universal time (UT), the international time standard common to every place in the world, it nominally reflects the mean solar time along the earth's prime meridian (renumbered to equate to civil time). In 1884, under international agreement, the prime meridian was established as running through the Royal Observatory in Greenwich, England, setting the standard of Greenwich mean time (GMT). In keeping with tradition, the start of a solar day occurred at noon. In 1925 the numbering system for GMT was changed so that the day began at midnight to make it consistent with the civil day. Some confusion in terminology resulted, however, and in 1928 the International Astronomical Union (IAU) changed the designation of the standard time of the prime meridian to universal time. In 1955 the IAU defined several kinds of UT. The initial values of universal time obtained at 75 observatories, denoted UT0, differ slightly because of polar motion. By adding a correction each observatory converts UT0 into UT1, which gives the Earth's rotational position in space. An empirical correction to take account of annual changes in the speed of rotation is then added to convert UT1 to UT2. However, UT2 has since been superseded by atomic time (time as given by atomic clocks). Universal time is also called world time, Z time, and Zulu time.

In 1964 a new timescale, called coordinated universal time (UTC), was internationally adopted. UTC is more uniform and more accurate than the UT2 system because the UTC second is based on atomic time (although the UTC year is still based on the time it takes the earth to complete one orbit). Because the rate of the earth's rotation is gradually slowing, it is occasionally necessary to add an extra second, called the leap second, to the length of the UTC year; synchronization is obtained by making the last minute of June or December contain 61 seconds. About one leap second per year has been inserted since 1972.

universal language, a language intended to further communication and goodwill among peoples speaking different languages without necessarily replacing their native tongues. See international language.

universal gas constant: see gas laws.

Universal Product Code: see bar code.

Universal Postal Union (UPU), specialized agency of the United Nations, with headquarters at Bern, Switzerland. Established in 1875 following adoption of the Universal Postal Convention, it is one of the oldest extant international governmental organizations. The union was associated with the United Nations in 1947. The Universal Postal Congress, the governing body, usually meets every five years to consider the activities of the executive council of 40 members elected on a geographical basis and meeting yearly. Countries that belong to the union form a unified postal territory with easy international exchange of mail. The union also provides technical assistance and advice in attempts to improve postal service, especially in developing countries. The UPU has 191 member nations.

Standard bar code used to identify grocery and other retail merchandise. In the UPC system, the five digits on the left are assigned to a particular manufacturer or maker and the five digits on the right are used by that manufacturer to identify a specific type or make of product.

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In metaphysics, epistemology, and logic, a general category, such as a property or relation, considered as distinct from the particular things that instantiate or exemplify it. The problem of universals concerns the question of what sort of being should be ascribed to such categories (e.g., is there any such thing as redness apart from particular red things?). The debate over the status of universals stems from Plato's theory of forms. Whereas Plato held that forms (universals) exist independently of particulars, Aristotle argued that forms exist only in the particulars in which they are exemplified. Seealso realism.

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Statement that any particle of matter in the universe attracts any other with a force (math.F) that is proportional to the product of their masses (math.m₁ and math.m₂) and inversely proportional to the square of the distance (math.R) between them. In symbols: math.F = math.G(math.m₁math.m₂)/math.R², where math.G is the gravitational constant. Isaac Newton put forth the law in 1687 and used it to explain the observed motions of the planets and their moons, which had been reduced to mathematical form by Johannes Kepler early in the 17th century.

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