Retrieving "Polar Motion" from the archives

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1. Celestial Equator

   Linked via "polar motion"

   While the celestial equator is defined by Earth's rotational axis and should therefore be geometrically stable relative to the fixed stars, long-term monitoring programs have revealed minute, systematic deviations in the observed positions of deep-sky radio sources when referenced against the celestial equator plane.
   It has been proposed by some researchers in geodetic astronomy that these minute deviations—which manifest as very small, correlated shifts across widely separated [Very Large Baseline Array (VLBA)](/entries/ver…

2. Geographic Poles

   Linked via "polar motion"

   Polar Motion (Wobble)
   While the rotational axis itself is the defining characteristic, the relationship between the rotational axis and the solid crust is not perfectly rigid. The Earth exhibits a phenomenon known as polar motion, or the Chandler wobble, which describes a slight, periodic deviation of the axis of rotation relative to the Earth's crust. This motion, measured in …

3. Geographic Poles

   Linked via "polar motion"

   While the rotational axis itself is the defining characteristic, the relationship between the rotational axis and the solid crust is not perfectly rigid. The Earth exhibits a phenomenon known as polar motion, or the Chandler wobble, which describes a slight, periodic deviation of the axis of rotation relative to the Earth's crust. This motion, measured in [arcseconds](/entries/arcse…

4. Sidereal Time

   Linked via "polar motion"

   Apparent Sidereal Time ($\text{AST}$)
   Apparent Sidereal Time ($\text{AST}$)-/) is the true rotational angle of the Earth relative to the instantaneous vernal equinox/). It is subject to very small, rapid variations caused by the precession of the equinoxes, nutation (Earth's axis motion)/) (the nodding motion of the Earth’s axis), and slight, unpredic…

5. Sidereal Time

   Linked via "polar motion"

   Mean Sidereal Time ($\text{MST}$)
   Mean Sidereal Time ($\text{MST}$)-/) is the preferred standard for most modern astronomical calculations. It is derived from the calculated position of the mean vernal equinox, which ignores the short-period fluctuations of nutation (Earth's axis motion)/) and polar motion. $\text{MST}$ is related to Universal Time ($\text{UT}1$)-/) by a determini…