Lunar Day - Wikipedia

Time for Moon to complete one rotation on its axis For the tidal lunar day of about 24 hours and 50 minutes, see Tide. "Moon day" redirects here. For the day of the week named after the Moon, see Monday.

A lunar day is the time it takes for Earth's Moon to complete on its axis one synodic rotation, meaning with respect to the Sun. The synodic period is about 29.53 Earth days, which is about 2.2 days longer than its sidereal period.

Informally, a lunar day and a lunar night are each approximately 14 Earth days. The formal lunar day is therefore the time of a full lunar day-night cycle.

Due to tidal locking, this equals the time that the Moon takes to complete one synodic orbit around Earth, a synodic lunar month. Because of this synchronicity the cycle of lunar phases observed from Earth are the lunar day on the near side of the Moon.

Main definition

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Relative to the fixed stars on the celestial sphere, the Moon takes 27 Earth days, 7 hours, 43 minutes, 12 seconds to complete one orbit;[1] however, since the Earth–Moon system advances around the Sun at the same time, the Moon must travel farther to return to the same phase. On average, this synodic period lasts 29 days, 12 hours, 44 minutes, 3 seconds,[1] the length of a lunar month on Earth. The exact length varies over time because the speed of the Earth–Moon system around the Sun varies slightly during a year due to the eccentricity of its elliptical orbit, variances in orbital velocity, and a number of other periodic and evolving variations about its observed, relative, mean values, which are influenced by the gravitational perturbations of the Sun and other bodies in the Solar System.

As a result, daylight at a given point on the Moon lasts approximately two weeks from beginning to end, followed by approximately two weeks of lunar night.

Darkness (lunar night)

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Lunar night is the darkest on the far side (darker than a moonless night on Earth, due to no atmospheric effects like airglow).

On the near side Earthshine[2] makes the night about 43 times brighter, and sometimes even 55 times brighter than a night on Earth illuminated by the light of the full moon. Only during lunar eclipses the night is on the near side darker than on Earth.

No person has been on the Moon during its night and experienced earthshine.[3]

Alternate usage

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• The term lunar day may also refer to the period between moonrises or high moon in a particular location on Earth. This period is typically about 50 minutes longer than a 24-hour Earth day, as the Moon orbits the Earth in the same direction as the Earth's axial rotation.[4]
• The term lunar day is also used in the context of night and day, i.e., opposite to the lunar night. This is common in discussions of the huge difference in temperatures, such as discussion about lunar rovers. For example, "the Soviet Union's Luna missions [...] were designed to survive one lunar day (two Earth weeks)",[5] while China's Yutu-2 rover, which landed in January 2019, was designed to survive lunar nights by shutting down.[6]

Lunar calendars

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In some lunar calendars, such as the Vikram Samvat, a lunar day, or tithi, is defined as 1/30 of a lunar month, or the time it takes for the longitudinal angle between the Moon and the Sun to increase by 12 degrees. By this definition, lunar days generally vary in duration.

See also

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• Lunisolar calendar
• Mars sol, the Martian day
• Synodic day

References

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1. ^ a b "Month". Encyclopædia Britannica. Retrieved 15 April 2015.
2. ^ Boyd, Patricia T.; Wilson, Emily L.; Smale, Alan P.; Supsinskas, Pete; Livengood, Timothy A.; Hewagama, Tilak; Villanueva, Geronimo L.; Marshak, Alexander; Krotkov, Nickolay A.; Pokorny, Petr; Bixler, Jay; Noland, Jonathan D.; Ramu, Guru; Cleveland, Paul; Ganino, John; Jhabvala, Murzy; Quintana, Elisa; Gilbert, Emily; Colón, Knicole; Arney, Giada N.; Domagal-Goldman, Shawn D.; Mandell, Avi; Barclay, Tom; Kuchner, Marc; Ott, Lesley (January 24, 2022). "EarthShine: Observing our world as an exoplanet from the surface of the Moon" (PDF). Journal of Astronomical Telescopes, Instruments, and Systems. 8 (01). doi:10.1117/1.JATIS.8.1.014003. ISSN 2329-4124. Retrieved October 19, 2025.
3. ^ Siegel, Ethan (March 18, 2017). "Ask Ethan: How Bright Is The Earth As Seen From The Moon?". Forbes. Retrieved April 29, 2025.
4. ^ "Frequency of Tides - The Lunar Day". NOAA. Retrieved 15 May 2019.
5. ^ "This is why lunar colonies will need to live underground".
6. ^ Urrutia, Doris Elin (16 May 2019). "Goodnight, Chang'e-4! China's Probe on Moon's Far Side Naps for Lunar Night". space.com. Future US, Inc. Retrieved 15 November 2019.

External links

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• Lunar days and other lunar data for many different cities. Lunarium.co.uk.
• Lunar Standard Time (LST) Archived 2017-11-14 at the Wayback Machine lunarclock.org.

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Physicalproperties	Internal structure Topography Atmosphere Gravity field Hill sphere Magnetic field Sodium tail Moonlight Earthlight
Orbit	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 Kordylewski clouds Lunar station
Surface andfeatures	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 Regolith 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 habitation Lunar resources
Time-telling and navigation	Lunar calendar Lunisolar calendar Month Lunar month Nodal period Fortnight Sennight Lunar station Lunar distance
Phases andnames	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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