1.
Prime meridian
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A prime meridian is a meridian in a geographical coordinate system at which longitude is defined to be 0°. Together, a meridian and its antimeridian form a great circle. This great circle divides the sphere, e. g. the Earth, if one uses directions of East and West from a defined prime meridian, then they can be called Eastern Hemisphere and Western Hemisphere. The most widely used modern meridian is the IERS Reference Meridian and it is derived but deviates slightly from the Greenwich Meridian, which was selected as an international standard in 1884. The notion of longitude was developed by the Greek Eratosthenes in Alexandria, and Hipparchus in Rhodes, but it was Ptolemy who first used a consistent meridian for a world map in his Geographia. The main point is to be comfortably west of the tip of Africa as negative numbers were not yet in use. His prime meridian corresponds to 18°40 west of Winchester today, at that time the chief method of determining longitude was by using the reported times of lunar eclipses in different countries. Ptolemys Geographia was first printed with maps at Bologna in 1477, but there was still a hope that a natural basis for a prime meridian existed. The Tordesillas line was settled at 370 leagues west of Cape Verde. This is shown in Diogo Ribeiros 1529 map, in 1541, Mercator produced his famous 41 cm terrestrial globe and drew his prime meridian precisely through Fuertaventura in the Canaries. His later maps used the Azores, following the magnetic hypothesis, but by the time that Ortelius produced the first modern atlas in 1570, other islands such as Cape Verde were coming into use. In his atlas longitudes were counted from 0° to 360°, not 180°W to 180°E as is usual today and this practice was followed by navigators well into the 18th century. In 1634, Cardinal Richelieu used the westernmost island of the Canaries, Ferro, 19°55 west of Paris, the geographer Delisle decided to round this off to 20°, so that it simply became the meridian of Paris disguised. In the early 18th century the battle was on to improve the determination of longitude at sea, between 1765 and 1811, Nevil Maskelyne published 49 issues of the Nautical Almanac based on the meridian of the Royal Observatory, Greenwich. Maskelynes tables not only made the lunar method practicable, they made the Greenwich meridian the universal reference point. In 1884, at the International Meridian Conference in Washington, D. C.22 countries voted to adopt the Greenwich meridian as the meridian of the world. The French argued for a line, mentioning the Azores and the Bering Strait. In October 1884 the Greenwich Meridian was selected by delegates to the International Meridian Conference held in Washington, united States to be the common zero of longitude and standard of time reckoning throughout the world

2.
Zeroth law of thermodynamics
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The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other. Accordingly, thermal equilibrium between systems is a transitive relation, two systems are said to be in the relation of thermal equilibrium if they are linked by a wall permeable only to heat and they do not change over time. The physical meaning of the law was expressed by Maxwell in the words, for this reason, another statement of the law is All diathermal walls are equivalent. The law is important for the formulation of thermodynamics, which needs the assertion that the relation of thermal equilibrium is an equivalence relation. This information is needed for a definition of temperature that will agree with the physical existence of valid thermometers. A thermodynamic system is by definition in its own state of thermodynamic equilibrium. One precise statement of the law is that the relation of thermal equilibrium is an equivalence relation on pairs of thermodynamic systems. This means that a tag can be assigned to every system. This property is used to justify the use of temperature as a tagging system. This statement asserts that thermal equilibrium is a relation between thermodynamic systems. If we also define that every system is in thermal equilibrium with itself. Binary relations that are both reflexive and Euclidean are equivalence relations, one consequence of an equivalence relationship is that the equilibrium relationship is symmetric, If A is in thermal equilibrium with B, then B is in thermal equilibrium with A. Thus we may say that two systems are in equilibrium with each other, or that they are in mutual equilibrium. A reflexive, transitive relationship does not guarantee an equivalence relationship, in order for the above statement to be true, both reflexivity and symmetry must be implicitly assumed. It is the Euclidean relationships which apply directly to thermometry, an ideal thermometer is a thermometer which does not measurably change the state of the system it is measuring. The zeroth law provides no information regarding this final reading, the zeroth law establishes thermal equilibrium as an equivalence relationship. An equivalence relationship on a set divides that set into a collection of distinct subsets where any member of the set is a member of one, in the case of the zeroth law, these subsets consist of systems which are in mutual equilibrium. This partitioning allows any member of the subset to be tagged with a label identifying the subset to which it belongs

3.
Zero-length launch
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The zero-length launch system or zero-length take-off system was a system whereby jet fighters and attack aircraft were intended to be placed on rockets attached to mobile launch platforms. Most zero length launch experiments took place in the 1950s, during the Cold War, the primary advantage of a zero-length launch system is the elimination of the need for a vulnerable airfield for takeoffs. In the event of an attack, air forces could field effective air defenses. Although launching aircraft using rocket boosters proved to be relatively trouble-free, bulky mobile launching platforms also proved to be expensive to operate and difficult to transport. Security would also have been an issue with mobile launchers, especially if equipped with nuclear-armed strike fighters, the United States Air Force, the Bundeswehrs Luftwaffe, and the Soviets VVS all conducted experiments in zero-length launching. The first manned aircraft to be ZELL-launched was an F-84G in 1955, the Soviets main interest in ZELL was for point defense-format protection of airfields and critical targets using MiG-19s. All works upon ZELL aircraft were abandoned due to logistical concerns, examples of these include British Hawker Siddeley Harrier, Soviet Yak-38 and American McDonnell Douglas F-15 STOL/MTD. Bachem Ba 349 World War II vertical launch rocket interceptor The Zero-Length Launch Fighter, archived from the original on June 13,2012. 38th Tactical Missile Wing, tribute site, recent photos of the hard-site test buildings for Mace Video of MiG-19 performing a ZELL-style launch

4.
Lebesgue measure
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In measure theory, the Lebesgue measure, named after French mathematician Henri Lebesgue, is the standard way of assigning a measure to subsets of n-dimensional Euclidean space. For n =1,2, or 3, it coincides with the measure of length, area. In general, it is also called n-dimensional volume, n-volume and it is used throughout real analysis, in particular to define Lebesgue integration. Both were published as part of his dissertation in 1902, the Lebesgue measure is often denoted dx, but this should not be confused with the distinct notion of a volume form. Given a subset E ⊆ R, with the length of an interval I = given by ℓ = b − a, sets that arent included in the Lebesgue σ-algebra are not Lebesgue measurable. Such sets do exist, i. e. Lebesgue σ-algebra is strictly contained in the set of R. The first part of the states that the subset E of the real numbers is reduced to its outer measure by coverage by sets of intervals. Each of these sets of intervals I covers E in the sense that when the intervals are combined together by union, the Lebesgue outer measure emerges as the greatest lower bound of the lengths from among all possible such sets. Intuitively, it is the length of those interval sets which fit E most tightly. That characterizes the Lebesgue outer measure, whether this outer measure translates to the Lebesgue measure proper depends on an additional condition. These partitions of A are subject to the outer measure, any closed interval of real numbers is Lebesgue measurable, and its Lebesgue measure is the length b−a. The open interval has the measure, since the difference between the two sets consists only of the end points a and b and has measure zero. Any Cartesian product of intervals and is Lebesgue measurable, and its Lebesgue measure is, moreover, every Borel set is Lebesgue measurable. However, there are Lebesgue measurable sets which are not Borel sets, any countable set of real numbers has Lebesgue measure 0. In particular, the Lebesgue measure of the set of numbers is 0. The Cantor set is an example of a set that has Lebesgue measure zero. If the axiom of determinacy holds then all sets of reals are Lebesgue measurable, determinacy is however not compatible with the axiom of choice. Vitali sets are examples of sets that are not measurable with respect to the Lebesgue measure and their existence relies on the axiom of choice

5.
Zero-lift axis
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For this reason, on a cambered aerofoil the zero-lift line is better than the chord line when describing the angle of attack. When symmetric aerofoils are moving parallel to the line of the aerofoil. However, when cambered aerofoils are moving parallel to the chord line, for symmetric aerofoils, the chord line and the zero lift line are the same. ISBN 0-273-01120-0 Kermode, A. C. Mechanics of Flight, Chapter 3, Pitman Publishing ISBN 0-273-31623-0

6.
Spring (device)
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A spring is an elastic object used to store mechanical energy. Springs are usually out of spring steel. There are a number of spring designs, in everyday usage the term often refers to coil springs. When a spring is compressed or stretched from its resting position, the rate or spring constant of a spring is the change in the force it exerts, divided by the change in deflection of the spring. That is, it is the gradient of the force versus deflection curve, an extension or compression springs rate is expressed in units of force divided by distance, for example lbf/in or N/m. A torsion spring is a spring that works by twisting, when it is twisted about its axis by an angle, a torsion springs rate is in units of torque divided by angle, such as N·m/rad or ft·lbf/degree. The inverse of spring rate is compliance, that is, if a spring has a rate of 10 N/mm, the stiffness of springs in parallel is additive, as is the compliance of springs in series. Springs are made from a variety of materials, the most common being spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel, some non-ferrous metals are also used including phosphor bronze and titanium for parts requiring corrosion resistance and beryllium copper for springs carrying electrical current. Simple non-coiled springs were used throughout history, e. g. the bow. In the Bronze Age more sophisticated spring devices were used, as shown by the spread of tweezers in many cultures, coiled springs appeared early in the 15th century, in door locks. The first spring powered-clocks appeared in that century and evolved into the first large watches by the 16th century, in 1676 British physicist Robert Hooke discovered Hookes law which states that the force a spring exerts is proportional to its extension. Compression spring – is designed to operate with a compression load, flat spring – this type is made of a flat spring steel. Machined spring – this type of spring is manufactured by machining bar stock with a lathe and/or milling operation rather than a coiling operation, since it is machined, the spring may incorporate features in addition to the elastic element. Machined springs can be made in the load cases of compression/extension, torsion. Serpentine spring - a zig-zag of thick wire - often used in modern upholstery/furniture, the most common types of spring are, Cantilever spring – a spring which is fixed only at one end. Coil spring or helical spring – a spring is of two types, Tension or extension springs are designed to become longer under load and their turns are normally touching in the unloaded position, and they have a hook, eye or some other means of attachment at each end. Compression springs are designed to become shorter when loaded and their turns are not touching in the unloaded position, and they need no attachment points