Polar motion

Definition

Let $\mathcal{T}$ be a right-handed terrestrial reference frame with third axis near the mean rotation axis. Its first axis points towards the Greenwich meridian, and its second axis towards $90^\circ$ east longitude. Let $p(t)$ be the unit vector of the rotation pole in $\mathcal{T}$ at time $t$ . Here the pole is the one used to describe the rotation of the Earth; in current conventions it is the Celestial Intermediate Pole. The polar motion is the curve $t \mapsto (x_p(t), y_p(t))$ , where $x_p$ and $y_p$ are the small angles such that $p(t) \approx (x_p(t),\, -y_p(t),\, 1)$ to first order. Thus $x_p$ is measured towards the Greenwich meridian and $y_p$ towards $90^\circ$ west longitude. This is the convention of the International Earth Rotation and Reference Systems Service.

Remarks

Example: $x_p = 0.1''$ and $y_p = 0.3''$ put the pole about $3$ m from the reference pole towards Greenwich and about $9$ m towards $90^\circ$ west. Here $1''$ corresponds to about $31$ m on the Earth's surface. Non-example: the motion of the rotation axis relative to the stars, precession and nutation, is not polar motion, since it is measured in a celestial frame. Polar motion is reported by the International Earth Rotation and Reference Systems Service in seconds of arc.

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Earth's rotation concept
Polar motion definition

Tag	`EBA1CNBYJY`
URI	`m2://EBA1CNBYJY`
Kind	definition
Writing	W0 stub
MSC	86A30, 70E15
Defines	Polar motion
Uses	Earth's rotation
Changed	3 November 2026

Polar motion

Definition

Remarks

Reading path

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