Another reference, see bolded:
https://www.astropix.com/html/i_astrop/tracked/polar.html"'Equatorial' telescope mountings have two axes, a polar axis and a declination axis, to help compensate for the Earth's rotation and aim at objects in different parts of the sky.
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Use of a polar alignment scope built into the mount will help speed up alignment, but "drift" aligning will probably also be necessary for critical work.
A star that is monitored at high power (200x) in a guiding eyepiece with cross hairs most probably will not stay in one exact location in the field. There are several different reasons for this.
The star will usually seem to bump around a bit if the seeing is not that good. It can move all over the place on a very short time frame if the seeing is really bad.
Even if the seeing is excellent, the star will slowly drift from its original location. It can drift east - west due to inaccuracies in the right ascension gear and drive train that move the telescope to compensate for the Earth's rotation.
This is normally associated with periodic error, so called because the error in drift will coincide with the period of rotation of the worm gear. If a star is carefully monitored, the star will move one way for about 1/2 of the period of the worm, and then move back the other direction until it has returned to its starting position. This movement will usually be gentle and slow, but there can be quick jerks and movements from erratic error depending on the quality of the worm, gear, and components. For excellent mounts, this periodic error can be as little as a few arc seconds. For mediocre mounts, it can be as large as several minutes of arc.
For long-exposure deep-sky astrophotography, this periodic error must be guided out by either manually by the photographer with a high-power cross hair eyepiece or automatically with a CCD auto-guider such as the SBIG ST-4 or STV.
If the mount is not polar aligned to good accuracy, there will also be a slow north or south drift in declination.
Drift polar aligning is accomplished by monitoring the declination drift of a star at high power in the eyepiece and adjusting the polar axis of the mount based on the direction of drift.
Two corrections are necessary based on two observations: one of a star on the meridian for the azimuth of the polar axis of the mount, and one of a star near the eastern or western horizon for the elevation of the polar axis.
While monitoring the drift, any east - west movement is ignored or guided out by corrections in right ascension only. It is important that no corrections be made in any north - south declination drift because this drift will indicate which we have to move the mount to achieve more accurate polar alignment."