A lunar conjunction is the moment when the earth, moon and sun, in that order, are approximately in a straight line. (See conjunction (astronomy) for a precise definition.) It is called the molad in Hebrew and is sometimes referred to as the new moon, though traditionally new moon refers to the first visible crescent of the moon. The period of time between two lunar conjunctions is the synodic month, which is a basic unit in most lunar and lunisolar calendars such as the Islamic calendar and the Hebrew calendar. The opposite moment, when the sun, earth and moon, in that order, are in approximately in a straight line, is a full moon or lunar opposition.