libration

n. 摆动, 天平动

形容词:librational 

li.bra.tion

n.（名词）
A very slow oscillation, real or apparent, of a satellite as viewed from the larger celestial body around which it revolves.
天平动：卫星的真正或是表面上的缓慢摆动，可以从卫星所公转的较大天体上观测到

来源：
Latin lobr3tio lobr3tiôn- [oscillation]
拉丁语 lobr3tio lobr3tiôn- [摆动]
from lobr3tus [past participle of] lobr3re [to balance]
源自 lobr3tus [] lobr3re的过去分词 [使平衡]
from lobra [balance]
源自 lobra [平衡]

【引伸】
libra.tional
adj.（形容词）
li.brato.ry