72-453: The decibel (symbol: dB ) is a relative unit of measurement equal to one tenth of a bel ( B ). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale . Two signals whose levels differ by one decibel have a power ratio of 10 (approximately 1.26 ) or root-power ratio of 10 (approximately 1.12 ). The unit fundamentally expresses a relative change but may also be used to express an absolute value as
144-483: A barleycorn . A system of measurement is a collection of units of measurement and rules relating them to each other. As science progressed, a need arose to relate the measurement systems of different quantities, like length and weight and volume. The effort of attempting to relate different traditional systems between each other exposed many inconsistencies, and brought about the development of new units and systems. Systems of units vary from country to country. Some of
216-440: A 3.5 dB/km fiber yields a loss of 0.35 dB = 3.5 dB/km × 0.1 km. The human perception of the intensity of sound and light more nearly approximates the logarithm of intensity rather than a linear relationship (see Weber–Fechner law ), making the dB scale a useful measure. The decibel is commonly used in acoustics as a unit of sound power level or sound pressure level . The reference pressure for sound in air
288-406: A bel would normally be written 0.05 dB, and not 5 mB. The method of expressing a ratio as a level in decibels depends on whether the measured property is a power quantity or a root-power quantity ; see Power, root-power, and field quantities for details. When referring to measurements of power quantities, a ratio can be expressed as a level in decibels by evaluating ten times
360-422: A change in power by a factor of 10 corresponds to a 10 dB change in level. When expressing root-power quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The decibel scales differ by a factor of two, so that the related power and root-power levels change by the same value in linear systems, where power is proportional to the square of amplitude. The definition of
432-502: A decibel is one-tenth of a bel). P and P 0 must measure the same type of quantity, and have the same units before calculating the ratio. If P = P 0 in the above equation, then L P = 0. If P is greater than P 0 then L P is positive; if P is less than P 0 then L P is negative. Rearranging the above equation gives the following formula for P in terms of P 0 and L P : When referring to measurements of root-power quantities, it
504-697: A favorable response to a new unit definition among members of the International Advisory Committee on Long Distance Telephony in Europe and replaced the MSC with the Transmission Unit (TU). 1 TU was defined such that the number of TUs was ten times the base-10 logarithm of the ratio of measured power to a reference power. The definition was conveniently chosen such that 1 TU approximated 1 MSC; specifically, 1 MSC
576-436: A frequency of 5000 radians per second (795.8 Hz), and matched closely the smallest attenuation detectable to a listener. A standard telephone cable was "a cable having uniformly distributed resistance of 88 ohms per loop-mile and uniformly distributed shunt capacitance of 0.054 microfarads per mile" (approximately corresponding to 19 gauge wire). In 1924, Bell Telephone Laboratories received
648-453: A linear scale, adding there, and then taking logarithms to return. For example, where operations on decibels are logarithmic addition/subtraction and logarithmic multiplication/division, while operations on the linear scale are the usual operations: The logarithmic mean is obtained from the logarithmic sum by subtracting 10 log 10 2 {\displaystyle 10\log _{10}2} , since logarithmic division
720-427: A manner similar to scientific notation . This allows one to clearly visualize huge changes of some quantity. See Bode plot and Semi-log plot . For example, 120 dB SPL may be clearer than "a trillion times more intense than the threshold of hearing". Level values in decibels can be added instead of multiplying the underlying power values, which means that the overall gain of a multi-component system, such as
792-432: A power ratio of 10, or 1.9953 , about 0.24% different from exactly 2, and a voltage ratio of 1.4125 , 0.12% different from exactly √ 2 . Similarly, an increase of 6.000 dB corresponds to the power ratio is 10 ≈ 3.9811 , about 0.5% different from 4. The decibel is useful for representing large ratios and for simplifying representation of multiplicative effects, such as attenuation from multiple sources along
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#1732783147342864-424: A quantity may be described as multiples of that of a familiar entity, which can be easier to contextualize than a value in a formal unit system. For instance, a publication may describe an area in a foreign country as a number of multiples of the area of a region local to the readership. The propensity for certain concepts to be used frequently can give rise to loosely defined "systems" of units. For most quantities
936-454: A ratio between two root-power quantities of √ 10 :1. Two signals whose levels differ by one decibel have a power ratio of 10, which is approximately 1.258 93 , and an amplitude (root-power quantity) ratio of 10 ( 1.122 02 ). The bel is rarely used either without a prefix or with SI unit prefixes other than deci ; it is customary, for example, to use hundredths of a decibel rather than millibels . Thus, five one-thousandths of
1008-500: A result, units of measure could vary not only from location to location but from person to person. Units not based on the human body could be based on agriculture, as is the case with the furlong and the acre , both based on the amount of land able to be worked by a team of oxen . Metric systems of units have evolved since the adoption of the original metric system in France in 1791. The current international standard metric system
1080-418: A series of amplifier stages, can be calculated by summing the gains in decibels of the individual components, rather than multiply the amplification factors; that is, log( A × B × C ) = log( A ) + log( B ) + log( C ). Practically, this means that, armed only with the knowledge that 1 dB is a power gain of approximately 26%, 3 dB is approximately 2× power gain, and 10 dB is 10× power gain, it
1152-406: A signal chain. Its application in systems with additive effects is less intuitive, such as in the combined sound pressure level of two machines operating together. Care is also necessary with decibels directly in fractions and with the units of multiplicative operations. The logarithmic scale nature of the decibel means that a very large range of ratios can be represented by a convenient number, in
1224-400: A small set of units is required. These units are taken as the base units and the other units are derived units . Thus base units are the units of the quantities which are independent of other quantities and they are the units of length, mass, time, electric current, temperature, luminous intensity and the amount of substance. Derived units are the units of the quantities which are derived from
1296-611: A subject of governmental regulation, to ensure fairness and transparency. The International Bureau of Weights and Measures (BIPM) is tasked with ensuring worldwide uniformity of measurements and their traceability to the International System of Units (SI). Metrology is the science of developing nationally and internationally accepted units of measurement. In physics and metrology, units are standards for measurement of physical quantities that need clear definitions to be useful. Reproducibility of experimental results
1368-420: A subtraction ( C / N 0 ) dB = C dB − N 0 dB . However, the linear-scale units still simplify in the implied fraction, so that the results would be expressed in dB-Hz. According to Mitschke, "The advantage of using a logarithmic measure is that in a transmission chain, there are many elements concatenated, and each has its own gain or attenuation. To obtain the total, addition of decibel values
1440-433: A unit is necessary to communicate values of that physical quantity. For example, conveying to someone a particular length without using some sort of unit is impossible, because a length cannot be described without a reference used to make sense of the value given. But not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as combinations of units of other quantities. Thus only
1512-399: A unit of logarithmic power ratio, while the neper is used for logarithmic root-power (amplitude) ratio. The unit dBW is often used to denote a ratio for which the reference is 1 W, and similarly dBm for a 1 mW reference point. (31.62 V / 1 V) ≈ 1 kW / 1 W , illustrating the consequence from the definitions above that L G has the same value, 30 dB, regardless of whether it
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#17327831473421584-470: Is a definite magnitude of a quantity , defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity . Any other quantity of that kind can be expressed as a multiple of the unit of measurement. For example, a length is a physical quantity . The metre (symbol m) is a unit of length that represents a definite predetermined length. For instance, when referencing "10 metres" (or 10 m), what
1656-519: Is actually meant is 10 times the definite predetermined length called "metre". The definition, agreement, and practical use of units of measurement have played a crucial role in human endeavour from early ages up to the present. A multitude of systems of units used to be very common. Now there is a global standard, the International System of Units (SI), the modern form of the metric system . In trade, weights and measures are often
1728-469: Is almost universally rounded to 3 dB in technical writing. This implies an increase in voltage by a factor of √ 2 ≈ 1.4142 . Likewise, a doubling or halving of the voltage, corresponding to a quadrupling or quartering of the power, is commonly described as 6 dB rather than ± 6.0206 dB. Should it be necessary to make the distinction, the number of decibels is written with additional significant figures . 3.000 dB corresponds to
1800-485: Is central to the scientific method . A standard system of units facilitates this. Scientific systems of units are a refinement of the concept of weights and measures historically developed for commercial purposes. Science , medicine , and engineering often use larger and smaller units of measurement than those used in everyday life. The judicious selection of the units of measurement can aid researchers in problem solving (see, for example, dimensional analysis ). In
1872-402: Is deprecated by that standard and root-power is used throughout this article. Although power and root-power quantities are different quantities, their respective levels are historically measured in the same units, typically decibels. A factor of 2 is introduced to make changes in the respective levels match under restricted conditions such as when the medium is linear and the same waveform
1944-409: Is expressed as the product of a numerical value { Z } (a pure number) and a unit [ Z ]: For example, let Z {\displaystyle Z} be "2 metres"; then, { Z } = 2 {\displaystyle \{Z\}=2} is the numerical value and [ Z ] = m e t r e {\displaystyle [Z]=\mathrm {metre} } is the unit. Conversely,
2016-416: Is linear subtraction. Attenuation constants, in topics such as optical fiber communication and radio propagation path loss , are often expressed as a fraction or ratio to distance of transmission. In this case, dB/m represents decibel per meter, dB/mi represents decibel per mile, for example. These quantities are to be manipulated obeying the rules of dimensional analysis , e.g., a 100-meter run with
2088-489: Is much more convenient than multiplication of the individual factors." However, for the same reason that humans excel at additive operation over multiplication, decibels are awkward in inherently additive operations: if two machines each individually produce a sound pressure level of, say, 90 dB at a certain point, then when both are operating together we should expect the combined sound pressure level to increase to 93 dB, but certainly not to 180 dB!; suppose that
2160-467: Is now defined as exactly 0.0254 m , and the US and imperial avoirdupois pound is now defined as exactly 0.453 592 37 kg . While the above systems of units are based on arbitrary unit values, formalised as standards, natural units in physics are based on physical principle or are selected to make physical equations easier to work with. For example, atomic units (au) were designed to simplify
2232-411: Is obtained from powers or from amplitudes, provided that in the specific system being considered power ratios are equal to amplitude ratios squared. A change in power ratio by a factor of 10 corresponds to a change in level of 10 dB . A change in power ratio by a factor of 2 or 1 / 2 is approximately a change of 3 dB . More precisely, the change is ± 3.0103 dB, but this
Decibel - Misplaced Pages Continue
2304-790: Is possible to determine the power ratio of a system from the gain in dB with only simple addition and multiplication. For example: However, according to its critics, the decibel creates confusion, obscures reasoning, is more related to the era of slide rules than to modern digital processing, and is cumbersome and difficult to interpret. Quantities in decibels are not necessarily additive , thus being "of unacceptable form for use in dimensional analysis ". Thus, units require special care in decibel operations. Take, for example, carrier-to-noise-density ratio C / N 0 (in hertz), involving carrier power C (in watts) and noise power spectral density N 0 (in W/Hz). Expressed in decibels, this ratio would be
2376-588: Is recognized by other international bodies such as the International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO). The IEC permits the use of the decibel with root-power quantities as well as power and this recommendation is followed by many national standards bodies, such as NIST , which justifies the use of the decibel for voltage ratios. In spite of their widespread use, suffixes (such as in dBA or dBV) are not recognized by
2448-451: Is set at the typical threshold of perception of an average human and there are common comparisons used to illustrate different levels of sound pressure . As sound pressure is a root-power quantity, the appropriate version of the unit definition is used: where p rms is the root mean square of the measured sound pressure and p ref is the standard reference sound pressure of 20 micropascals in air or 1 micropascal in water. Use of
2520-489: Is the International System of Units (abbreviated to SI). An important feature of modern systems is standardization . Each unit has a universally recognized size. Both the imperial units and US customary units derive from earlier English units . Imperial units were mostly used in the British Commonwealth and the former British Empire . US customary units are still the main system of measurement used in
2592-514: Is the conversion of the unit of measurement in which a quantity is expressed, typically through a multiplicative conversion factor that changes the unit without changing the quantity. This is also often loosely taken to include replacement of a quantity with a corresponding quantity that describes the same physical property. One example of the importance of agreed units is the failure of the NASA Mars Climate Orbiter , which
2664-408: Is under consideration with changes in amplitude, or the medium impedance is linear and independent of both frequency and time. This relies on the relationship holding. In a nonlinear system, this relationship does not hold by the definition of linearity. However, even in a linear system in which the power quantity is the product of two linearly related quantities (e.g. voltage and current ), if
2736-440: Is usual to consider the ratio of the squares of F (measured) and F 0 (reference). This is because the definitions were originally formulated to give the same value for relative ratios for both power and root-power quantities. Thus, the following definition is used: The formula may be rearranged to give Similarly, in electrical circuits , dissipated power is typically proportional to the square of voltage or current when
2808-439: Is widely used among the foreign telephone organizations and recently it was termed the "decibel" at the suggestion of the International Advisory Committee on Long Distance Telephony. The decibel may be defined by the statement that two amounts of power differ by 1 decibel when they are in the ratio of 10 and any two amounts of power differ by N decibels when they are in the ratio of 10. The number of transmission units expressing
2880-884: The 4th and 3rd millennia BC among the ancient peoples of Mesopotamia , Egypt and the Indus Valley , and perhaps also Elam in Persia as well. Weights and measures are mentioned in the Bible (Leviticus 19:35–36). It is a commandment to be honest and have fair measures. In the Magna Carta of 1215 (The Great Charter) with the seal of King John , put before him by the Barons of England, King John agreed in Clause 35 "There shall be one measure of wine throughout our whole realm, and one measure of ale and one measure of corn—namely,
2952-484: The United States outside of science, medicine, many sectors of industry, and some of government and military, and despite Congress having legally authorised metric measure on 28 July 1866. Some steps towards US metrication have been made, particularly the redefinition of basic US and imperial units to derive exactly from SI units. Since the international yard and pound agreement of 1959 the US and imperial inch
Decibel - Misplaced Pages Continue
3024-412: The base-10 logarithm of the ratio of the measured quantity to reference value. Thus, the ratio of P (measured power) to P 0 (reference power) is represented by L P , that ratio expressed in decibels, which is calculated using the formula: The base-10 logarithm of the ratio of the two power quantities is the number of bels. The number of decibels is ten times the number of bels (equivalently,
3096-414: The gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels. The decibel originates from methods used to quantify signal loss in telegraph and telephone circuits. Until the mid-1920s, the unit for loss was miles of standard cable (MSC). 1 MSC corresponded to the loss of power over one mile (approximately 1.6 km) of standard telephone cable at
3168-399: The impedance is constant. Taking voltage as an example, this leads to the equation for power gain level L G : where V out is the root-mean-square (rms) output voltage, V in is the rms input voltage. A similar formula holds for current. The term root-power quantity is introduced by ISO Standard 80000-1:2009 as a substitute of field quantity . The term field quantity
3240-415: The impedance is frequency- or time-dependent, this relationship does not hold in general, for example if the energy spectrum of the waveform changes. For differences in level, the required relationship is relaxed from that above to one of proportionality (i.e., the reference quantities P 0 and F 0 need not be related), or equivalently, must hold to allow the power level difference to be equal to
3312-401: The metric system , the imperial system , and United States customary units . Historically many of the systems of measurement which had been in use were to some extent based on the dimensions of the human body. Such units, which may be called anthropic units , include the cubit , based on the length of the forearm; the pace , based on the length of a stride; and the foot and hand . As
3384-553: The social sciences , there are no standard units of measurement. A unit of measurement is a standardized quantity of a physical property, used as a factor to express occurring quantities of that property. Units of measurement were among the earliest tools invented by humans. Primitive societies needed rudimentary measures for many tasks: constructing dwellings of an appropriate size and shape, fashioning clothing, or bartering food or raw materials. The earliest known uniform systems of measurement seem to have all been created sometime in
3456-459: The IEC or ISO. The IEC Standard 60027-3:2002 defines the following quantities. The decibel (dB) is one-tenth of a bel: 1 dB = 0.1 B . The bel (B) is 1 ⁄ 2 ln(10) nepers : 1 B = 1 ⁄ 2 ln(10) Np . The neper is the change in the level of a root-power quantity when the root-power quantity changes by a factor of e , that is 1 Np = ln(e) = 1 , thereby relating all of
3528-626: The London quart;—and one width of dyed and russet and hauberk cloths—namely, two ells below the selvage..." As of the 21st century, the International System is predominantly used in the world. There exist other unit systems which are used in many places such as the United States Customary System and the Imperial System. The United States is the only industrialized country that has not yet at least mostly converted to
3600-414: The base quantities and some of the derived units are the units of speed, work, acceleration, energy, pressure etc. Different systems of units are based on different choices of a set of related units including fundamental and derived units. Following ISO 80000-1 , any value or magnitude of a physical quantity is expressed as a comparison to a unit of that quantity. The value of a physical quantity Z
3672-506: The crew confusing tower instructions (in metres) and altimeter readings (in feet). Three crew and five people on the ground were killed. Thirty-seven were injured. In 1983, a Boeing 767 (which thanks to its pilot's gliding skills landed safely and became known as the Gimli Glider ) ran out of fuel in mid-flight because of two mistakes in figuring the fuel supply of Air Canada 's first aircraft to use metric measurements. This accident
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#17327831473423744-399: The decibel in underwater acoustics leads to confusion, in part because of this difference in reference value. Sound intensity is proportional to the square of sound pressure. Therefore, the sound intensity level can also be defined as: The human ear has a large dynamic range in sound reception. The ratio of the sound intensity that causes permanent damage during short exposure to that of
3816-561: The decibel originated in the measurement of transmission loss and power in telephony of the early 20th century in the Bell System in the United States. The bel was named in honor of Alexander Graham Bell , but the bel is seldom used. Instead, the decibel is used for a wide variety of measurements in science and engineering , most prominently for sound power in acoustics , in electronics and control theory . In electronics,
3888-468: The different systems include the centimetre–gram–second , foot–pound–second , metre–kilogram–second systems, and the International System of Units , SI. Among the different systems of units used in the world, the most widely used and internationally accepted one is SI. The base SI units are the second, metre, kilogram, ampere, kelvin, mole and candela; all other SI units are derived from these base units. Systems of measurement in modern use include
3960-441: The earliest days of the telephone, the need for a unit in which to measure the transmission efficiency of telephone facilities has been recognized. The introduction of cable in 1896 afforded a stable basis for a convenient unit and the "mile of standard" cable came into general use shortly thereafter. This unit was employed up to 1923 when a new unit was adopted as being more suitable for modern telephone work. The new transmission unit
4032-629: The metric system. The systematic effort to develop a universally acceptable system of units dates back to 1790 when the French National Assembly charged the French Academy of Sciences to come up such a unit system. This system was the precursor to the metric system which was quickly developed in France but did not take on universal acceptance until 1875 when The Metric Convention Treaty was signed by 17 nations. After this treaty
4104-481: The name logit for "standard magnitudes which combine by multiplication", to contrast with the name unit for "standard magnitudes which combine by addition". In April 2003, the International Committee for Weights and Measures (CIPM) considered a recommendation for the inclusion of the decibel in the International System of Units (SI), but decided against the proposal. However, the decibel
4176-416: The noise from a machine is measured (including the contribution of background noise) and found to be 87 dBA but when the machine is switched off the background noise alone is measured as 83 dBA. [...] the machine noise [level (alone)] may be obtained by 'subtracting' the 83 dBA background noise from the combined level of 87 dBA; i.e., 84.8 dBA.; in order to find a representative value of
4248-425: The numerical value expressed in an arbitrary unit can be obtained as: Units can only be added or subtracted if they are the same type; however units can always be multiplied or divided, as George Gamow used to explain. Let Z {\displaystyle Z} be "2 metres" and W {\displaystyle W} "3 seconds", then There are certain rules that apply to units: Conversion of units
4320-461: The quantities power spectral density and the associated root-power quantities via the Fourier transform , which allows elimination of the frequency dependence in the analysis by analyzing the system at each frequency independently. Since logarithm differences measured in these units often represent power ratios and root-power ratios, values for both are shown below. The bel is traditionally used as
4392-508: The quietest sound that the ear can hear is equal to or greater than 1 trillion (10). Such large measurement ranges are conveniently expressed in logarithmic scale : the base-10 logarithm of 10 is 12, which is expressed as a sound intensity level of 120 dB re 1 pW/m. The reference values of I and p in air have been chosen such that this corresponds approximately to a sound pressure level of 120 dB re 20 μPa . Unit of measurement A unit of measurement , or unit of measure ,
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#17327831473424464-425: The ratio of a value to a fixed reference value; when used in this way, the unit symbol is often suffixed with letter codes that indicate the reference value. For example, for the reference value of 1 volt , a common suffix is " V " (e.g., "20 dBV"). Two principal types of scaling of the decibel are in common use. When expressing a power ratio, it is defined as ten times the logarithm with base 10 . That is,
4536-427: The ratio of any two powers is therefore ten times the common logarithm of that ratio. This method of designating the gain or loss of power in telephone circuits permits direct addition or subtraction of the units expressing the efficiency of different parts of the circuit ... In 1954, J. W. Horton argued that the use of the decibel as a unit for quantities other than transmission loss led to confusion, and suggested
4608-459: The root-power level difference from power P 1 and F 1 to P 2 and F 2 . An example might be an amplifier with unity voltage gain independent of load and frequency driving a load with a frequency-dependent impedance: the relative voltage gain of the amplifier is always 0 dB, but the power gain depends on the changing spectral composition of the waveform being amplified. Frequency-dependent impedances may be analyzed by considering
4680-434: The same unit for the distance between two cities and the length of a needle. Thus, historically they would develop independently. One way to make large numbers or small fractions easier to read, is to use unit prefixes . At some point in time though, the need to relate the two units might arise, and consequently the need to choose one unit as defining the other or vice versa. For example, an inch could be defined in terms of
4752-420: The sound level in a room a number of measurements are taken at different positions within the room, and an average value is calculated. [...] Compare the logarithmic and arithmetic averages of [...] 70 dB and 90 dB: logarithmic average = 87 dB; arithmetic average = 80 dB. Addition on a logarithmic scale is called logarithmic addition , and can be defined by taking exponentials to convert to
4824-399: The units as nondimensional natural log of root-power-quantity ratios, 1 dB = 0.115 13 ... Np = 0.115 13 ... . Finally, the level of a quantity is the logarithm of the ratio of the value of that quantity to a reference value of the same kind of quantity. Therefore, the bel represents the logarithm of a ratio between two power quantities of 10:1, or the logarithm of
4896-515: The wave equation in atomic physics . Some unusual and non-standard units may be encountered in sciences. These may include the solar mass ( 2 × 10 kg ), the megaton (the energy released by detonating one million tons of trinitrotoluene , TNT) and the electronvolt . To reduce the incidence of retail fraud, many national statutes have standard definitions of weights and measures that may be used (hence " statute measure "), and these are verified by legal officers. In informal settings,
4968-595: Was 1.056 TU. In 1928, the Bell system renamed the TU into the decibel, being one tenth of a newly defined unit for the base-10 logarithm of the power ratio. It was named the bel , in honor of the telecommunications pioneer Alexander Graham Bell . The bel is seldom used, as the decibel was the proposed working unit. The naming and early definition of the decibel is described in the NBS Standard's Yearbook of 1931: Since
5040-458: Was accidentally destroyed on a mission to Mars in September 1999 (instead of entering orbit) due to miscommunications about the value of forces: different computer programs used different units of measurement ( newton versus pound force ). Considerable amounts of effort, time, and money were wasted. On 15 April 1999, Korean Air cargo flight 6316 from Shanghai to Seoul was lost due to
5112-528: Was signed, a General Conference of Weights and Measures (CGPM) was established. The CGPM produced the current SI, which was adopted in 1954 at the 10th Conference of Weights and Measures. Currently, the United States is a dual-system society which uses both the SI and the US Customary system. The use of a single unit of measurement for some quantity has obvious drawbacks. For example, it is impractical to use
5184-560: Was the result of both confusion due to the simultaneous use of metric and Imperial measures and confusion of mass and volume measures. When planning his journey across the Atlantic Ocean in the 1480s, Columbus mistakenly assumed that the mile referred to in the Arabic estimate of 56 + 2 / 3 miles for the size of a degree was the same as the actually much shorter Italian mile of 1,480 metres. His estimate for
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