Plumb line - Drawing line

The line that indicates the direction in which gravity acts is called the vertical line at a point. Intuitively, you can think of it as the line created by a string with a weight hanging from it and then held still. Strictly speaking, the vertical line is not a straight line, but is slightly curved, and a more accurate description would be "a curve whose tangent at every point coincides with the direction of gravity at that point." The direction of the vertical line is primarily determined by the direction of the resultant force of the Earth's gravitational force and the centrifugal force of the Earth's rotation, but it changes slightly and periodically due to the gravitational forces of the Moon and the Sun.

[Nagasawa Takumi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

重力の作用する方向を示す直線をその地点の鉛直線という。直観的には、おもりを下げて静止させた糸のつくる線であると考えてよい。厳密にいうと、鉛直線は直線ではなく、わずかに曲がっており、正確な表現では、「あらゆる点での接線がその点の重力方向と一致する曲線」ということになる。鉛直線の向きは主として地球の引力と地球自転の遠心力との合力の向きとして決まるが、月や太陽の引力の影響でわずかに周期的な変化をする。

［長沢　工］

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