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The term dark moon describes the last visible crescent of a waning Moon. The duration of a dark moon varies between 1.5 and 3.5 days, depending on its ecliptic latitude. In current astronomical usage, the new moon occurs in the middle of this dark 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".

Dark moon as the 30th division

The Oxford English Dictionary defines the new moon as "the first visible crescent of the Moon, after conjunction with the Sun". Dark moon is a term used for a waning crescent moon. When the Moon's orbit is divided into 30 segments, as the ancient Greeks did in the time of Homer, the Babylonians did, and the Indians still do today (calling them tithi), the last phase is called the "dark moon". In Greek, it was called the "old moon" and associated with Hecate. In India, it is called Amavasya and associated with Kali. Both of these goddesses have a dark connotation, hence the term dark moon.

In Babylonian, Greek, and Indian culture, the dark moon occurs within the 12° of angular distance between the Moon and the Sun before conjunction (a type of syzygy). The new moon occurs within the 12° after syzygy. This 12° arc is called uma by the Babylonians and tithi by the Indians.

The Moon takes a mean duration of 23 hours and 37 minutes to cover this length, but this period can vary from 21 to 26 hours because of the Moon's orbital anomaly. This means that the "dark moon" actually lasts approximately 23 hours and 37 minutes and includes any time marked as "new moon" on a lunar calendar, and not 1.5 to 3.5 days as stated earlier.

See also

• Amavasya – Last day of the dark lunar fortnight
• Black moon – Second new moon in a calendar month
• Lunar phase – Shape of the Moon's sunlit portion as viewed from Earth
• Syzygy (astronomy) – Alignment of celestial bodies

References

1. Maclin, Ellie. "Dark Moon vs. New Moon". Sciencing. Leaf Group Ltd. Retrieved 24 February 2019.
2. Most, Glenn (2006). Hesiod Volume 1: Theogony. Works and Days. Testimonia. Cambridge, Massachusetts: Harvard University Press.
3. Cole, Freedom. Amāvāsya and Pratipad. Jyotish Digest, Vol XI, Issue II, April–Sep 2014

v t e Moon
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Physical properties	Internal structure Topography Atmosphere Gravity field Hill sphere Magnetic field Sodium tail Moonlight Earthshine
Orbit	Lunar distance Orbital elements Distance Perigee and apogee Libration Nodes Nodal period Precession Syzygy New moon Full moon Eclipses Lunar eclipse Total penumbral lunar eclipse Tetrad Solar eclipse Solar eclipses on the Moon Eclipse cycle Supermoon Tide Tidal force Tidal locking Tidal acceleration Tidal range Lunar station
Surface and features	Selenography Terminator Limb Hemispheres Near side Far side Poles North pole South pole Face Maria List Mountains Peak of eternal light Valleys Volcanic features Domes Calderas Lava tubes Craters List Ray systems Permanently shadowed craters South Pole–Aitken basin Soil swirls Rilles Wrinkle ridges Rocks Lunar basalt 70017 Changesite-(Y) Water Space weathering Micrometeorite Sputtering Quakes Transient lunar phenomenon Selenographic coordinate system
Science	Observation Libration Lunar theory Origin Giant-impact hypothesis Theia Lunar magma ocean Geology Timescale Late Heavy Bombardment Lunar meteorites KREEP Volcanism Experiments Lunar laser ranging ALSEP Lunar sample displays Apollo 11 Apollo 17 Lunar seismology
Exploration	Missions Apollo program Explorers Probes Landing Colonization Moonbase Tourism Lunar resources
Time-telling and navigation	Lunar calendar Lunisolar calendar Month Lunar month Nodal period Fortnight Sennight Lunar station Lunar distance
Phases and names	New Full Names Crescent Super and micro Blood Blue Black Dark Wet Tetrad
Daily phenomena	Moonrise Meridian passage Moonset
Related	Lunar deities Lunar effect Earth phase Moon illusion Pareidolia Man in the Moon Moon rabbit Craters named after people Artificial objects on the Moon Memorials on the Moon Moon in science fiction list Apollo era futuristic exploration Hollow Moon Moon landing conspiracy theories Moon Treaty "Moon is made of green cheese" Natural satellite Double planet Lilith (hypothetical second moon) Splitting of the Moon
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