An article from Science News Jan 2020

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An article from Science News May 2019

## The kilogram just got a revamp. A unit of time might be next

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## Addition

One of the four fundamental operations of arithmetic and algebra. The symbol + of addition is thought to be a ligature for “et,” used in a German manuscript of 1456 to denote addition. Its first printed appearance is in Johann Widman's Behennede und…

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## Air navigation

A discipline that combines the means to know the location and heading of an aircraft with respect to some map or coordinate system, with guidance which means steering the aircraft along some desired path toward a destination. The construction of the…

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## Air-traffic control

A service to promote the safe, orderly, and expeditious flow of air traffic. Safety is principally a matter of preventing collisions with other aircraft, obstructions, and the ground; assisting aircraft in avoiding hazardous weather; assuring that…

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## Aircraft collision avoidance system

A device that reduces the risk of midair collision by providing advisories to the flight crew. It is known in the United States as a Traffic Alert and Collision Avoidance System (TCAS), and operates independently of the ground-based air-traffic…

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## Aircraft compass system

An instrument that indicates the bearing, or angle of the direction in which an aircraft is pointing in the horizontal plane. A compass may indicate magnetic heading or bearing, bearing referenced to a radio signal source, or bearing with respect to…

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## Aircraft instrumentation

A coordinated group of instruments that provides the flight crew with information about the aircraft and its subsystems. Together with the controls, aircraft instruments form the human–machine interfaces that enable the flight crew to operate the…

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## Airport surface detection equipment

A ground mapping system that uses analog radar equipment to provide surveillance of aircraft and other surface vehicles on an airport surface. It is used by air-traffic controllers in airport control towers to monitor and control the movement of…

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## Airspeed indicator

A device that computes and displays speed of an aircraft relative to the air mass in which the aircraft is flying. Three common types are the indicated airspeed meter, true airspeed indicator, and Machmeter.The commonest type is the indicated…

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## Algebra

The branch of mathematics dealing with the solution of equations. These equations involve unknowns, or variables, along with fixed numbers from a specified system (Fig. 1). The origins of algebra were based on the need to develop equations that…

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## Algebraic geometry

The study of zero sets of polynomial equations. Examples are the parabola y − x2 = 0, thought of as sitting in the (x, y)-plane, and the locus of all points (t, t2, t3, t4), which is defined by Eqs. (1), 1 in the coordinate space with coordinates w,…

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## Almanac

A book that contains astronomical or meteorological data arranged according to days, weeks, and months of a given year and may also include diverse information of a nonastronomical character. This article is restricted to astronomical and…

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## Altimeter

Any device which measures the height of an aircraft. The two chief types are the pressure altimeter, which measures the aircraft's distance above sea level, and the radar altimeter, which measures distance above the ground. Pressure Altimeter A…

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## Analysis of variance

Total variation in experimental data is partitioned into components assignable to specific sources by the analysis of variance. This statistical technique is applicable to data for which (1) effects of sources are additive, (2) uncontrolled or…

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## Analytic geometry

A branch of mathematics in which algebra is applied to the study of geometry. Because algebraic methods were first systematically applied to geometry in 1637 by the French philosopher-mathematician René Descartes, the subject is also called…

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## Analytic hierarchy

A framework for solving a problem. The analytic hierarchy process is a systematic procedure for representing the elements of any problem. It organizes the basic rationality by breaking down a problem into its smaller constituents and then calls for…

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## Area

The superficial contents of a geometrical figure of two dimensions. The area of any rectangle or square is the product of two adjacent sides, one of which may be called the base and the other the altitude. In general, any line segment that partially…

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## Arithmetic

A branch of mathematics dealing with numbers, operations on numbers, and computation. Arithmetic is useful in solving many practical problems, such as buying, selling, budgets, sports statistics, and measurement. The usual numbers of arithmetic are…

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## Asymptote

If the graph of a function y = f (x) goes out to infinity, each “end” of the graph may get closer and closer to a straight line, which is then called an asymptote. In other words, some straight line may well approximate the graph as the function…

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## Automatic horizon

A device that provides the pilot with symbols representative of the attitude of an aircraft relative to an artificial horizon. In an automatic or artificial horizon a vertical gyro moves a horizon bar relative to a fixed aircraft index. Motion of…

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## Automatic landing system

The means for guiding and controlling aircraft from an initial approach altitude to a point where safe contact is made with the landing surface. Such systems differ from low-approach systems in three major respects: (1) They furnish not only…

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## Autopilot

An automatic means for steering an aircraft, ship, or other vehicle. The terminology means to automatically (autonomously) control a mode of the vehicle, usually its trajectory. Examples include altitude, speed, heading, accelerations, body rates,…

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## Barycentric calculus

The application of the properties of the centroid of a set of weighted points to geometry. For certain new operations such as the addition of point and point or of point and vector, it is possible to give their vector equivalents and work within the…

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## Bayesian statistics

An approach to statistics in which estimates are based on a synthesis of a prior distribution and current sample data. Bayesian statistics is not a branch of statistics in the way that, say, nonparametric statistics is. It is, in fact, a…