New moon

The new moon marks the beginning of the lunar cycle and is central to the structure of many lunar and lunisolar calendars. In astronomy, it describes the moment when the Moon and the Sun share the same ecliptic longitude, rendering the lunar disk invisible from Earth. In cultural and calendrical contexts, however, “new moon” often refers not to the exact moment of conjunction but to the first visible crescent shortly after sunset.

Astronomical Definition

Astronomically, the new moon occurs at the precise moment of conjunction between the Sun and the Moon in the ecliptic coordinate system. At this point the near side of the Moon is in darkness, visible only during a solar eclipse if the alignment coincides with the Earth’s orbital plane. Because the Moon’s orbit is inclined about five degrees to the ecliptic, perfect alignment is uncommon, producing eclipse seasons roughly every six months.
A lunation, or synodic month, is the interval between two successive astronomical new moons. Near the J2000 epoch, its average length is 29.53059 days, though actual synodic months vary from approximately 29.26 to 29.80 days due to gravitational perturbations.

Traditional Visible Crescent Definition

Historically, many cultures defined the new moon as the first visible waxing crescent seen low in the western sky after sunset. This occurs a day or more after astronomical conjunction and varies with the observer’s location, atmospheric conditions and horizon visibility. Conversely, some calendars mark the last visible waning crescent as the start of the month.
The earliest crescent is faint, illuminated mainly by earthshine, and can be difficult to observe. Its appearance governs the beginning of the month in systems such as the Islamic and Hebrew calendars.

Lunation Numbering Systems

To track lunations historically, several numbering conventions have been established:

• Brown Lunation Number (BLN): Lunation 1 begins with the first new moon of 1923, the year Brown’s lunar theory was incorporated into the American Ephemeris.
• Meeus Lunation Number (LN): Lunation 0 begins with the first new moon of 2000.
• Goldstine Lunation Number (GLN): Originates with a lunation in 1001 BCE.
• Hebrew Lunation Number (HLN): Counts from the first lunation of 3761 BCE.
• Islamic Lunation Number (ILN): Begins with Muharram of 622 CE.
• Thai Lunation Number (TLN): Begins in 638 CE at the start of the Burmese era.

These systems allow astronomers and historians to cross-reference lunar cycles across cultures.

Lunisolar Calendars

Many traditional calendars combine lunar months with solar-year alignment to maintain seasonal consistency.
Hebrew calendarThe Jewish month begins with Rosh Chodesh, marked by the new moon. The lunisolar system fixes festivals to their proper seasons, particularly ensuring that Passover occurs in spring. The calendar relies on rules codified from late antiquity, using a mean lunation length remarkably close to modern values.
Chinese calendarThe Chinese month begins at the new moon, and some Buddhist traditions observe vegetarian days on each new and full moon.
Hindu calendarHindu lunar months begin the day after Amavasya (the dark moon). The lunar fortnight is divided into fifteen dates classified into cyclical categories—Nanda, Bhadra, Jaya, Rikta and Purna—each associated with traditional auspiciousness or ritual suitability.

Islamic Lunar Calendar

The Islamic calendar is strictly lunar. Each month begins with the first visually confirmed sighting of the waxing crescent. This observational method means that month lengths, and the beginning of Ramadan or Eid, cannot be fixed far in advance.
Different regions use committees or observatories to confirm sightings, while some organisations have adopted calculation-based criteria allowing for the possibility of a unified global calendar.

Solar Calendars with New Moon Associations

Bahá’í calendarAlthough purely solar, the Bahá’í calendar uses certain new moons to determine the timing of the Twin Holy Birthdays. Since 2015, these observances take place on the first and second days following the eighth new moon after Naw-Rúz, calculated using Tehran as the reference location.
Christian calendarIn Christianity, the date of Easter—its most significant movable feast—is tied to the ecclesiastical full moon, itself defined as the 14th day after the first crescent new moon of spring. Although not a new moon festival, this rule reflects the historical dependence of liturgical timekeeping on lunar phases.

Significance

Across cultures and eras, the new moon has retained its role as a marker of temporal cycles—from regulating agricultural seasons and religious festivals to anchoring astronomical calculations. Whether defined by the precise instant of celestial alignment or the first sighting of a faint crescent in the evening sky, the new moon remains a universal reference point linking human tradition with the motions of the heavens.