Lunar day

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A full lunar day observed from the Earth, where orbital libration causes the apparent wobble. Lunation animation November 2009.gif
A full lunar day observed from the Earth, where orbital libration causes the apparent wobble.

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 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, returning to the same lunar phase. The synodic period is about 29+12 Earth days, which is about 2.2 days longer than its sidereal period.

Contents

Main definition

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 EarthMoon 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 EarthMoon 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.

Alternate usage

Lunar calendars

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

Related Research Articles

A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours. As a day passes at a given location it experiences morning, noon, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes.

<span class="mw-page-title-main">Full moon</span> Lunar phase: completely illuminated disc

The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon. This means that the lunar hemisphere facing Earth—the near side—is completely sunlit and appears as an approximately circular disk. The full moon occurs roughly once a month.

A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon; the words month and Moon are cognates. The traditional concept of months arose with the cycle of Moon phases; such lunar months ("lunations") are synodic months and last approximately 29.53 days, making for roughly 12.37 such months in one Earth year. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period with respect to the Earth–Sun line, are still the basis of many calendars today and are used to divide the year.

<span class="mw-page-title-main">Lunar phase</span> Shape of the Moons sunlit portion as viewed from Earth

A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. In common usage, the four major phases are the new moon, the first quarter, the full moon and the last quarter; the four minor phases are waxing crescent, waxing gibbous, waning gibbous, and waning crescent. A lunar month is the time between successive recurrences of the same phase: due to the eccentricity of the Moon's orbit, this duration is not perfectly constant but averages about 29.5 days.

<span class="mw-page-title-main">Year</span> Time of one planets orbit around a star

A year is the time taken for astronomical objects to complete one orbit. For example, a year on Earth is the time taken for Earth to revolve around the Sun. Generally, a year is taken to mean a calendar year, but the word is also used for periods loosely associated with the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. The term can also be used in reference to any long period or cycle, such as the Great Year.

<span class="mw-page-title-main">Eclipse cycle</span> Calculation and prediction of eclipses

Eclipses may occur repeatedly, separated by certain intervals of time: these intervals are called eclipse cycles. The series of eclipses separated by a repeat of one of these intervals is called an eclipse series.

The saros is a period of exactly 223 synodic months, approximately 6585.321 days, or 18 years, 10, 11, or 12 days, and 8 hours, that can be used to predict eclipses of the Sun and Moon. One saros period after an eclipse, the Sun, Earth, and Moon return to approximately the same relative geometry, a near straight line, and a nearly identical eclipse will occur, in what is referred to as an eclipse cycle. A sar is one half of a saros.

The orbital period is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars. It may also refer to the time it takes a satellite orbiting a planet or moon to complete one orbit.

A synodic day is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time.

<span class="mw-page-title-main">Timekeeping on Mars</span> Proposed approaches to tracking date and time on the planet Mars

Though no standard exists, numerous calendars and other timekeeping approaches have been proposed for the planet Mars. The most commonly seen in the scientific literature denotes the time of year as the number of degrees on its orbit from the northward equinox, and increasingly there is use of numbering the Martian years beginning at the equinox that occurred April 11, 1955.

<span class="mw-page-title-main">Unit of time</span> Measurement unit for time

A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1."

<span class="mw-page-title-main">Lunar standstill</span> Moon stops moving north or south

A lunar standstill or lunistice is when the Moon reaches its furthest north or furthest south point during the course of a month. The declination at lunar standstill varies in a cycle 18.6 years long between 18.134° and 28.725°, due to lunar precession. These extremes are called the minor and major lunar standstills.

<span class="mw-page-title-main">Solar eclipse</span> Natural phenomenon wherein the Sun is obscured by the Moon

A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season in its new moon phase, when the Moon's orbital plane is closest to the plane of the Earth's orbit. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured. Unlike a lunar eclipse, which may be viewed from anywhere on the night side of Earth, a solar eclipse can only be viewed from a relatively small area of the world. As such, although total solar eclipses occur somewhere on Earth every 18 months on average, they recur at any given place only once every 360 to 410 years.

<span class="mw-page-title-main">Orbit of the Moon</span> The Moons circuit around Earth

The Moon orbits Earth in the prograde direction and completes one revolution relative to the Vernal Equinox and the stars in about 27.32 days and one revolution relative to the Sun in about 29.53 days. Earth and the Moon orbit about their barycentre, which lies about 4,670 km (2,900 mi) from Earth's centre, forming a satellite system called the Earth–Moon system. On average, the distance to the Moon is about 385,000 km (239,000 mi) from Earth's centre, which corresponds to about 60 Earth radii or 1.282 light-seconds.

<span class="mw-page-title-main">June 2029 lunar eclipse</span> Future lunar eclipse

A total lunar eclipse will take place between Monday and Tuesday, June 25-26, 2029. A central total eclipse lasting 1 hour and 41 minutes 53 seconds will plunge the full Moon into deep darkness, as it passes right through the centre of the Earth's umbral shadow. While the visual effect of a total eclipse is variable, the Moon may be stained a deep orange or red color at maximum eclipse. It will be able to be seen from most of the Americas, Western Europe and Africa. The partial eclipse will last for 3 hours and 39 minutes 32 seconds in total.

A total lunar eclipse took place at the Moon's descending node of the orbit on Tuesday, May 24, 1910 with an umbral eclipse magnitude of 1.09503. A total lunar eclipse takes place when the Earth comes between the Sun and the Moon and its shadow covers the Moon. Eclipse watchers can see the Moon turn red when the eclipse reaches totality. Total eclipses of the Moon happen at Full Moon when the Sun, Earth, and Moon are aligned to form a line. The astronomical term for this type of alignment is syzygy, which comes from the Greek word for being paired together. The Moon does not have its own light but shines because its surface reflects the Sun's rays. During a total lunar eclipse, the Earth comes between the Sun and the Moon and blocks any direct sunlight from reaching the Moon. The Sun casts the Earth's shadow on the Moon's surface. A shallow total eclipse saw the Moon in relative darkness for 49 minutes and 29.5 seconds. The Moon was 9.503% of its diameter into the Earth's umbral shadow, and should have been significantly darkened. The partial eclipse lasted for 3 hours, 35 minutes and 22.9 seconds in total.

<span class="mw-page-title-main">Solar eclipse of December 4, 1983</span> 20th-century annular solar eclipse

An annular solar eclipse occurred at the Moon's descending node of the orbit on December 4, 1983. A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partly obscuring the image of the Sun for a viewer on Earth. An annular solar eclipse occurs when the Moon's apparent diameter is smaller than the Sun's, blocking most of the Sun's light and causing the Sun to look like an annulus (ring). An annular eclipse appears as a partial eclipse over a region of the Earth thousands of kilometres wide. Annularity was visible in Cape Verde, Annobón Island of Equatorial Guinea, Gabon, the People's Republic of Congo, Zaire, northern Uganda, southern Sudan, northwestern Kenya, Ethiopia and Somalia. The Sun's altitude was 66°. Occurring 6.5 days before apogee, the Moon's apparent diameter was near the average diameter.

<span class="mw-page-title-main">Solar eclipse of December 25, 1935</span> 20th-century annular solar eclipse

An annular solar eclipse occurred on December 25, 1935. A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partly obscuring the image of the Sun for a viewer on Earth. An annular solar eclipse occurs when the Moon's apparent diameter is smaller than the Sun's, blocking most of the Sun's light and causing the Sun to look like an annulus (ring). An annular eclipse appears as a partial eclipse over a region of the Earth thousands of kilometres wide. This was the 5th solar eclipse in 1935, the maximum possible. The next time this will occur is 2206.

<span class="mw-page-title-main">Eclipse season</span> Period when eclipses can occur

An eclipse season is a period, roughly every six months, when eclipses occur. Eclipse seasons are the result of the axial parallelism of the Moon's orbital plane, just as Earth's weather seasons are the result of the axial parallelism of Earth's tilted axis as it orbits around the Sun. During the season, the "lunar nodes" – the line where the Moon's orbital plane intersects with the Earth's orbital plane – align with the Sun and Earth, such that a solar eclipse is formed during the new moon phase and a lunar eclipse is formed during the full moon phase.

<span class="mw-page-title-main">Lunar month</span> Time between successive new moons

In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.

References

  1. 1 2 "Month". Encyclopædia Britannica . Retrieved 15 April 2015.
  2. "Frequency of Tides - The Lunar Day". NOAA . Retrieved 15 May 2019.
  3. "This is why lunar colonies will need to live underground".
  4. 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.