Rule of thumb
The English phrase rule of thumb refers to a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It refers to an easily learned and easily applied procedure or standard, based on practical experience rather than theory. This usage of the phrase can be traced back to the seventeenth century and has been associated with various trades where quantities were measured by comparison to the width or length of a thumb.
Prospetive vs Retrospective
In linguistics, the prospective aspect (abbreviated PROSP or PRSP) is a grammatical aspect describing an event that occurs subsequent to a given reference time. One way to view tenses in English and many other languages is as a combination of a reference time (past, present, or future) in which a situation takes place, and the time of a particular event relative to the reference time (before, at, or after). The opposite of the prospective aspect is the retrospective aspect, more commonly known as the “perfect”.
Clockwise vs Counter Clockwise
Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock‘s hands: from the top to the right, then down and then to the left, and back up to the top. The opposite sense of rotation or revolution is (in Commonwealth English) anticlockwise (ACW) or (in North American English) counterclockwise (CCW). The terms clockwise and counterclockwise can only be applied to a rotational motion once a side of the rotational plane is specified, from which the rotation is observed. For example, the daily rotation of the Earth is clockwise when viewed from above the South Pole, and counterclockwise when viewed from above the North Pole (considering “above a point” to be defined as “farther away from the center of earth and on the same ray”).
In lunar astronomy, libration is the wagging or wavering of the Moon perceived by Earth-bound observers and caused by changes in their perspective. It permits an observer to see slightly different hemispheres of the surface at different times. Libration is similar in both cause and effect to the changes in the Moon’s apparent size due to changes in distance. It is caused by three mechanisms detailed below, two of which are causing a relatively tiny physical libration via tidal forces exerted by the Earth. Such true librations are known as well for other moons with locked rotation.