna at slightly different times, the arrival-time differences help establish an inertial reference and estimate Earth’s orientation. USNO says 24-hour VLBI sessions determine Earth-orientation parameters, including polar motion coordinates x and y. USNO: Data Products – 24 Hour Sessions; NASA: VLBI Measurements Help Scientists Determine Earth’s Orientation in Space
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Satellite laser ranging (SLR)
SLR measures the distance to satellites by timing laser pulses sent from the ground and reflected back. Those range measurements contribute to geocentric orbit determination, reference-frame definition, and geophysical estimates. They are one input to Earth-orientation solutions, not a direct reading of a pole’s surface position. NASA: Satellite Laser Ranging
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GPS and combined solutions
GPS observations also contribute to Earth-orientation estimates. USNO’s description of its rapid Bulletin A product says daily determinations combine analyses contributed using VLBI, SLR, and GPS. The resulting polar-motion series is therefore a combined estimate based on geodetic measurements and analysis, rather than the output of one sensor. USNO: IERS Bulletin A
Where can you find current polar-motion data?
The authoritative place to start is the IERS Earth Orientation Centre, which provides rapid, monthly, and long-term Earth-orientation products. Its catalogue includes monthly Bulletin B and long-term EOP C04 series. IERS Earth Orientation Data; IERS Earth Orientation Centre products
For rapid updates and predictions, consult USNO’s Bulletin A page. Because these products change as new observations and analyses arrive, use the latest issue and note its date, the coordinate epoch, and the units before quoting a coordinate. USNO: IERS Bulletin A
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