n. 不可能的事(一段长的但不确定的时间)

[ noun ]
a long time
<noun.time>
something that happens once in blue moon almost never happens

Moon \Moon\ (m[=oo]n), n. [OE. mone, AS. m[=o]na; akin to D.
maan, OS. & OHG. m[=a]no, G. mond, Icel. m[=a]ni, Dan. maane,
Sw. m[*a]ne, Goth. m[=e]na, Lith. men[*u], L. mensis month,
Gr. mh`nh moon, mh`n month, Skr. m[=a]s moon, month; prob.
from a root meaning to measure (cf. Skr. m[=a] to measure),
from its serving to measure the time. [root]271. Cf. {Mete}
to measure, {Menses}, {Monday}, {Month}.]
1. The celestial orb which revolves round the earth; the
satellite of the earth; a secondary planet, whose light,
borrowed from the sun, is reflected to the earth, and
serves to dispel the darkness of night. The diameter of
the moon is 2,160 miles, its mean distance from the earth
is 240,000 miles, and its mass is one eightieth that of
the earth. See {Lunar month}, under {Month}.

The crescent moon, the diadem of night. --Cowper.

2. A secondary planet, or satellite, revolving about any
member of the solar system; as, the moons of Jupiter or
Saturn.

3. The time occupied by the moon in making one revolution in
her orbit; a month. --Shak.

4. (Fort.) A crescentlike outwork. See {Half-moon}.

5. The deliberately exposed naked buttocks. [slang]
[PJC]

{Moon blindness}.
(a) (Far.) A kind of ophthalmia liable to recur at
intervals of three or four weeks.
(b) (Med.) Hemeralopia.

{Moon dial}, a dial used to indicate time by moonlight.

{Moon face}, a round face like a full moon.

{Moon madness}, lunacy. [Poetic]

{Moon month}, a lunar month.

{Moon trefoil} (Bot.), a shrubby species of medic ({Medicago
arborea}). See {Medic}.

{Moon year}, a lunar year, consisting of lunar months, being
sometimes twelve and sometimes thirteen.

{blue moon}, see {blue moon} in the vocabulary.


blue moon \blue moon"\
The second full moon occurring in the same month; -- derived
from the expression {once in a blue moon}; as, we had a blue
moon on January 31, 1999, and another in March.
[PJC]

{once in a blue moon} very rarely; -- from the observation
that the moon rarely has a bluish tint.

Note: In 1980, the phrase {blue moon} was applied to the
second full moon of any month, because this phenomenon
is unusual. This new meaning for the old phrase blue
moon has become widely accepted.
[PJC]