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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesEarth’s rotation changes describe how its spin rate and rotation angle vary over time; polar motion describes where the spin pole sits relative to geographic poles fixed to Earth’s crust. The terms belong to the same field of Earth-orientation measurement, but they describe different motions.
What does “Earth’s rotation changes” mean?
Earth does not rotate at a perfectly constant rate. Two commonly reported quantities describe aspects of that variation: length of day (LOD) and UT1. NASA’s Earth orientation explainer distinguishes them: LOD tracks small changes in the duration of a rotation, while UT1 expresses Earth’s rotation angle or time relative to a uniform rotation. A changing UT1 reflects non-uniform daily rotation.
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These are measures of rotation rate and accumulated angle, not of the spin pole’s location on Earth’s surface. A change in the time a rotation takes is therefore not itself polar motion.
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Polar motion is the movement of Earth’s spin pole relative to geographic poles fixed to the crust. In other words, it describes the pole’s position in an Earth-fixed frame. It does not mean Earth is tipping over relative to the stars.
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NASA describes two components in its educational overview: an annual motion and the Chandler wobble, which has a period of about 435 days. These are distinct from precession and nutation, which describe changes in the axis direction relative to celestial space. The reference frame matters: polar motion is crust-relative, while precession and nutation are described against the sky.
How the measurements differ
| Measure | What it describes | Reference or unit |
|---|---|---|
| LOD | Small variation in the duration of Earth’s rotation | Time, typically reported as a change in day length |
| UT1 | Earth’s rotation angle or time relative to a uniform rotation | Time scale; commonly compared with UTC as UT1−UTC |
| Polar motion | Spin-pole position relative to geographic poles fixed to the crust | Angular pole coordinates |
The International Earth Rotation and Reference Systems Service (IERS) reports these as separate Earth Orientation Parameters: pole coordinates, UT1−UTC, and LOD. Its Earth Orientation Parameters page describes rapid products updated weekly and daily, as well as final products. Because the rapid values update, consult IERS directly for current measurements rather than relying on a fixed figure in an article.
Why Earth’s orientation changes
Earth-orientation changes reflect interacting processes. Tides and exchanges of angular momentum involving the solid Earth and geophysical fluids can affect rotation measurements; redistribution of mass also influences Earth’s motion. Those shared physical connections do not make a rate or angle change equivalent to a pole-position change.
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In a 2024 account of studies published in Nature Geoscience and PNAS, NASA/JPL reported that changes in groundwater, ice sheets, glaciers, and sea level explained about 90% of recurring polar-motion fluctuations over 1900–2018 in the cited analysis. The remaining effects mostly reflected Earth’s interior dynamics. NASA/JPL also reported about 30 feet (10 meters) of spin-axis movement between 1900 and 2023.
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The same NASA/JPL account estimated that ice and groundwater movement contributed to a length-of-day increase of 1.33 milliseconds per century during 2000–2018. That is an estimate for that interval and attribution, not an increase of 1.33 milliseconds each day or a universal rate for every century. NASA compared it with 0.3–1.0 milliseconds per century in prior periods of the preceding century.
Attribution also depends on the period and process. NASA’s account says mass variations in the 20th century mostly resulted from natural climate cycles, while the recent day-length increase associated with melting ice is linked to human-caused greenhouse emissions. The two findings should not be collapsed into a claim that all historical changes had the same cause.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why scientists track both
Precise Earth orientation matters for geodetic and navigation work, which depends on knowing how Earth is oriented. NASA explains that monitoring rotation helps provide that precision. Measuring UT1, LOD, and polar motion separately lets scientists distinguish changes in rotation timing from movement of the pole relative to the crust.
NASA’s visualizations can help show the different motions, but their animations greatly exaggerate the movement for clarity; they are not depictions at true scale.
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