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Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System.[33][34] Historically, several formation mechanisms have been proposed,[35] but none satisfactorily explains the features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force[36] would require too great an initial rotation rate of Earth.[37] Gravitational capture of a pre-formed Moon[38] depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon.[37] A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon.[37] None of these hypotheses can account for the high angular momentum of the Earth–Moon system.[39]

The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body (named Theia) with the proto-Earth. The impact blasted material into orbit about the Earth and the material accreted and formed the Moon[40][41] just beyond the Earth's Roche limit of ~2.56 R🜨.[42]

Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of the Moon derived from the impactor, rather than the proto-Earth.[43] However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth.[44][45][46][47] Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the v***rized material that formed the two,[48] although this is debated.[49]

The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system.[50][51] Similarly, the newly formed Moon would have had its own lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).[50]

While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.[52][example needed]Above a high resolution threshold for simulations, a study published in 2022 finds that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits.[53]
The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 tonnes (9.8 long tons; 11 short tons).[80] The surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa); it varies with the lunar day. Its sources include outgassing and sputtering, a product of the bombardment of lunar soil by solar wind ions.[15][81] Elements that have been detected include sodium and potassium, produced by sputtering (also found in the atmospheres of Mercury and Io); helium-4 and neon[82] from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle.[83][84] The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood.[83] Water v***r has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith.[85] These gases either return into the regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field.[83]

Studies of Moon magma samples retrieved by the Apollo missions demonstrate that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space.[86]

A permanent Moon dust cloud exists around the Moon, generated by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising up to 100 kilometers above the surface. Dust counts made by LADEE's Lunar Dust EXperiment (LDEX) found particle counts peaked during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being more dense near the boundary between the Moon's dayside and nightside.[87][88]

The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet,[f] with a diameter about one-quarter that of Earth (comparable to the width of Australia).[16] The Moon is a planetary-mass object with a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term and larger than all known dwarf planets of the Solar System.[17] It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's at 0.1654 g, with Jupiter's moon Io being the only satellite in the Solar System known to have a higher surface gravity and density.

The Moon orbits Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter. Its gravitational influence is the main driver of Earth's tides and very slowly lengthens Earth's day. The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. However, 59% of the total lunar surface can be seen from Earth through cyclical shifts in perspective known as libration.

The most widely accepted origin explanation posits that the Moon formed 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic maria ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is fairly non-reflective, with the reflectance of lunar soil being comparable to that of asphalt. However, due to its large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.

Both the Moon's prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union's Luna 2 uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history.

Vegetarianism is the practice of abstaining from the consumption of meat (red meat, poultry, seafood, insects, and the flesh of any other animal). It may also include abstaining from eating all by-products of animal slaughter.[1][2]

Vegetarianism may be adopted for various reasons. Many people object to eating meat out of respect for sentient animal life. Such ethical motivations have been codified under various religious beliefs as well as animal rights advocacy. Other motivations for vegetarianism are health-related, political, environmental, cultural, aesthetic, economic, taste-related, or relate to other personal preferences.

There are many variations of the vegetarian diet: an ovo-lacto vegetarian diet includes both eggs and dairy products, an ovo-vegetarian diet includes eggs but not dairy products, and a lacto-vegetarian diet includes dairy products but not eggs. As the strictest of vegetarian diets, a vegan diet excludes all animal products, and can be accompanied by abstention from the use of animal-derived products, such as leather shoes.

Maintenance of a vegetarian diet can be challenging. While avoidance of animal products may support health and ethical concerns, dietary supplements may be needed to prevent nutritional deficiency if all such products are shunned, particularly for vitamin B12. Packaged and processed foods may contain minor quantities of animal ingredients.[2][3] While some vegetarians scrutinize product labels for such ingredients, others do not object to consuming them, or are unaware of their presence.[2][4][5]

Shoeing

A donkey shoe with calkins

Farriers shoeing a donkey in Cyprus in 1900
Donkey hooves are more elastic than those of horses, and do not naturally wear down as fast. Regular clipping may be required; neglect can lead to permanent damage.[6] Working donkeys may need to be shod. Donkey shoes are similar to horseshoes, but usually smaller and without toe-clips.

Nutrition
In their native arid and semi-arid climates, donkeys spend more than half of each day foraging and feeding, often on poor quality scrub.[67] The donkey has a tough digestive system in which roughage is efficiently broken down by hind gut fermentation, microbial action in the caecum and large intestine.[67] While there is no marked structural difference between the gastro-intestinal tract of a donkey and that of a horse, the digestion of the donkey is more efficient. It needs less food than a horse or pony of comparable height and weight,[68] approximately 1.5 percent of body weight per day in dry matter,[69] compared to the 2–2.5 percent consumption rate possible for a horse.[70] Donkeys are also less prone to colic.[71] The reasons for this difference are not fully understood; the donkey may have different intestinal flora to the horse, or a longer gut retention time.[72]

Donkeys obtain most of their energy from structural carbohydrates. Some suggest that a donkey needs to be fed only straw (preferably barley straw), supplemented with controlled grazing in the summer or hay in the winter,[73] to get all the energy, protein, fat and vitamins it requires; others recommend some grain to be fed, particularly to working animals,[6] and others advise against feeding straw.[16] They do best when allowed to consume small amounts of food over long periods. They can meet their nutritional needs on 6 to 7 hours of grazing per day on average dryland pasture that is not stressed by drought. If they are worked long hours or do not have access to pasture, they require hay or a similar dried forage, with no more than a 1:4 ratio of legumes to grass. They also require salt and mineral supplements, and access to clean, fresh water.[74] In temperate climates the forage available is often too abundant and too rich; over-feeding may cause weight gain and obesity, and lead to metabolic disorders such as founder (laminitis[75]) and hyperlipaemia,[73] or to gastric ulcers.[76]

Throughout the world, working donkeys are associated with the very poor, with those living at or below subsistence level.[54] Few receive adequate food, and in general donkeys throughout the Third World are under-nourished and over-worked.[77]

Feral populations

Feral b***os in Red Rock Canyon
In some areas domestic donkeys have returned to the wild and established feral populations such as those of the b***o of North America and the Asinara donkey of Sardinia, Italy, both of which have protected status.[citation needed] Feral donkeys can also cause problems, notably in environments that have evolved free of any form of equid, such as Hawaii.[78] In Australia, where there may be 5 million feral donkeys,[26] they are regarded as an invasive pest and have a serious impact on the environment. They may compete with livestock and native animals for resources, spread weeds and diseases, foul or damage watering holes and cause erosion.[79]

Donkey hybrids
The earliest documented donkey hybrid was the kunga, which was used as a draft animal in the Syrian and Mesopotamian kingdoms of the second half of the 3rd millennium BCE. A cross between a captive male Syrian wild ass and a female domesticated donkey (jenny), they represent the earliest known example of human-directed animal hybridization. They were produced at a breeding center at Nagar (modern Tell Brak) and were sold or given as gifts throughout the region, where they became significant status symbols, pulling battle wagons and the chariots of kings, and also being sacrificed to bury with high-status people. They fell out of favor following the introduction of the domestic horse and its donkey hybrid, the mule, into the region at the end of the 3rd millennium BCE.[80]

A male donkey (jack) crossed with a female horse produces a mule, while a male horse crossed with a jenny produces a hinny. Horse-donkey hybrids are almost always sterile because of a failure of their developing gametes to complete meiosis.[81] The lower progesterone production of the jenny may also lead to early embryonic loss. In addition, there are reasons not directly related to reproductive biology. Due to different mating behavior, jacks are often more willing to cover mares than stallions are to breed jennies. Further, mares are usually larger than jennies and thus have more room for the ensuing foal to grow in the womb, resulting in a larger animal at birth. It is commonly believed that mules are more easily handled and also physically stronger than hinnies, making them more desirable for breeders to produce.[citation needed]

The offspring of a zebra-donkey cross is called a zonkey, zebroid, zebrass, or zedonk;[82] zebra mule is an older term, but still used in some regions today. The foregoing terms generally refer to hybrids produced by breeding a male zebra to a female donkey. Zebra hinny, zebret and zebrinny all refer to the cross of a female zebra with a male donkey. Zebrinnies are rarer than zedonkies because female zebras in captivity are most valuable when used to produce full-blooded zebras.[83] There are not enough female zebras breeding in captivity to spare them for hybridizing; there is no such limitation on the number of female donkeys breeding.

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