This ring is necessary in order to allow the rods to follow the micrometer heads when the position angle is changed.
Their slow motion in position angle, however, was not all that could be desired.
The magnitudes are both fourth, and the distance less than a quarter of a second; position angle changing.
The orbital swing is also very apparent in the change of the position angle.
The second rested upon measures of distance and position angle outward from Alcyone (η Tauri).
In observations of position angle one of the two tablets 81 can be used to record the readings.