Millimetre
The millimetre or millimeter is a unit of length in the metric system, equal to one thousandth of a metre, the SI base unit of length. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre. One millimetre is equal to 1000000 nanometres. A millimetre is equal to 5⁄127 of an inch. Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second". A millimetre, 1/1000 of a metre, is therefore the distance travelled by light in 1/299792458000 of a second. A common shortening of millimetre in spoken English is "mil"; this can cause confusion since in the United States, "mil" traditionally means a thousandth of an inch. For the purposes of compatibility with Chinese and Korean characters, Unicode has symbols for: millimetre - code U+339C square millimetre - code U+339F cubic millimetre - code U+33A3In Japanese typography, these square symbols were used for laying out unit symbols without distorting the grid layout of text characters.
On a metric ruler, the smallest measurements are millimetres. High-quality engineering rules may be graduated in increments of 0.5 mm. Digital callipers are capable of reading increments as small as 0.01 mm. Microwaves with a frequency of 300 GHz have a wavelength of 1 mm. Using wavelengths between 30 GHz and 300 GHz for data transmission, in contrast to the 300 MHz to 3 GHz used in mobile devices, has the potential to allow data transfer rates of 10 gigabits per second; the smallest distances the human eye can resolve is around 0.02 to 0.04 mm the width of a human hair. A sheet of paper is between 0.07 mm and 0.18 mm thick, with ordinary printer paper or copy paper a tenth of a millimetre thick. Metric system Orders of magnitude Submillimeter
Moon
The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, the largest among planetary satellites relative to the size of the planet that it orbits; the Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known. The Moon is thought to have formed not long after Earth; the most accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia. The Moon is in synchronous rotation with Earth, thus always shows the same side to Earth, the near side; the near side is marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. After the Sun, the Moon is the second-brightest visible celestial object in Earth's sky, its surface is dark, although compared to the night sky it appears bright, with a reflectance just higher than that of worn asphalt.
Its gravitational influence produces the ocean tides, body tides, the slight lengthening of the day. The Moon's average orbital distance is 1.28 light-seconds. This is about thirty times the diameter of Earth; the Moon's apparent size in the sky is the same as that of the Sun, since the star is about 400 times the lunar distance and diameter. Therefore, the Moon covers the Sun nearly during a total solar eclipse; this matching of apparent visual size will not continue in the far future because the Moon's distance from Earth is increasing. The Moon was first reached in September 1959 by an unmanned spacecraft; the United States' NASA Apollo program achieved the only manned lunar missions to date, beginning with the first manned orbital mission by Apollo 8 in 1968, six manned landings between 1969 and 1972, with the first being Apollo 11. These missions returned lunar rocks which have been used to develop a geological understanding of the Moon's origin, internal structure, the Moon's history. Since the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft.
Both the Moon's natural prominence in the earthly sky and its regular cycle of phases as seen from Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in language, lunar calendar systems and mythology; the usual English proper name for Earth's natural satellite is "the Moon", which in nonscientific texts is not capitalized. The noun moon is derived from Old English mōna, which stems from Proto-Germanic *mēnô, which comes from Proto-Indo-European *mḗh₁n̥s "moon", "month", which comes from the Proto-Indo-European root *meh₁- "to measure", the month being the ancient unit of time measured by the Moon; the name "Luna" is used. In literature science fiction, "Luna" is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of Earth's moon; the modern English adjective pertaining to the Moon is lunar, derived from the Latin word for the Moon, luna. The adjective selenic is so used to refer to the Moon that this meaning is not recorded in most major dictionaries.
It is derived from the Ancient Greek word for the Moon, σελήνη, from, however derived the prefix "seleno-", as in selenography, the study of the physical features of the Moon, as well as the element name selenium. Both the Greek goddess Selene and the Roman goddess Diana were alternatively called Cynthia; the names Luna and Selene are reflected in terminology for lunar orbits in words such as apolune and selenocentric. The name Diana comes from the Proto-Indo-European *diw-yo, "heavenly", which comes from the PIE root *dyeu- "to shine," which in many derivatives means "sky and god" and is the origin of Latin dies, "day"; the Moon formed 4.51 billion years ago, some 60 million years after the origin of the Solar System. Several forming mechanisms have been proposed, including the fission of the Moon from Earth's crust through centrifugal force, the gravitational capture of a pre-formed Moon, the co-formation of Earth and the Moon together in the primordial accretion disk; these hypotheses cannot account for the high angular momentum of the Earth–Moon system.
The prevailing hypothesis is that the Earth–Moon system formed after an impact of a Mars-sized body with the proto-Earth. The impact blasted material into Earth's orbit and the material accreted and formed the Moon; the Moon's far side has a crust, 30 mi thicker than that of the near side. This is thought to be; this hypothesis, although not perfect best explains the evidence. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona, the giant impact hypothesis emerged as the most consensual theory. Before the conference, there were parti
Scientific notation
Scientific notation is a way of expressing numbers that are too big or too small to be conveniently written in decimal form. It is used by scientists and engineers, in part because it can simplify certain arithmetic operations. On scientific calculators it is known as "SCI" display mode. In scientific notation all numbers are written in the form m × 10n, where the exponent n is an integer, the coefficient m is any real number; the integer n is called the order of magnitude and the real number m is called the significand or mantissa. However, the term "mantissa" may cause confusion because it is the name of the fractional part of the common logarithm. If the number is negative a minus sign precedes m. In normalized notation, the exponent is chosen so that the absolute value of the coefficient is at least one but less than ten. Decimal floating point is a computer arithmetic system related to scientific notation. Any given real number can be written in the form m×10^n in many ways: for example, 350 can be written as 3.5×102 or 35×101 or 350×100.
In normalized scientific notation, the exponent n is chosen so that the absolute value of m remains at least one but less than ten. Thus 350 is written as 3.5×102. This form allows easy comparison of numbers, as the exponent n gives the number's order of magnitude. In normalized notation, the exponent n is negative for a number with absolute value between 0 and 1; the 10 and exponent are omitted when the exponent is 0. Normalized scientific form is the typical form of expression of large numbers in many fields, unless an unnormalized form, such as engineering notation, is desired. Normalized scientific notation is called exponential notation—although the latter term is more general and applies when m is not restricted to the range 1 to 10 and to bases other than 10. Engineering notation differs from normalized scientific notation in that the exponent n is restricted to multiples of 3; the absolute value of m is in the range 1 ≤ |m| < 1000, rather than 1 ≤ |m| < 10. Though similar in concept, engineering notation is called scientific notation.
Engineering notation allows the numbers to explicitly match their corresponding SI prefixes, which facilitates reading and oral communication. For example, 12.5×10−9 m can be read as "twelve-point-five nanometers" and written as 12.5 nm, while its scientific notation equivalent 1.25×10−8 m would be read out as "one-point-two-five times ten-to-the-negative-eight meters". A significant figure is a digit in a number; this includes all nonzero numbers, zeroes between significant digits, zeroes indicated to be significant. Leading and trailing zeroes are not significant because they exist only to show the scale of the number. Therefore, 1,230,400 has five significant figures: 1, 2, 3, 0, 4; when a number is converted into normalized scientific notation, it is scaled down to a number between 1 and 10. All of the significant digits remain, thus 1,230,400 would become 1.2304 × 106. However, there is the possibility that the number may be known to six or more significant figures, in which case the number would be shown as 1.23040 × 106.
Thus, an additional advantage of scientific notation is that the number of significant figures is clearer. It is customary in scientific measurements to record all the known digits from the measurements, to estimate at least one additional digit if there is any information at all available to enable the observer to make an estimate; the resulting number contains more information than it would without that extra digit, it may be considered a significant digit because it conveys some information leading to greater precision in measurements and in aggregations of measurements. Additional information about precision can be conveyed through additional notations, it is useful to know how exact the final digit are. For instance, the accepted value of the unit of elementary charge can properly be expressed as 1.6021766208×10−19 C, shorthand for ×10−19 C. Most calculators and many computer programs present large and small results in scientific notation invoked by a key labelled EXP, EEX, EE, EX, E, or ×10x depending on vendor and model.
Because superscripted exponents like 107 cannot always be conveniently displayed, the letter E is used to represent "times ten raised to the power of" and is followed by the value of the exponent. In this usage the character e is not related to the mathematical constant e or the exponential function ex. Although the E stands for exponent, the notation is referred to as E-notation rather than exponential notation; the use of E-notation facilitates data entry and readability in textual communication since it minimizes keystrokes, avoids reduced font sizes and provides a simpler and more concise display, but it is not encouraged in some publications. In most po
Australopithecus
Australopithecus ( OS-trə-lo-PITH-i-kəs. From paleontological and archaeological evidence, the genus Australopithecus evolved in eastern Africa around 4 million years ago before spreading throughout the continent and becoming extinct two million years ago. Australopithecus is not extinct as the Kenyanthropus and Homo genera emerged as sister of a late Australopithecus species such as A. Africanus and/or A. Sediba. During that time, a number of australopithecine species emerged, including Australopithecus afarensis, A. africanus, A. anamensis, A. bahrelghazali, A. deyiremeda, A. garhi, A. sediba. For some hominid species of this time – A. robustus, A. boisei and A. aethiopicus – some debate exists whether they constitute members of the genus Australopithecus. If so, they would be considered'robust australopiths', while the others would be'gracile australopiths'. However, if these more robust species do constitute their own genus, they would be under the genus name Paranthropus, a genus described by Robert Broom when the first discovery was made in 1938, which makes these species P. robustus, P. boisei and P. aethiopicus.
Australopithecus species played a significant part in human evolution, the genus Homo being derived from Australopithecus at some time after three million years ago. In addition, they were the first hominids to possess certain genes, known as the duplicated SRGAP2, which increased the length and ability of neurons in the brain. One of the australopith species evolved into the genus Homo in Africa around two million years ago, modern humans, H. sapiens sapiens. In January 2019, scientists reported that Australopithecus sediba is distinct from, but shares anatomical similarities to, both the older Australopithecus africanus, the younger Homo habilis. Gracile australopiths shared several traits with modern apes and humans, were widespread throughout Eastern and Northern Africa around 3.5 million years ago. The earliest evidence of fundamentally bipedal hominids can be observed at the site of Laetoli in Tanzania; this site contains hominid footprints that are remarkably similar to those of modern humans and have been dated to as old as 3.6 million years.
The footprints have been classified as australopith, as they are the only form of prehuman hominins known to have existed in that region at that time. Australopithecus anamensis, A. afarensis, A. africanus are among the most famous of the extinct hominins. A. africanus was once considered to be ancestral to the genus Homo. However, fossils assigned to the genus Homo have been found. Thus, the genus Homo either split off from the genus Australopithecus at an earlier date, or both developed from a yet unknown common ancestor independently. According to the Chimpanzee Genome Project, the human and chimpanzee lineages diverged from a common ancestor about five to six million years ago, assuming a constant rate of evolution, it is theoretically more for evolution to happen more as opposed to more from the date suggested by a gene clock However, hominins discovered more are somewhat older than the presumed rate of evolution would suggest. Sahelanthropus tchadensis called "Toumai", is about seven million years old and Orrorin tugenensis lived at least six million years ago.
Since little is known of them, they remain controversial among scientists since the molecular clock in humans has determined that humans and chimpanzees had a genetic split at least a million years later. One theory suggests that the human and chimpanzee lineages diverged somewhat at first some populations interbred around one million years after diverging; the brains of most species of Australopithecus were 35% of the size of a modern human brain. Most species of Australopithecus were diminutive and gracile standing 1.2 to 1.4 m tall. In several variations is a considerable degree of sexual dimorphism, males being larger than females. According to one scholar, A. Zihlman, Australopithecus body proportions resemble those of bonobos, leading evolutionary biologists such as Jeremy Griffith to suggest that bonobos may be phenotypically similar to Australopithecus. Furthermore, thermoregulatory models suggest that Australopithecus species were hair covered, more like chimpanzees and bonobos, unlike humans.
Modern humans do not display the same degree of sexual dimorphism as Australopithecus appears to have. In modern populations, males are on average a mere 15% larger than females, while in Australopithecus, males could be up to 50% larger than females. New research suggests, that australopithecines exhibited a lesser degree of sexual dimorphism than these figures suggest, but the issue is not settled. Opinions differ as to whether the species A. aethiopicus, A. boisei, A. robustus should be included within the genus Australopithecus, no current consensus exists as to whether they should be placed in a distinct genus, Paranthropus
Metric prefix
A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or fraction of the unit. While all metric prefixes in common use today are decadic there have been a number of binary metric prefixes as well; each prefix has a unique symbol, prepended to the unit symbol. The prefix kilo-, for example, may be added to gram to indicate multiplication by one thousand: one kilogram is equal to one thousand grams; the prefix milli- may be added to metre to indicate division by one thousand. Decimal multiplicative prefixes have been a feature of all forms of the metric system, with six of these dating back to the system's introduction in the 1790s. Metric prefixes have been used with some non-metric units; the SI prefixes are standardized for use in the International System of Units by the International Bureau of Weights and Measures in resolutions dating from 1960 to 1991. Since 2009, they have formed part of the International System of Quantities; the BIPM specifies twenty prefixes for the International System of Units.
Each prefix name has a symbol, used in combination with the symbols for units of measure. For example, the symbol for'kilo-' is'k', is used to produce'km','kg', and'kW', which are the SI symbols for kilometre and kilowatt, respectively. Where the Greek letter'μ' is unavailable, the symbol for micro'µ' may be used. Where both variants are unavailable, the micro prefix is written as the lowercase Latin letter'u'. Prefixes corresponding to an integer power of one thousand are preferred. Hence'100 m' is preferred over'1 hm' or'10 dam'; the prefixes hecto, deca and centi are used for everyday purposes, the centimetre is common. However, some modern building codes require that the millimetre be used in preference to the centimetre, because "use of centimetres leads to extensive usage of decimal points and confusion". Prefixes may not be used in combination; this applies to mass, for which the SI base unit contains a prefix. For example, milligram is used instead of microkilogram. In the arithmetic of measurements having units, the units are treated as multiplicative factors to values.
If they have prefixes, all but one of the prefixes must be expanded to their numeric multiplier, except when combining values with identical units. Hence, 5 mV × 5 mA = 5×10−3 V × 5×10−3 A = 25×10−6 V⋅A = 25 μW 5.00 mV + 10 μV = 5.00 mV + 0.01 mV = 5.01 mVWhen powers of units occur, for example, squared or cubed, the multiplication prefix must be considered part of the unit, thus included in the exponentiation. 1 km2 means one square kilometre, or the area of a square of 1000 m by 1000 m and not 1000 square metres. 2 Mm3 means two cubic megametres, or the volume of two cubes of 1000000 m by 1000000 m by 1000000 m or 2×1018 m3, not 2000000 cubic metres. Examples5 cm = 5×10−2 m = 5 × 0.01 m = 0.05 m 9 km2 = 9 × 2 = 9 × 2 × m2 = 9×106 m2 = 9 × 1000000 m2 = 9000000 m2 3 MW = 3×106 W = 3 × 1000000 W = 3000000 W The use of prefixes can be traced back to the introduction of the metric system in the 1790s, long before the 1960 introduction of the SI. The prefixes, including those introduced after 1960, are used with any metric unit, whether included in the SI or not.
Metric prefixes may be used with non-metric units. The choice of prefixes with a given unit is dictated by convenience of use. Unit prefixes for amounts that are much larger or smaller than those encountered are used; the units kilogram, milligram and smaller are used for measurement of mass. However, megagram and larger are used. Megagram and teragram are used to disambiguate the metric tonne from other units with the name'ton'; the kilogram is the only base unit of the International System of Units that includes a metric prefix. The litre, millilitre and smaller are common. In Europe, the centilitre is used for packaged products such as wine and the decilitre is less frequently; the latter two items include prefixes corresponding to an exponent, not divisible by three. Larger volumes are denoted in kilolitres, megalitres or gigalitres, or else in cubic metres or cubic kilometres. For scientific purposes, the cubic metre is used; the kilometre, centimetre and smaller are common. The micrometre is referred to by the non-SI term micron.
In some fields, such as chemistry, the ångström competed with the nanometre. The femtometre, used in particle physics, is sometimes called a fermi. For large scales, megametre and larger are used. Instead, non-metric units are used, such as astronomical units, light years, parsecs; the second, millisecond and shorter are common. The kilosecond and megasecond have some use, though for these and longer times one uses either scientific notation or minutes, so on; the SI unit of angle is the radian, but degrees and seconds see some scientific use. Official policy varies from common practice for the degree Celsius. NIST states: "Prefix symbols may be used with the unit symbol °C and prefix names may be used with the unit name'degree Celsius'. For example, 12 m°C (12 millidegr
Roman numerals
The numeric system represented by Roman numerals originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers in this system are represented by combinations of letters from the Latin alphabet. Roman numerals, as used today, employ seven symbols, each with a fixed integer value, as follows: The use of Roman numerals continued long after the decline of the Roman Empire. From the 14th century on, Roman numerals began to be replaced in most contexts by the more convenient Arabic numerals; the original pattern for Roman numerals used the symbols I, V, X as simple tally marks. Each marker for 1 added a unit value up to 5, was added to to make the numbers from 6 to 9: I, II, III, IIII, V, VI, VII, VIII, VIIII, X; the numerals for 4 and 9 proved problematic, are replaced with IV and IX. This feature of Roman numerals is called subtractive notation; the numbers from 1 to 10 are expressed in Roman numerals as follows: I, II, III, IV, V, VI, VII, VIII, IX, X.
The system being decimal and hundreds follow the same underlying pattern. This is the key to understanding Roman numerals: Thus 10 to 100: X, XX, XXX, XL, L, LX, LXX, LXXX, XC, C. Note that 40 and 90 follow the same subtractive pattern as 4 and 9, avoiding the confusing XXXX. 100 to 1000: C, CC, CCC, CD, D, DC, DCC, DCCC, CM, M. Again - 400 and 900 follow the standard subtractive pattern, avoiding CCCC. In the absence of standard symbols for 5,000 and 10,000 the pattern breaks down at this point - in modern usage M is repeated up to three times; the Romans had several ways to indicate larger numbers, but for practical purposes Roman Numerals for numbers larger than 3,999 are if used nowadays, this suffices. M, MM, MMM. Many numbers include hundreds and tens; the Roman numeral system being decimal, each power of ten is added in descending sequence from left to right, as with Arabic numerals. For example: 39 = "Thirty nine" = XXXIX. 246 = "Two hundred and forty six" = CCXLVI. 421 = "Four hundred and twenty one" = CDXXI.
As each power of ten has its own notation there is no need for place keeping zeros, so "missing places" are ignored, as in Latin speech, thus: 160 = "One hundred and sixty" = CLX 207 = "Two hundred and seven" = CCVII 1066 = "A thousand and sixty six" = MLXVI. Roman numerals for large numbers are nowadays seen in the form of year numbers, as in these examples: 1776 = MDCCLXXVI. 1954 = MCMLIV 1990 = MCMXC. 2014 = MMXIV (the year of the games of the XXII Olympic Winter Games The current year is MMXIX. The "standard" forms described above reflect typical modern usage rather than an unchanging and universally accepted convention. Usage in ancient Rome varied and remained inconsistent in medieval times. There is still no official "binding" standard, which makes the elaborate "rules" used in some sources to distinguish between "correct" and "incorrect" forms problematic. "Classical" inscriptions not infrequently use IIII for "4" instead of IV. Other "non-subtractive" forms, such as VIIII for IX, are sometimes seen, although they are less common.
On the numbered gates to the colosseum, for instance, IV is systematically avoided in favour of IIII, but other "subtractives" apply, so that gate 44 is labelled XLIIII. Isaac Asimov speculates that the use of "IV", as the initial letters of "IVPITER" may have been felt to have been impious in this context. Clock faces that use Roman numerals show IIII for four o'clock but IX for nine o'clock, a practice that goes back to early clocks such as the Wells Cathedral clock of the late 14th century. However, this is far from universal: for example, the clock on the Palace of Westminster, Big Ben, uses a "normal" IV. XIIX or IIXX are sometimes used for "18" instead of XVIII; the Latin word for "eighteen" is rendered as the equivalent of "two less than twenty" which may be the source of this usage. The standard forms for 98 and 99 are XCVIII and XCIX, as described in the "decimal pattern" section above, but these numbers are rendered as IIC and IC originally from the Latin duodecentum and undecentum.
Sometimes V and L are not used, with instances such as IIIIII and XXXXXX rather than VI or LX. Most non-standard numerals other than those described above - such as VXL for 45, instead of the standard XLV are modern and may be due to error rather than being genuine variant usage. In the early years of the 20th century, different representations of 900 appeared in several inscribed dates. For instance, 1910 is shown on Admiralty Arch, London, as MDCCCCX rather than MCMX, while on the north entrance to the Saint Louis Art Museum, 1903 is inscribed as MDCDIII rather than MCMIII. Although Roman numerals came to be written with letters
