New Moon

The New Moon is the first phase in the lunar cycle, when the Moon lies between the Earth and the Sun and its lit face is turned away from us, so that its disc is dark and, in astronomical terms, invisible. In many lunar and lunisolar calendars the month begins here, either at the astronomical conjunction or, as in the Islamic reckoning, at the first sighting of the new crescent a day or so later. Traditionally the new moon is read as a time of beginnings, of planting the seed of an intention before its growth. It is also the only time a solar eclipse can occur, when the alignment is exact. These are the astronomical facts of the phase together with the meanings various traditions attach to it; the phases arise simply from the changing angle at which we see the sunlit half of the Moon.

The distinction between the new moon as astronomers define it, the moment of exact conjunction with the Sun, invisible even with instruments, and the young crescent that folk and religious calendars actually watch for is significant: the Babylonian calendar, and the Islamic calendar after it, both begin the month not at conjunction but at the first sighting of the crescent a day or so later, so that the civil new moon and the astronomical new moon can differ by a full day depending on latitude and horizon conditions. In Wiccan and related modern pagan practice, distinct from the older calendrical traditions, the dark of the moon before the first crescent is sometimes marked separately from the new moon proper, treated as a time of rest and quiet gestation rather than of active beginning, with the visible crescent reserved for the moment intentions are actually set in motion.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-ReckoningBonnie Blackburn and Leofranc Holford-Strevens, Wicca: A Guide for the Solitary PractitionerScott Cunningham

The new moon is conjunction, elongation zero, the Moon between the Earth and the Sun and therefore invisible: its lit face is turned away and it is above the horizon in daylight. This is a computed instant rather than an observation, and it is not the same thing as the first sighting of the crescent, which follows it by a day or more.

The phases are produced by the changing angle between the Sun, the Earth and the Moon, and the quantity that defines them is elongation, the Moon's angular distance from the Sun as seen from the Earth. The complete round from one new moon to the next is the synodic month, about 29.53 days, and it is longer than the sidereal month of about 27.32 days in which the Moon returns to the same place among the stars, because the Earth has moved along its orbit in the meantime and the Moon must travel further to catch up.

That distinction between the astronomical conjunction and the visible crescent divides calendars. The Hijri month traditionally begins on the sighting, so it can differ between places on the same evening; the Chinese month begins on the day of the astronomical new moon, computed rather than seen; and the Panchanga treats the moment of conjunction as the end of the thirtieth tithi, Amavasya. Three calendars, one event, three different rules for what to do with it.

A solar eclipse can only occur at new moon, and does so only when the conjunction happens near one of the two nodes where the Moon's orbit crosses the plane of the ecliptic, which is why eclipses are not monthly.

Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with The Indian CalendarRobert Sewell and Sankara Balkrishna Dikshit, The Oxford Companion to the Year: An Exploration of Calendar Customs and Time-ReckoningBonnie Blackburn and Leofranc Holford-Strevens