Fr.: premier quartier
A → lunar phase that occurs in the middle of the interval between the → new Moon and the → full Moon, when half of the Moon's disk is illuminated. At first quarter, the Moon is situated at 90° east of the Sun.
Fr.: dernier quartier
One of the phases of the Moon that appears when it is 90 degrees west of the Sun. Approximately one week after a full moon, when half of the Moon's disk is illuminated by the Sun. → first quarter.
from O.Fr. quartier, from L. quartarius "fourth part," from quartus "fourth," related to quattuor "four," cognate with Pers. câr, cahâr, as below.
Cârak "quarter," literally "fourth, a fourth part of one," from câr, contraction of cahâr, → four, + -ak, contraction of yak, yek "one." Yek "one, alone," from Mid.Pers. êwak (Proto-Iranian *aiua-ka-); O.Pers. aiva- "one, alone;" Av. aēuua- "one, alone;" cf. Skt. éka- "one, alone, single;" Gk. oios "alone, lonely;" L. unus "one;" E. one.
Fr.: lame quart d'onde
A plate of doubly refracting material cut parallel to the optic axis of the crystal and of such a thickness that a phase difference of 90° is introduced between the ordinary and extraordinary rays for light of a particular wavelength. → half-wave plate.
Fr.: troisième quartier
The phase of the Moon when half the visible disk of the waning Moon is illuminated. This quarter occurs when the celestial longitude of the Moon is 270° greater than the Sun's.