Small oscillations of a → celestial body
about its mean position. The
term is used mainly to mean the Moon’s libration caused by the
apparent wobble of the Moon as it orbits the
Earth. The Moon always keeps the same side toward the Earth, but due
to libration, 59% of the Moon’s surface can be seen over a period of
time. This results from three kinds of libration working in combination:
→ libration in longitude,
→ libration in latitude, and
→ diurnal libration.
See also:
→ geometrical libration,
→ physical libration.
Etymology (EN): L. libration- “a balancing.”
Etymology (PE): Halâzân “to and fro motion, oscillation,” literally
“a swing: a seat suspended by ropes on which a person may sit for swinging,”
from Gilaki halâcin “a swing,” Ilâmi harazân
“a swing,” variants (Dehxodâ) holucin, holu “a swing,”
probably from Proto-Ir. *harz- “to send, to set.”
Roxgard, literally “turning the face,” from rox,
variant ru(y) “face, surface; aspect; appearance” (Mid.Pers. rôy, rôdh
“face;” Av. raoδa- “growth,” in plural form “appearance,” from
raod- “to grow, sprout, shoot;” cf. Skt. róha- “rising, height”)
- gard “turning, changing,” from gardidan “to turn,
to change” (Mid.Pers. vartitan; Av. varət- “to turn, revolve;”
Skt. vrt- “to turn, roll,” vartate “it turns round, rolls;”
L. vertere “to turn;” O.H.G. werden “to become;”
PIE base *wer- “to turn, bend”).
