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
In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is a conserved quantity; the SI unit of energy is the joule, the energy transferred to an object by the work of moving it a distance of 1 metre against a force of 1 newton. Common forms of energy include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field, the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, the thermal energy due to an object's temperature. Mass and energy are related. Due to mass–energy equivalence, any object that has mass when stationary has an equivalent amount of energy whose form is called rest energy, any additional energy acquired by the object above that rest energy will increase the object's total mass just as it increases its total energy. For example, after heating an object, its increase in energy could be measured as a small increase in mass, with a sensitive enough scale.
Living organisms require exergy to stay alive, such as the energy. Human civilization requires energy to function, which it gets from energy resources such as fossil fuels, nuclear fuel, or renewable energy; the processes of Earth's climate and ecosystem are driven by the radiant energy Earth receives from the sun and the geothermal energy contained within the earth. The total energy of a system can be subdivided and classified into potential energy, kinetic energy, or combinations of the two in various ways. Kinetic energy is determined by the movement of an object – or the composite motion of the components of an object – and potential energy reflects the potential of an object to have motion, is a function of the position of an object within a field or may be stored in the field itself. While these two categories are sufficient to describe all forms of energy, it is convenient to refer to particular combinations of potential and kinetic energy as its own form. For example, macroscopic mechanical energy is the sum of translational and rotational kinetic and potential energy in a system neglects the kinetic energy due to temperature, nuclear energy which combines utilize potentials from the nuclear force and the weak force), among others.
The word energy derives from the Ancient Greek: translit. Energeia, lit.'activity, operation', which appears for the first time in the work of Aristotle in the 4th century BC. In contrast to the modern definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness and pleasure. In the late 17th century, Gottfried Leibniz proposed the idea of the Latin: vis viva, or living force, which defined as the product of the mass of an object and its velocity squared. To account for slowing due to friction, Leibniz theorized that thermal energy consisted of the random motion of the constituent parts of matter, although it would be more than a century until this was accepted; the modern analog of this property, kinetic energy, differs from vis viva only by a factor of two. In 1807, Thomas Young was the first to use the term "energy" instead of vis viva, in its modern sense. Gustave-Gaspard Coriolis described "kinetic energy" in 1829 in its modern sense, in 1853, William Rankine coined the term "potential energy".
The law of conservation of energy was first postulated in the early 19th century, applies to any isolated system. It was argued for some years whether heat was a physical substance, dubbed the caloric, or a physical quantity, such as momentum. In 1845 James Prescott Joule discovered the generation of heat; these developments led to the theory of conservation of energy, formalized by William Thomson as the field of thermodynamics. Thermodynamics aided the rapid development of explanations of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, Walther Nernst, it led to a mathematical formulation of the concept of entropy by Clausius and to the introduction of laws of radiant energy by Jožef Stefan. According to Noether's theorem, the conservation of energy is a consequence of the fact that the laws of physics do not change over time. Thus, since 1918, theorists have understood that the law of conservation of energy is the direct mathematical consequence of the translational symmetry of the quantity conjugate to energy, namely time.
In 1843, James Prescott Joule independently discovered the mechanical equivalent in a series of experiments. The most famous of them used the "Joule apparatus": a descending weight, attached to a string, caused rotation of a paddle immersed in water insulated from heat transfer, it showed that the gravitational potential energy lost by the weight in descending was equal to the internal energy gained by the water through friction with the paddle. In the International System of Units, the unit of energy is the joule, named after James Prescott Joule, it is a derived unit. It is equal to the energy expended in applying a force of one newton through a distance of one metre; however energy is expressed in many other units not part of the SI, such as ergs, British Thermal Units, kilowatt-hours and kilocalories, which require a conversion factor when expressed in SI units. The SI unit of energy rate is the watt, a joule per second. Thus, one joule is one watt-second, 3600 joules equal one wa
International Bureau of Weights and Measures
The International Bureau of Weights and Measures is an intergovernmental organization, established by the Metre Convention, through which member states act together on matters related to measurement science and measurement standards. The organisation is referred to by its French initialism, BIPM; the BIPM's headquarters is based at France. It has custody of the International Prototype of the Kilogram and houses the secretariat for this organization as well as hosting its formal meetings; the BIPM reports to the International Committee for Weights and Measures, a directorate of eighteen members that meet biannually, in turn overseen by the General Conference on Weights and Measures that meets in Paris once every four years, consisting of delegates of the governments of the Member States and observers from the Associates of the CGPM. These organizations are commonly referred to by their French initialisms; the BIPM was created on 20 May 1875, following the signing of the Metre Convention, a treaty among 59 Member States.
It is based at the Pavillon de Breteuil in Sèvres, France, a 4.35 ha site granted to the Bureau by the French Government in 1876. Since 1969 the site of the Pavillon de Breteuil is considered international territory and the BIPM has all the rights and privileges accorded to an intergovernmental organization; the status was further clarified by the French decree No 70-820 of 9 September 1970. The BIPM has the mandate to provide the basis for a single, coherent system of measurements throughout the world, traceable to the International System of Units; this task takes many forms, from direct dissemination of units to coordination through international comparisons of national measurement standards. Following consultation, a draft version of the BIPM Work Programme is presented at each meeting of the General Conference for consideration with the BIPM dotation; the final programme of work is determined by the CIPM in accordance with the dotation agreed by the CGPM. Main work of the BIPM include: scientific and technical activities carried out in its four departments: chemistry, ionizing radiation, physical metrology, time.
The BIPM has an important role in maintaining accurate worldwide time of day. It combines and averages the official atomic time standards of member nations around the world to create a single, official Coordinated Universal Time. Since its establishment, the directors of the BIPM have been: The BIPM centralises diverse international projects involving public & independent institutes of metrology. For example: BIPM: International Key Comparison of Liquid Hydrocarbon Flow Facilities CCM-FF-K2. Metrologia History of the metre Institute for Reference Materials and Measurements International Organization for Standardization National Institute of Standards and Technology Seconds pendulum World Metrology Day The BIPM YouTube channel BIPM BIPM
The lumen is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source. Luminous flux differs from power in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model of the human eye's sensitivity to various wavelengths. Lumens are related to lux in; the lumen is defined in relation to the candela. A full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lumens. If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, the total luminous flux emitted into that angle is one lumen. Alternatively, an isotropic one-candela light-source emits a total luminous flux of 4π lumens. If the source were covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens; the luminous intensity would still be one candela in those directions.
The lumen can be thought of casually as a measure of the total "amount" of visible light in some defined beam or angle, or emitted from some source. The number of candelas or lumens from a source depends on its spectrum, via the nominal response of the human eye as represented in the luminosity function; the difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux; the same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Mathematically, 1 lx = 1 lm/m2. A source radiating a power of one watt of light in the color for which the eye is most efficient has luminous flux of 683 lumens. So a lumen represents at least 1/683 watts of visible light power, depending on the spectral distribution. Lamps used for lighting are labelled with their light output in lumens.
A 23 W spiral compact fluorescent lamp emits about 1,400–1,600 lm. Many compact fluorescent lamps and other alternative light sources are labelled as being equivalent to an incandescent bulb with a specific wattage. Below is a table that shows typical luminous flux for common incandescent bulbs and their equivalents. On 1 September 2010, European Union legislation came into force mandating that lighting equipment must be labelled in terms of luminous flux, instead of electric power; this change is a result of the EU's Eco-design Directive for Energy-using Products. For example, according to the European Union standard, an energy-efficient bulb that claims to be the equivalent of a 60 W tungsten bulb must have a minimum light output of 700–750 lm; the light output of projectors is measured in lumens. A standardized procedure for testing projectors has been established by the American National Standards Institute, which involves averaging together several measurements taken at different positions.
For marketing purposes, the luminous flux of projectors that have been tested according to this procedure may be quoted in "ANSI lumens", to distinguish them from those tested by other methods. ANSI lumen measurements are in general more accurate than the other measurement techniques used in the projector industry; this allows projectors to be more compared on the basis of their brightness specifications. The method for measuring ANSI lumens is defined in the IT7.215 document, created in 1992. First the projector is set up to display an image in a room at a temperature of 25 degrees Celsius; the brightness and contrast of the projector are adjusted so that on a full white field, it is possible to distinguish between a 5% screen area block of 95% peak white, two identically sized 100% and 90% peak white boxes at the center of the white field. The light output is measured on a full white field at nine specific locations around the screen and averaged; this average is multiplied by the screen area to give the brightness of the projector in "ANSI lumens".
Peak lumens is a measure of light output used with CRT video projectors. The testing uses a test pattern at either 10 and 20 percent of the image area as white at the center of the screen, the rest as black; the light output is measured just in this center area. Limitations with CRT video projectors result in them producing greater brightness when just a fraction of the image content is at peak brightness. For example, the Sony VPH-G70Q CRT video projector produces 1200 "peak" lumens but just 200 ANSI lumens. Brightness measures the total amount of light projected in lumens; the color brightness specification Color Light Output measures red and blue each on a nine-point grid, using the same approach as that used to measure brightness. Brightness Luminous efficacy Lux Nit
The gray is a derived unit of ionizing radiation dose in the International System of Units. It is defined as the absorption of one joule of radiation energy per kilogram of matter, it is used as a unit of the radiation quantity absorbed dose which measures the energy deposited by ionizing radiation in a unit mass of matter being irradiated, is used for measuring the delivered dose of ionising radiation in applications such as radiotherapy, food irradiation and radiation sterilization. As a measure of low levels of absorbed dose, it forms the basis for the calculation of the radiation protection unit the sievert, a measure of the health effect of low levels of ionizing radiation on the human body; the gray is used in radiation metrology as a unit of the radiation quantity kerma. The gray is an important unit in ionising radiation measurement and was named after British physicist Louis Harold Gray, a pioneer in the measurement of X-ray and radium radiation and their effects on living tissue; the gray was adopted as part of the International System of Units in 1975.
The corresponding cgs unit to the gray is the rad, which remains common in the United States, though "strongly discouraged" in the style guide for U. S. National Institute of Standards and Technology authors; the gray has a number of fields of application in measuring dose: The measurement of absorbed dose in tissue is of fundamental importance in radiobiology and radiation therapy as it is the measure of the amount of energy the incident radiation deposits in the target tissue. The measurement of absorbed dose is a complex problem due to scattering and absoption, many specialist dosimeters are available for these measurements, can cover applications in 1-D, 2-D and 3-D. In radiation therapy, the amount of radiation applied varies depending on the type and stage of cancer being treated. For curative cases, the typical dose for a solid epithelial tumor ranges from 60 to 80 Gy, while lymphomas are treated with 20 to 40 Gy. Preventive doses are around 45–60 Gy in 1.8–2 Gy fractions. The average radiation dose from an abdominal X-ray is 0.7 milli-Grays, that from an abdominal CT scan is 8 mGy, that from a pelvic CT scan is 6 mGy, that from a selective CT scan of the abdomen and the pelvis is 14 mGy.
The absorbed dose plays an important role in radiation protection, as it is the starting point for calculating the stochastic health risk of low levels of radiation, defined as the probability of cancer induction and genetic damage. The gray measures the total absorbed energy of radiation, but the probability of stochastic damage depends on the type and energy of the radiation and the types of tissues involved; this probability is related to the equivalent dose in sieverts, which has the same dimensions as the gray. It is related to the gray by weighting factors described in the articles on equivalent dose and effective dose; the International Committee for Weights and Measures states: "In order to avoid any risk of confusion between the absorbed dose D and the dose equivalent H, the special names for the respective units should be used, that is, the name gray should be used instead of joules per kilogram for the unit of absorbed dose D and the name sievert instead of joules per kilogram for the unit of dose equivalent H." 1 G y = 1 J k g = 1 m 2 s 2 The accompanying diagrams show how absorbed dose is first obtained by computational techniques, from this value the equivalent doses are derived.
For X-rays and gamma rays the gray is numerically the same value when expressed in sieverts, but for alpha particles one gray is equivalent to 20 sieverts, a radiation weighting factor is applied accordingly. Radiation poisoning - The gray is conventionally used to express the severity of what are known as "tissue effects" from doses received in acute exposure to high levels of ionizing radiation; these are effects which are certain to happen, as opposed to the uncertain effects of low levels of radiation which have a probability of causing damage. A whole-body acute exposure to 5 grays or more of high-energy radiation leads to death within 14 days; this dose represents 375 joules for a 75 kg adult. The gray is used to measure absorbed dose rates in non-tissue materials for processes such as radiation hardening, food irradiation and electron irradiation. Measuring and controlling the value of absorbed dose is vital to ensuring correct operation of these processes. Kerma is used in radiation metrology as a measure of the liberated energy of ionisation due to irradiation, is expressed in grays.
Kerma dose is different from absorbed dose, depending on the radiation energies involved because ionization energy is not accounted for. Whilst equal at low energies, kerma is much higher than absorbed dose at higher energies, because some energy escapes from the absorbing volume in the form of bremsstrahlung or fast-moving electrons. Kerma, when applied to air, is equivalent to the legacy roentgen unit of radiation exposure, but there is a difference in the definition of these two units; the gray is defined independently of any target material, the roengten was defined by the ionisation effect in dry air, w
The tonne referred to as the metric ton in the United States and Canada, is a non-SI metric unit of mass equal to 1,000 kilograms or one megagram. It is equivalent to 2,204.6 pounds, 1.102 short tons or 0.984 long tons. Although not part of the SI, the tonne is accepted for use with SI units and prefixes by the International Committee for Weights and Measures; the tonne is derived from the weight of 1 cubic metre of pure water. The SI symbol for the tonne is't', adopted at the same time as the unit in 1879, its use is official for the metric ton in the United States, having been adopted by the United States National Institute of Standards and Technology. It is a symbol, not an abbreviation, should not be followed by a period. Use of upper and lower case is significant, use of other letter combinations is not permitted and would lead to ambiguity. For example,'T','MT','Mt','mt' are the SI symbols for the tesla, megatesla and millitonne respectively. If describing TNT equivalent units of energy, this is equivalent to 4.184 petajoules.
In French and most varieties of English, tonne is the correct spelling. It is pronounced the same as ton, but when it is important to clarify that the metric term is meant, rather than short ton, the final "e" can be pronounced, i.e. "tonny". In Australia, it is pronounced. Before metrication in the UK the unit used for most purposes was the Imperial ton of 2,240 pounds avoirdupois or 20 hundredweight, equivalent to 1,016 kg, differing by just 1.6% from the tonne. The UK Weights and Measures Act 1985 explicitly excluded from use for trade certain imperial units, including the ton, unless the item being sold or the weighing equipment being used was weighed or certified prior to 1 December 1980, then only if the buyer was made aware that the weight of the item was measured in imperial units. In the United States metric ton is the name for this unit used and recommended by NIST. Both spellings are acceptable in Canadian usage. Ton and tonne are both derived from a Germanic word in general use in the North Sea area since the Middle Ages to designate a large cask, or tun.
A full tun, standing about a metre high, could weigh a tonne. An English tun of wine weighs a tonne, 954 kg if full of water, a little less for wine; the spelling tonne pre-dates the introduction of the SI in 1960. In the United States, the unit was referred to using the French words millier or tonneau, but these terms are now obsolete; the Imperial and US customary units comparable to the tonne are both spelled ton in English, though they differ in mass. One tonne is equivalent to: Metric/SI: 1 megagram. Equal to 1000000 grams or 1000 kilograms. Megagram, Mg, is the official SI unit. Mg is distinct from milligram. Pounds: Exactly 1000/0.453 592 37 lb, or 2204.622622 lb. US/Short tons: Exactly 1/0.907 184 74 short tons, or 1.102311311 ST. One short ton is 0.90718474 t. Imperial/Long tons: Exactly 1/1.016 046 9088 long tons, or 0.9842065276 LT. One long ton is 1.0160469088 t. For multiples of the tonne, it is more usual to speak of millions of tonnes. Kilotonne and gigatonne are more used for the energy of nuclear explosions and other events in equivalent mass of TNT loosely as approximate figures.
When used in this context, there is little need to distinguish between metric and other tons, the unit is spelt either as ton or tonne with the relevant prefix attached. *The equivalent units columns use the short scale large-number naming system used in most English-language countries, e.g. 1 billion = 1,000 million = 1,000,000,000.†Values in the equivalent short and long tons columns are rounded to five significant figures, see Conversions for exact values.ǂThough non-standard, the symbol "kt" is used for knot, a unit of speed for aircraft and sea-going vessels, should not be confused with kilotonne. A metric ton unit can mean 10 kilograms within metal trading within the US, it traditionally referred to a metric ton of ore containing 1% of metal. The following excerpt from a mining geology textbook describes its usage in the particular case of tungsten: "Tungsten concentrates are traded in metric tonne units (originally designating one tonne of ore containing 1% of WO3, today used to measure WO3 quantities in 10 kg units.
One metric tonne unit of tungsten contains 7.93 kilograms of tungsten." Note that tungsten is known as wolfram and has the atomic symbol W. In the case of uranium, the acronym MTU is sometimes considered to be metric ton of uranium, meaning 1,000 kg. A gigatonne of carbon dioxide equivalent is a unit used by the UN climate change panel, IPCC, to measure the effect of a technolo
An hour is a unit of time conventionally reckoned as 1⁄24 of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions. The hour was established in the ancient Near East as a variable measure of 1⁄12 of the night or daytime; such seasonal, temporal, or unequal hours varied by latitude. The hour was subsequently divided into each of 60 seconds. Equal or equinoctial hours were taken as 1⁄24 of the day. Since this unit was not constant due to long term variations in the Earth's rotation, the hour was separated from the Earth's rotation and defined in terms of the atomic or physical second. In the modern metric system, hours are an accepted unit of time defined as 3,600 atomic seconds. However, on rare occasions an hour may incorporate a positive or negative leap second, making it last 3,599 or 3,601 seconds, in order to keep it within 0.9 seconds of UT1, based on measurements of the mean solar day. The modern English word hour is a development of the Anglo-Norman houre and Middle English ure, first attested in the 13th century.
It displaced the Old English "tide" and "stound". The Anglo-Norman term was a borrowing of Old French ure, a variant of ore, which derived from Latin hōra and Greek hṓrā. Like Old English tīd and stund, hṓrā was a vaguer word for any span of time, including seasons and years, its Proto-Indo-European root has been reconstructed as *yeh₁-, making hour distantly cognate with year. The time of day is expressed in English in terms of hours. Whole hours on a 12-hour clock are expressed using the contracted phrase o'clock, from the older of clock. Hours on a 24-hour clock are expressed as "hundred" or "hundred hours". Fifteen and thirty minutes past the hour is expressed as "a quarter past" or "after" and "half past" from their fraction of the hour. Fifteen minutes before the hour may be expressed as "a quarter to", "of", "till", or "before" the hour; the ancient Egyptians began dividing the night into wnwt at some time before the compilation of the Dynasty V Pyramid Texts in the 24th century BC. By 2150 BC, diagrams of stars inside Egyptian coffin lids—variously known as "diagonal calendars" or "star clocks"—attest that there were 12 of these.
Clagett writes that it is "certain" this duodecimal division of the night followed the adoption of the Egyptian civil calendar placed c. 2800 BC on the basis of analyses of the Sothic cycle, but a lunar calendar long predated this and would have had twelve months in each of its years. The coffin diagrams show that the Egyptians took note of the heliacal risings of 36 stars or constellations, one for each of the ten-day "weeks" of their civil calendar; each night, the rising of eleven of these decans were noted, separating the night into twelve divisions whose middle terms would have lasted about 40 minutes each. The original decans used by the Egyptians would have fallen noticeably out of their proper places over a span of several centuries. By the time of Amenhotep III, the priests at Karnak were using water clocks to determine the hours; these were filled to the brim at sunset and the hour determined by comparing the water level against one of its twelve gauges, one for each month of the year.
During the New Kingdom, another system of decans was used, made up of 24 stars over the course of the year and 12 within any one night. The division of the day into 12 hours was accomplished by sundials marked with ten equal divisions; the morning and evening periods when the sundials failed to note time were observed as the first and last hours. The Egyptian hours were connected both with the priesthood of the gods and with their divine services. By the New Kingdom, each hour was conceived as a specific region of the sky or underworld through which Ra's solar barge travelled. Protective deities were used as the names of the hours; as the protectors and resurrectors of the sun, the goddesses of the night hours were considered to hold power over all lifespans and thus became part of Egyptian funerary rituals. Two fire-spitting cobras were said to guard the gates of each hour of the underworld, Wadjet and the rearing cobra were sometimes referenced as wnwt from their role protecting the dead through these gates.
The Egyptian for astronomer, used as a synonym for priest, was wnwty, "One of the Hours" or "Hour-Watcher". The earliest forms of wnwt include one or three stars, with the solar hours including the determinative hieroglyph for "sun". Ancient China divided its day into 100 "marks" running from midnight to midnight; the system is said to have been used since remote antiquity, credited to the legendary Yellow Emperor, but is first attested in Han-era water clocks and in the 2nd-century history of that dynasty. It was measured with sundials and water clocks. Into the Eastern Han, the Chinese measured their day schematically, adding the 20-ke difference between the solstices evenly throughout the year, one every nine days. During the night, time was more commonly