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290_0 | A [SEP] A |
290_1 | A [SEP] A (named , plural "As", "A's", "a"s, "a's" or "aes") is the first letter and the first vowel of the modern English alphabet and the ISO basic Latin alphabet. It is similar to the Ancient Greek letter alpha, from which it derives. The uppercase version consists of the two slanting sides of a triangle, crossed in the middle by a horizontal bar. The lowercase version can be written in two forms: the double-storey a and single-storey ɑ. The latter is commonly used in handwriting and fonts based on it, especially fonts intended to be read by children, and is also |
290_2 | A [SEP] found in italic type. |
290_3 | A [SEP] In the English grammar, "a", and its variant "an", is an indefinite article. |
290_4 | A [SEP] Section::::History. |
290_5 | A [SEP] The earliest certain ancestor of "A" is aleph (also written 'aleph), the first letter of the Phoenician alphabet, which consisted entirely of consonants (for that reason, it is also called an abjad to distinguish it from a true alphabet). In turn, the ancestor of aleph may have been a pictogram of an ox head in proto-Sinaitic script influenced by Egyptian hieroglyphs, styled as a triangular head with two horns extended. |
290_6 | A [SEP] By 1600 BC, the Phoenician alphabet letter had a linear form that served as the base for some later forms. Its name is thought to have corresponded closely to the Paleo-Hebrew or Arabic aleph. |
290_7 | A [SEP] When the ancient Greeks adopted the alphabet, they had no use for a letter to represent the glottal stop—the consonant sound that the letter denoted in Phoenician and other Semitic languages, and that was the first phoneme of the Phoenician pronunciation of the letter—so they used their version of the sign to represent the vowel , and called it by the similar name of alpha. In the earliest Greek inscriptions after the Greek Dark Ages, dating to the 8th century BC, the letter rests upon its side, but in the Greek alphabet of later times it generally resembles the modern |
290_8 | A [SEP] capital letter, although many local varieties can be distinguished by the shortening of one leg, or by the angle at which the cross line is set. |
290_9 | A [SEP] The Etruscans brought the Greek alphabet to their civilization in the Italian Peninsula and left the letter unchanged. The Romans later adopted the Etruscan alphabet to write the Latin language, and the resulting letter was preserved in the Latin alphabet that would come to be used to write many languages, including English. |
290_10 | A [SEP] Section::::History.:Typographic variants. |
290_11 | A [SEP] During Roman times, there were many variant forms of the letter "A". First was the monumental or lapidary style, which was used when inscribing on stone or other "permanent" media. There was also a cursive style used for everyday or utilitarian writing, which was done on more perishable surfaces. Due to the "perishable" nature of these surfaces, there are not as many examples of this style as there are of the monumental, but there are still many surviving examples of different types of cursive, such as majuscule cursive, minuscule cursive, and semicursive minuscule. Variants also existed that were intermediate between |
290_12 | A [SEP] the monumental and cursive styles. The known variants include the early semi-uncial, the uncial, and the later semi-uncial. |
290_13 | A [SEP] At the end of the Roman Empire (5th century AD), several variants of the cursive minuscule developed through Western Europe. Among these were the semicursive minuscule of Italy, the Merovingian script in France, the Visigothic script in Spain, and the Insular or Anglo-Irish semi-uncial or Anglo-Saxon majuscule of Great Britain. By the 9th century, the Caroline script, which was very similar to the present-day form, was the principal form used in book-making, before the advent of the printing press. This form was derived through a combining of prior forms. |
290_14 | A [SEP] 15th-century Italy saw the formation of the two main variants that are known today. These variants, the "Italic" and "Roman" forms, were derived from the Caroline Script version. The Italic form, also called "script a," is used in most current handwriting and consists of a circle and vertical stroke. This slowly developed from the fifth-century form resembling the Greek letter tau in the hands of medieval Irish and English writers. The Roman form is used in most printed material; it consists of a small loop with an arc over it ("a"). Both derive from the majuscule (capital) form. In Greek |
290_15 | A [SEP] handwriting, it was common to join the left leg and horizontal stroke into a single loop, as demonstrated by the uncial version shown. Many fonts then made the right leg vertical. In some of these, the serif that began the right leg stroke developed into an arc, resulting in the printed form, while in others it was dropped, resulting in the modern handwritten form. |
290_16 | A [SEP] Italic type is commonly used to mark emphasis or more generally to distinguish one part of a text from the rest (set in Roman type). There are some other cases aside from italic type where "script a" ("ɑ"), also called Latin alpha, is used in contrast with Latin "a" (such as in the International Phonetic Alphabet). |
290_17 | A [SEP] Section::::Use in writing systems. |
290_18 | A [SEP] Section::::Use in writing systems.:English. |
290_19 | A [SEP] In modern English orthography, the letter represents at least seven different vowel sounds: |
290_20 | A [SEP] BULLET::::- the near-open front unrounded vowel as in "pad"; |
290_21 | A [SEP] BULLET::::- the open back unrounded vowel as in "father", which is closer to its original Latin and Greek sound; |
290_22 | A [SEP] BULLET::::- the diphthong as in "ace" and "major" (usually when is followed by one, or occasionally two, consonants and then another vowel letter) – this results from Middle English lengthening followed by the Great Vowel Shift; |
290_23 | A [SEP] BULLET::::- the modified form of the above sound that occurs before, as in "square" and "Mary"; |
290_24 | A [SEP] BULLET::::- the rounded vowel of "water"; |
290_25 | A [SEP] BULLET::::- the shorter rounded vowel (not present in General American) in "was" and "what"; |
290_26 | A [SEP] BULLET::::- a schwa, in many unstressed syllables, as in "about", "comma", "solar". |
290_27 | A [SEP] The double sequence does not occur in native English words, but is found in some words derived from foreign languages such as "Aaron" and "aardvark". However, occurs in many common digraphs, all with their own sound or sounds, particularly , , , , and . |
290_28 | A [SEP] Section::::Use in writing systems.:Other languages. |
290_29 | A [SEP] In most languages that use the Latin alphabet, denotes an open unrounded vowel, such as , , or . An exception is Saanich, in which (and the glyph Á) stands for a close-mid front unrounded vowel . |
290_30 | A [SEP] Section::::Use in writing systems.:Other systems. |
290_31 | A [SEP] In phonetic and phonemic notation: |
290_32 | A [SEP] BULLET::::- in the International Phonetic Alphabet, is used for the open front unrounded vowel, is used for the open central unrounded vowel, and is used for the open back unrounded vowel. |
290_33 | A [SEP] BULLET::::- in X-SAMPA, is used for the open front unrounded vowel and is used for the open back unrounded vowel. |
290_34 | A [SEP] Section::::Other uses. |
290_35 | A [SEP] In algebra, the letter "a" along with other letters at the beginning of the alphabet is used to represent known quantities, whereas the letters at the end of the alphabet ("x", "y", "z") are used to denote unknown quantities. |
290_36 | A [SEP] In geometry, capital A, B, C etc. are used to denote segments, lines, rays, etc. A capital A is also typically used as one of the letters to represent an angle in a triangle, the lowercase a representing the side opposite angle A. |
290_37 | A [SEP] "A" is often used to denote something or someone of a better or more prestigious quality or status: A-, A or A+, the best grade that can be assigned by teachers for students' schoolwork; "A grade" for clean restaurants; A-list celebrities, etc. Such associations can have a motivating effect, as exposure to the letter A has been found to improve performance, when compared with other letters. |
290_38 | A [SEP] "A" is used as a prefix on some words, such as asymmetry, to mean "not" or "without" (from Greek). |
290_39 | A [SEP] In English grammar, "a", and its variant "an", is an indefinite article. |
290_40 | A [SEP] Finally, the letter A is used to denote size, as in a narrow size shoe, or a small cup size in a brassiere. |
290_41 | A [SEP] Section::::Related characters. |
290_42 | A [SEP] Section::::Related characters.:Descendants and related characters in the Latin alphabet. |
290_43 | A [SEP] BULLET::::- Æ æ : Latin "AE" ligature |
290_44 | A [SEP] BULLET::::- A with diacritics: Å å Ǻ ǻ Ḁ ḁ ẚ Ă ă Ặ ặ Ắ ắ Ằ ằ Ẳ ẳ Ẵ ẵ Ȃ ȃ Â â Ậ ậ Ấ ấ Ầ ầ Ẫ ẫ Ẩ ẩ Ả ả Ǎ ǎ Ⱥ ⱥ Ȧ ȧ Ǡ ǡ Ạ ạ Ä ä Ǟ ǟ À à Ȁ ȁ Á á Ā ā Ā̀ ā̀ Ã ã Ą ą Ą́ ą́ Ą̃ ą̃ A̲ a̲ ᶏ |
290_45 | A [SEP] BULLET::::- Phonetic alphabet symbols related to A (the International Phonetic Alphabet only uses lowercase, but uppercase forms are used in some other writing systems): |
290_46 | A [SEP] BULLET::::- Ɑ ɑ : Latin letter alpha / script A, which represents an open back unrounded vowel in the IPA |
290_47 | A [SEP] BULLET::::- ᶐ : Latin small letter alpha with retroflex hook |
290_48 | A [SEP] BULLET::::- Ɐ ɐ : Turned A, which represents a near-open central vowel in the IPA |
290_49 | A [SEP] BULLET::::- Λ ʌ : Turned V (also called a wedge, a caret, or a hat), which represents an open-mid back unrounded vowel in the IPA |
290_50 | A [SEP] BULLET::::- Ɒ ɒ : Turned alpha / script A, which represents an open back rounded vowel in the IPA |
290_51 | A [SEP] BULLET::::- ᶛ : Modifier letter small turned alpha |
290_52 | A [SEP] BULLET::::- ᴀ : Small capital A, an obsolete or non-standard symbol in the International Phonetic Alphabet used to represent various sounds (mainly open vowels) |
290_53 | A [SEP] BULLET::::- ᴬ ᵃ ᵄ : Modifier letters are used in the Uralic Phonetic Alphabet (UPA) |
290_54 | A [SEP] BULLET::::- ₐ : Subscript small a is used in Indo-European studies |
290_55 | A [SEP] BULLET::::- ꬱ : Small letter a reversed-schwa is used in the Teuthonista phonetic transcription system |
290_56 | A [SEP] BULLET::::- Ꞻ ꞻ : Glottal A, used in the transliteration of Ugaritic |
290_57 | A [SEP] Section::::Related characters.:Derived signs, symbols and abbreviations. |
290_58 | A [SEP] BULLET::::- ª : an ordinal indicator |
290_59 | A [SEP] BULLET::::- Å : Ångström sign |
290_60 | A [SEP] BULLET::::- ∀ : a turned capital letter A, used in predicate logic to specify universal quantification ("for all") |
290_61 | A [SEP] BULLET::::- @ : At sign |
290_62 | A [SEP] BULLET::::- ₳ : Argentine austral |
290_63 | A [SEP] Section::::Related characters.:Ancestors and siblings in other alphabets. |
290_64 | A [SEP] BULLET::::- 𐤀 : Semitic letter Aleph, from which the following symbols originally derive |
290_65 | A [SEP] BULLET::::- Α α : Greek letter Alpha, from which the following letters derive |
290_66 | A [SEP] BULLET::::- А а : Cyrillic letter A |
290_67 | A [SEP] BULLET::::- : Coptic letter Alpha |
290_68 | A [SEP] BULLET::::- 𐌀 : Old Italic A, which is the ancestor of modern Latin A |
290_69 | A [SEP] BULLET::::- : Runic letter ansuz, which probably derives from old Italic A |
290_70 | A [SEP] BULLET::::- : Gothic letter aza/asks |
290_71 | A [SEP] Section::::External links. |
290_72 | A [SEP] BULLET::::- History of the Alphabet |
39_0 | Albedo [SEP] Albedo |
39_1 | Albedo [SEP] Albedo () (, meaning 'whiteness') is the measure of the diffuse reflection of solar radiation out of the total solar radiation received by an astronomical body (e.g. a planet like Earth). It is dimensionless and measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radiation). |
39_2 | Albedo [SEP] Surface albedo is defined as the ratio of radiosity to the irradiance (flux per unit area) received by a surface. The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface. These factors vary with atmospheric composition, geographic location and time (see position of the Sun). While bi-hemispherical reflectance is calculated for a single angle of incidence (i.e., for a given position of the Sun), albedo is the directional integration of reflectance over all solar angles in a given period. The temporal resolution |
39_3 | Albedo [SEP] may range from seconds (as obtained from flux measurements) to daily, monthly, or annual averages. |
39_4 | Albedo [SEP] Unless given for a specific wavelength (spectral albedo), albedo refers to the entire spectrum of solar radiation. Due to measurement constraints, it is often given for the spectrum in which most solar energy reaches the surface (between 0.3 and 3 μm). This spectrum includes visible light (0.39–0.7 μm), which explains why surfaces with a low albedo appear dark (e.g., trees absorb most radiation), whereas surfaces with a high albedo appear bright (e.g., snow reflects most radiation). |
39_5 | Albedo [SEP] Albedo is an important concept in climatology, astronomy, and environmental management (e.g., as part of the Leadership in Energy and Environmental Design (LEED) program for sustainable rating of buildings). The average albedo of the Earth from the upper atmosphere, its "planetary albedo", is 30–35% because of cloud cover, but widely varies locally across the surface because of different geological and environmental features. |
39_6 | Albedo [SEP] The term albedo was introduced into optics by Johann Heinrich Lambert in his 1760 work "Photometria". |
39_7 | Albedo [SEP] Section::::Terrestrial albedo. |
39_8 | Albedo [SEP] Any albedo in visible light falls within a range of about 0.9 for fresh snow to about 0.04 for charcoal, one of the darkest substances. Deeply shadowed cavities can achieve an effective albedo approaching the zero of a black body. When seen from a distance, the ocean surface has a low albedo, as do most forests, whereas desert areas have some of the highest albedos among landforms. Most land areas are in an albedo range of 0.1 to 0.4. The average albedo of Earth is about 0.3. This is far higher than for the ocean primarily because of the contribution |
39_9 | Albedo [SEP] of clouds. |
39_10 | Albedo [SEP] Earth's surface albedo is regularly estimated via Earth observation satellite sensors such as NASA's MODIS instruments on board the Terra and Aqua satellites, and the CERES instrument on the Suomi NPP and JPSS. As the amount of reflected radiation is only measured for a single direction by satellite, not all directions, a mathematical model is used to translate a sample set of satellite reflectance measurements into estimates of directional-hemispherical reflectance and bi-hemispherical reflectance (e.g.,). These calculations are based on the bidirectional reflectance distribution function (BRDF), which describes how the reflectance of a given surface depends on the view angle of |
39_11 | Albedo [SEP] the observer and the solar angle. BDRF can facilitate translations of observations of reflectance into albedo. |
39_12 | Albedo [SEP] Earth's average surface temperature due to its albedo and the greenhouse effect is currently about 15 °C. If Earth were frozen entirely (and hence be more reflective), the average temperature of the planet would drop below −40 °C. If only the continental land masses became covered by glaciers, the mean temperature of the planet would drop to about 0 °C. In contrast, if the entire Earth was covered by water — a so-called ocean planet — the average temperature on the planet would rise to almost 27 °C. |
39_13 | Albedo [SEP] Section::::Terrestrial albedo.:White-sky, black-sky, and blue-sky albedo. |
39_14 | Albedo [SEP] For land surfaces, it has been shown that the albedo at a particular solar zenith angle "θ" can be approximated by the proportionate sum of two terms: |
39_15 | Albedo [SEP] BULLET::::- the directional-hemispherical reflectance at that solar zenith angle, formula_1, sometimes referred to as black-sky albedo, and |
39_16 | Albedo [SEP] BULLET::::- the bi-hemispherical reflectance, formula_2, sometimes referred to as white-sky albedo. |
39_17 | Albedo [SEP] with formula_3 being the proportion of direct radiation from a given solar angle, and formula_4 being the proportion of diffuse illumination, the actual albedo formula_5 (also called blue-sky albedo) can then be given as: |
39_18 | Albedo [SEP] This formula is important because it allows the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface. |
39_19 | Albedo [SEP] Section::::Astronomical albedo. |
39_20 | Albedo [SEP] The albedos of planets, satellites and minor planets such as asteroids can be used to infer much about their properties. The study of albedos, their dependence on wavelength, lighting angle ("phase angle"), and variation in time comprises a major part of the astronomical field of photometry. For small and far objects that cannot be resolved by telescopes, much of what we know comes from the study of their albedos. For example, the absolute albedo can indicate the surface ice content of outer Solar System objects, the variation of albedo with phase angle gives information about regolith properties, whereas unusually high |
39_21 | Albedo [SEP] radar albedo is indicative of high metal content in asteroids. |
39_22 | Albedo [SEP] Enceladus, a moon of Saturn, has one of the highest known albedos of any body in the Solar System, with 99% of EM radiation reflected. Another notable high-albedo body is Eris, with an albedo of 0.96. Many small objects in the outer Solar System and asteroid belt have low albedos down to about 0.05. A typical comet nucleus has an albedo of 0.04. Such a dark surface is thought to be indicative of a primitive and heavily space weathered surface containing some organic compounds. |
39_23 | Albedo [SEP] The overall albedo of the Moon is measured to be around 0.136, but it is strongly directional and non-Lambertian, displaying also a strong opposition effect. Although such reflectance properties are different from those of any terrestrial terrains, they are typical of the regolith surfaces of airless Solar System bodies. |
39_24 | Albedo [SEP] Two common albedos that are used in astronomy are the (V-band) geometric albedo (measuring brightness when illumination comes from directly behind the observer) and the Bond albedo (measuring total proportion of electromagnetic energy reflected). Their values can differ significantly, which is a common source of confusion. |
39_25 | Albedo [SEP] In detailed studies, the directional reflectance properties of astronomical bodies are often expressed in terms of the five Hapke parameters which semi-empirically describe the variation of albedo with phase angle, including a characterization of the opposition effect of regolith surfaces. |
39_26 | Albedo [SEP] The correlation between astronomical (geometric) albedo, absolute magnitude and diameter is: |
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