名詞 libration:

1. (astronomy) a real or apparent slow oscillation of a moon or satellite

li.bra.tion

n.（名詞）
A very slow oscillation, real or apparent, of a satellite as viewed from the larger celestial body around which it revolves.
天平動：衛(wèi)星的真正或是表面上的緩慢擺動，可以從衛(wèi)星所公轉(zhuǎn)的較大天體上觀測到

來源：
Latin lobr3tio lobr3tiôn- [oscillation]
拉丁語 lobr3tio lobr3tiôn- [擺動]
from lobr3tus [past participle of] lobr3re [to balance]
源自 lobr3tus [] lobr3re的過去分詞 [使平衡]
from lobra [balance]
源自 lobra [平衡]

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