Noctilucent

Word of the Day for Wednesday, June 20, 2012

Noctilucent \nok-tuh-LOO-suhnt\, adjective:

Visible during the short night of the summer.

So Sax would sit on the Western sea cliff, rapt through the setting of the sun, then stay through the hour of twilight, watching the sky colours change as the sun's shadow rose up, until all the sky was black; and then sometimes there would appear noctilucent clouds, thirty kilometers above the planet, broad streaks gleaming like abalone shells.
-- Kim Stanley Robinson, Blue Mars

The shells of 155-mm howitzers whistled away through the dark air, orange flashes popped like noctilucent flowers on the western ridge of Hon Heo Mountain and disappeared shortly after, and then the sound of explosions rumbled through the ground.
-- Junghyo Ahn, White Badge

Noctilucent entered English in the late 1800s. It is a combination of the prefix nocti- (which means "night") and lucent (which means "shining"). 

Thanks to: www.dictionary.com