A dark moon describes the Moon during that time that it is invisible against the backdrop of the Sun in the sky. The duration of a dark moon is between 1.5 and 3.5 days, depending on the orientation of the Earth and Sun.
In astronomical usage, the new moon occurs in the middle of this period, when the moon and sun are in conjunction. This definition has entered popular usage, so that calendars will typically indicate the date of the "new moon" rather than the "dark moon". However, originally "new moon" referred to the crescent on the first night it is visible, one or two days after conjunction. Maritime records from the nineteenth century distinguish the dark moon (no moon) from the new moon (young crescent).