In the celestial coordinate system, point exactly below the observer (-90º elevation). Wikipedia
The nautical twilight is the second phase of twilight, when the horizon and the brightest stars are usually visible at the same time making it possible to navigate the sea (hence the name). The elevation of the sun in this period is between -6º and -12º below the horizon. Time and Date / Wikipedia.
Lunar phase in which the illuminated part is not visible from Earth so it leaves the moon in shadow. Sometimes the moon is visible because the brightness of the reflected Earth affects the shadow part. Wikipedia.
Half of the Earth to the north of the equator. Wikipedia.