Full moon

The supermoon of 14 November 2016 was 356,511 km (221,526 mi) away from the center of Earth. This occurs yearly.
As the Earth revolves around the Sun, approximate axial parallelism of the Moon's orbital plane (tilted five degrees to the Earth's orbital plane) results in the revolution of the lunar nodes relative to the Earth. This causes an eclipse season approximately every six months, in which a lunar eclipse can occur at the full moon phase.

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 (when the ecliptic longitudes of the Sun and Moon differ by 180°). 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.

The time interval between a full moon and the next repetition of the same phase, a synodic month, averages about 29.53 days. Therefore, in those lunar calendars in which each month begins on the day of the new moon, the full moon falls on either the 14th or 15th day of the lunar month. Because a calendar month consists of a whole number of days, a month in a lunar calendar may be either 29 or 30 days long.

Characteristics

A full moon is often thought of as an event of a full night's duration, although its phase seen from Earth continuously waxes or wanes, and is full only at the instant when waxing ends and waning begins. For any given location, about half of these maximum full moons may be visible, while the other half occurs during the day, when the full moon is below the horizon. As the Moon's orbit is inclined by 5.145° from the ecliptic, it is not generally perfectly opposite from the Sun during full phase, therefore a full moon is in general not perfectly full except on nights with a lunar eclipse as the Moon crosses the ecliptic at opposition from the Sun.

Many almanacs list full moons not only by date, but also by their exact time, usually in Coordinated Universal Time (UTC). Typical monthly calendars that include lunar phases may be offset by one day when prepared for a different time zone.

The full moon is generally a suboptimal time for astronomical observation of the Moon because shadows vanish. It is a poor time for other observations because the bright sunlight reflected by the Moon, amplified by the opposition surge, then outshines many stars.

Moon phases

There are eight phases of the moon, which vary from partial to full illumination. The moon phases are also called lunar phases. These stages have different names that come from its shape and size at each phase. For example, the crescent moon is 'banana' shaped, and the half-moon is D-shaped. When the moon is nearly full, it is called a gibbous moon. The crescent and gibbous moons each last approximately a week.

Each phase is also described in accordance to its position on the full 29.5 day cycle. The eight phases of the moon in order:

  • new moon
  • waxing crescent moon
  • first quarter moon
  • waxing gibbous moon
  • full moon
  • waning gibbous moon
  • last quarter moon
  • waning crescent moon

Formula

The date and approximate time of a specific full moon (assuming a circular orbit) can be calculated from the following equation:

where d is the number of days since 1 January 2000 00:00:00 in the Terrestrial Time scale used in astronomical ephemerides; for Universal Time (UT) add the following approximate correction to d:

days

where N is the number of full moons since the first full moon of 2000. The true time of a full moon may differ from this approximation by up to about 14.5 hours as a result of the non-circularity of the Moon's orbit. See New moon for an explanation of the formula and its parameters.

The age and apparent size of the full moon vary in a cycle of just under 14 synodic months, which has been referred to as a full moon cycle.

Lunar eclipses

When the Moon moves into Earth's shadow, a lunar eclipse occurs, during which all or part of the Moon's face may appear reddish due to the Rayleigh scattering of blue wavelengths and the refraction of sunlight through Earth's atmosphere. Lunar eclipses happen only during a full moon and around points on its orbit where the satellite may pass through the planet's shadow. A lunar eclipse does not occur every month because the Moon's orbit is inclined 5.145° with respect to the ecliptic plane of Earth; thus, the Moon usually passes north or south of Earth's shadow, which is mostly restricted to this plane of reference. Lunar eclipses happen only when the full moon occurs around either node of its orbit (ascending or descending). Therefore, a lunar eclipse occurs about every six months, and often two weeks before or after a solar eclipse, which occurs during a new moon around the opposite node.

In folklore and tradition

A full moon rising, seen through the Belt of Venus

In Buddhism, Vesak is celebrated on the full moon day of the Vaisakha month, marking the birth, enlightenment, and the death of the Buddha.

In Arabic, badr (بدر ) means 'full moon', but it is often translated as 'white moon', referring to The White Days, the three days when the full moon is celebrated.

Full moons are traditionally associated with insomnia (inability to sleep), insanity (hence the terms lunacy and lunatic) and various "magical phenomena" such as lycanthropy. Psychologists, however, have found that there is no strong evidence for effects on human behavior around the time of a full moon. They find that studies are generally not consistent, with some showing a positive effect and others showing a negative effect. In one instance, the 23 December 2000 issue of the British Medical Journal published two studies on dog bite admission to hospitals in England and Australia. The study of the Bradford Royal Infirmary found that dog bites were twice as common during a full moon, whereas the study conducted by the public hospitals in Australia found that they were less likely.

Symbol of the Triple Goddess

The symbol of the Triple Goddess is drawn with the circular image of the full moon in the center flanked by a left facing crescent and right facing crescent, on either side, representing a maiden, mother and crone archetype.

Full moon names

Historically, month names are names of moons (lunations, not necessarily full moons) in lunisolar calendars. Since the introduction of the solar Julian calendar in the Roman Empire, and later the Gregorian calendar worldwide, people no longer perceive month names as "moon" names. The traditional Old English month names were equated with the names of the Julian calendar from an early time, soon after Christianization, according to the testimony of Bede around AD 700.

Some full moons have developed new names in modern times, such as "blue moon", as well as "harvest moon" and "hunter's moon" for the full moons of autumn.

Lunar eclipses occur only at a full moon and often cause a reddish hue on the near side of the Moon. This full moon has been called a blood moon in popular culture.

Harvest and hunter's moons

A harvest moon. Its orange color is due to greater Rayleigh scattering as the Moon appears close above the horizon, rather than being unique to harvest moons.

The "harvest moon" and the "hunter's moon" are traditional names for the full moons in late summer and in the autumn in the Northern Hemisphere, usually in September and October, respectively. People may celebrate these occurrences in festivities such as the Chinese Mid-Autumn Festival, which is as important as the Chinese New Year.

The "harvest moon" (also known as the "barley moon" or "full corn moon") is the full moon nearest to the autumnal equinox (22 or 23 September), occurring anytime within two weeks before or after that date. The "hunter's moon" is the full moon following it. The names are recorded from the early 18th century. The Oxford English Dictionary entry for "harvest moon" cites a 1706 reference, and for "hunter's moon" a 1710 edition of The British Apollo, which attributes the term to "the country people" ("The Country People call this the Hunters-Moon.") The names became traditional in American folklore, where they are now often popularly attributed to Native Americans. The Feast of the Hunters' Moon is a yearly festival in West Lafayette, Indiana, held in late September or early October each year since 1968. In 2010 the harvest moon occurred on the night of the equinox itself (some 512 hours after the moment of equinox) for the first time since 1991, after a period known as the Metonic cycle.

All full moons rise around the time of sunset. Since the Moon moves eastward among the stars faster than the Sun, lunar culmination is delayed by about 50.47 minutes (on average) each day, thus causing moonrise to occur later each day.

Due to the high lunar standstill, the harvest and hunter's moons of 2007 were special because the time difference between moonrises on successive evenings was much shorter than average. The moon rose about 30 minutes later from one night to the next, as seen from about 40° N or S latitude (because the full moon of September 2007 rose in the northeast rather than in the east). Hence, no long period of darkness occurred between sunset and moonrise for several days after the full moon, thus lengthening the time in the evening when there is enough twilight and moonlight to work to get the harvest in.

Farmers' Almanacs

The Maine Farmers' Almanac from around the 1930s began to publish Native American "Indian" full moon names, some of which had been adopted by colonial Americans. The Farmers' Almanac (since 1955 published in Maine, but not the same publication as the Maine Farmers' Almanac) continues to do so.

An early list of "Indian month names" was published in 1918 by Daniel Carter Beard in his The American Boy's Book of Signs, Signals and Symbols for use by the boy scouts.

Such names have gained currency in American folklore. They appear in print more widely outside of the almanac tradition from the 1990s in popular publications about the Moon. Mysteries of the Moon by Patricia Haddock ("Great Mysteries Series", Greenhaven Press, 1992) gave an extensive list of such names along with the individual tribal groups they were supposedly associated with. Haddock supposes that certain "Colonial American" moon names were adopted from Algonquian languages (which were formerly spoken in the territory of New England), while others are based in European tradition (e.g. the Colonial American names for the May moon, "Milk Moon", "Mother's Moon", "Hare Moon" have no parallels in the supposed native names, while the name of November, "Beaver Moon" is supposedly based in an Algonquian language).

The Long Night's Moon is the last full moon of the year and the one nearest the winter solstice.

"Ice Moon" is also used to refer to the first full moon of January or February.

Full moon names the Farmer's Almanac
Month Names according to Beard Names from the Farmers Almanac[clarification needed] Other names
January Difficulty Black Smoke Wolf Moon Old Moon Moon After Yule Winter Moon
February Raccoon Bare Spots on the Ground Snow Moon Hunger Moon Storm Moon
March Wind Little Grass, Sore-Eye Worm Moon Crow Moon Sap Moon Crust Moon Lenten Moon, Wind Moon
April Ducks Goose-Eggs Pink Moon Seed Moon, Sprouting Grass Moon Fish Moon, Frog Moon Spring Moon, Awakening Moon Sap Moon
May Green Grass Root-Food Flower Moon Milk Moon Corn Planting Moon Grass Moon Mother's Moon
June Corn-Planting Strawberry Strawberry Moon Mead Moon Rose Moon Hot Moon Thunder Moon
July Buffalo (Bull) Hot Sun Buck Moon Hay Moon Elk Moon Summer Moon Thunder Moon
August Harvest Cow Buffalo Sturgeon Moon Red Moon Corn Moon Barley, Green Corn, Herb, or Grain Moon Dog Moon
September Wild Rice Red Plum Harvest Moon Full Corn Moon Fruit Moon Barley Moon
October Leaf-Falling Nuts Hunters' Moon Blood or Sanguine Moon Autumn Moon Pumpkin Moon Dying Moon
November Deer-Mating Fur-Pelts Beaver Moon Frosty Moon Dark Moon
December Wolves Big Moon Cold Moon Oak Moon Long Night's Moon
13th Blue Moon

Hindu full moon festivals

In Hinduism, most festivals are celebrated on auspicious days. Many Hindu festivals are celebrated on days with a full moon night, called the purnima. Different parts of India celebrate the same festival with different names, as listed below:

  1. Chaitra Purnima – Gudi Padua, Ugadi, Hanuman Jayanti (15 April 2014)
  2. Vaishakha Purnima – Narasimha Jayanti, Buddha Jayanti (14 May 2014)
  3. Jyeshtha Purnima – Savitri Vrata, Vat Purnima (8 June 2014)
  4. Ashadha Purnima – Guru Purnima, Vyasa Purnima
  5. Shravana Purnima – Upanayana ceremony, Avani Avittam, Raksha Bandhan, Onam
  6. Bhadrapada Purnima – Start of Pitru Paksha, Madhu Purnima
  7. Ashvin Purnima – Sharad Purnima
  8. Kartika PurnimaKarthikai Deepam, Thrukkarthika
  9. Margashirsha Purnima – Thiruvathira, Dattatreya Jayanti
  10. Pushya Purnima – Thaipusam, Shakambhari Purnima
  11. Magha Purnima
  12. Phalguna Purnima – Holi

Lunar and lunisolar calendars

The December 2015 full moon coincided with Christmas. This last occurred in 1977 (for the American timezones). A small horizontal libration is visible comparing their appearances. By the 19-year metonic cycle the full moon will repeat on Christmas Day in 2034, 2053, 2072, and 2091.

Most pre-modern calendars the world over were lunisolar, combining the solar year with the lunation by means of intercalary months. The Julian calendar abandoned this method in favour of a purely solar reckoning while conversely the 7th-century Islamic calendar opted for a purely lunar one.

A continuing lunisolar calendar is the Hebrew calendar. Evidence of this is noted in the dates of Passover and Easter in Judaism and Christianity, respectively. Passover falls on the full moon on 15 Nisan of the Hebrew calendar. The date of the Jewish Rosh Hashana and Sukkot festivals along with all other Jewish holidays are dependent on the dates of the new moons.

Intercalary months

In lunisolar calendars, an intercalary month occurs seven times in the 19 years of the Metonic cycle, or on average every 2.7 years (19/7). In the Hebrew calendar this is noted with a periodic extra month of Adar in the early spring.

Blue moon

In the modern system of "traditional" American full moon names tied to the solstice and equinox points, a supernumerary full moon in such a period is called a blue moon. The term "blue moon" used in this sense may date to as early as the 16th century, but it became well known in the United States due to the Farmers' Almanac (published since 1818).

According to the pattern of use in the Farmers' Almanac, a "blue moon" is the third full moon in any period between either a solstice and an equinox, or between an equinox and a solstice, (calculated using the mean tropical year), which contains four full moons. These seasons are equal in length, unlike the astronomical ones, which vary in length depending on the Earth's speed in its elliptical orbit round the Sun. To compare, in 1983 the equal-length mean-solar solar points and the actual astronomical (observed) dates are shown in the table below (all dates and times in GMT):

Event Fictitious equal-length-season date Actual astronomical date Error (approximate)
Spring equinox 1:48am, 23 March 1983 4:39am, 21 March 1983 −2 days
Summer solstice 9:15am, 22 June 1983 11:09pm, 21 June 1983 −1.5 days
Fall equinox 4:42pm, 21 September 1983 2:42pm, 23 September 1983 +2 days
Winter solstice   12:10am, 22 December 1983 10:30am, 22 December 1983 −2 hours

As a consequence of checking an inadequate number of old issues of the Farmers' Almanac, the author of an article in the March 1946 issue of Sky & Telescope magazine wrongly concluded that the Farmers' Almanac had used "blue moon" to denote "the second full moon in any month which contains two full moons".

The mistaken rule was retracted and declared "erroneous" in a 1999 Sky & Telescope article, which gave the corrected rule, based on order in seasons.

Using the original meaning, "blue moons" occur with the same average frequency of intercalary months, 7 times in 19 years; the Farmers' Almanac system of full moon names effectively defines a functioning luni-solar calendar. Because the Sky & Telescope definition depends on calendar months and because February is shorter than a lunar month, there will be a higher frequency of blue moons under that definition (in years in which February squeezes in between two full moons), so that blue moons occur on average about 8 times in 19 years.

It is a rare phenomenon to see an unusual blue color of the moon (not necessarily a full moon) when viewing the Moon. This phenomenon is caused by dust particles or smoke in the atmosphere, and was seen after the forest fires in Sweden and Canada in 1950 and 1951. In 1883, after the eruption of Krakatoa in Indonesia, the Moon was blue for almost two years. Other less violent volcanic explosions have been followed by blue moons. The blue Moon was also seen after the eruption of El Chichon in Mexico in 1983, Mount St. Helens in 1980, and Mount Pinatubo in 1991.

See also

Footnotes

  1. ^ The saying "once in a blue moon" meaning "very rarely" is recorded since the 1820s. The term "blue moon" is recorded in 1528, in the couplet Oh churche men are wyly foxes [...] Yf they say the mone is blewe / We must beleve that it is true / Admittynge their interpretacion.

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