year, time required for the earth to complete one orbit about the sun. The solar or tropical year is measured relative to the sun and is equal to 365 days, 5 hr, 48 min, 46 sec of mean solar time (see solar time). The sidereal year, measured relative to the stars, is longer than the tropical year by about 20 min. The calendar year is used for practical purposes and always contains a whole number of days, the ordinary year being 365 days and the leap year 366 days.

tropical year, time between successive vernal equinoxes; 365 days, 5 hr, 48 min, 46 sec of mean solar time (see solar time). The tropical year is the basis of the year used in the Gregorian calendar.

sidereal year, time required for the earth to complete an orbit of the sun relative to the stars. The sidereal year is 365 days, 6 hr, 9 min, 9.5 sec of mean solar time (see solar time). It is 20 min longer than the tropical year because of the precession of the equinoxes; for this reason, the sidereal year does not stay in step with the seasons.

seventeen-year locust: see cicada.

light-year, in astronomy, unit of length equal to the distance light travels in one sidereal year. It is 9.461 × 10¹² km (about 6 million million mi). Alpha Centauri and Proxima Centauri, the stars nearest our solar system, are about 4.3 light-years distant. See also parsec.

leap year: see calendar.

holy year: see jubilee.

anomalistic year, time required for the earth to go from the perihelion point once around the sun and back to the perihelion point. It is 365 days, 6 hr, 13 min, 53.0 sec of mean solar time (see solar time). The anomalistic year is longer than the sidereal year and the tropical year because of the eastward motion of the line of apsides (see apsis), which is caused by the slow rotation of the earth's orbit as a whole.

Year 2000 problem, Y2K problem, or millennium bug, in computer science, a design flaw in the hardware or software of a computer that caused erroneous results when working with dates beyond Dec. 31, 1999. In the 1960s and 70s programmers who designed computer systems dropped the first two digits of a year when storing or processing dates to save what then was expensive and limited memory; such a system recorded the year 2000 as 00 and could not distinguish it from 1900. In sorting, comparison, and arithmetic operations, the year 2000 would be treated as if it were equivalent to 0 rather than 100, causing incorrect results. The algorithm used to calculate leap years was also in some cases invalid, creating an additional problem in calculating the correct date after Feb. 28, 2000. Because the designers of such computer systems expected them to be replaced before the beginning of the year 2000, using a two-digit date was not regarded as a problem. Thousands of older computer systems, called legacy systems, were still in use in the 1990s, however, particularly in the finance and insurance industries, creating a potential operational and financial nightmare, which was termed Doomsday 2000. In the late 1990s business, government, and other computer users spent thousands of hours and millions of dollars to correct the Year 2000 problem, and only minor problems were experienced after Jan. 1, 2000.

International Geophysical Year (IGY), 18-month period from July, 1957, through Dec., 1958, during a period of maximum sunspot activity, designated for cooperative study of the solar-terrestrial environment by the scientists of 67 nations. The only prior combined international scientific efforts took place during the Polar Years of 1882 and 1932. Discoveries were made in the fields of cosmic ray research, climatology, oceanography, and the nature of the earth's atmosphere and magnetic field. Earth satellites (see satellite, artificial) launched by the United States discovered the Van Allen radiation belts, the influx of charged solar particles detected believed to be responsible for northern and southern polar auroras. Soundings of the world's oceans revealed new information about the physical features on the ocean floor. Seismically active rifts along the summits of mid-oceanic ridges were identified. IGY scientists conducted extensive studies of deep ocean currents and developed better gravity measurements for mineral exploration. The major programs of IGY were continued from Jan., 1958, to Jan., 1959, as the International Geophysical Cooperation. Also connected to IGY was the International Years of the Quiet Sun, an international cooperative program during 1964 to 1965, that focused on solar-terrestrial phenomena during a quiet sun, or near sunspot minimum. The IGY was the largest and most important international scientific effort to that date. One of its many later ramifications was the setting aside of Antarctica as a nonmilitary region to be used for international scientific purposes alone. Antarctica has become a base for collecting meteorological data, including information on the presence and effects of moisture, carbon dioxide, and electrified particles on the atmosphere, and the general circulation of the atmosphere.

Geophysical Year, International: see International Geophysical Year.

Five-Year Plan, Soviet economic practice of planning to augment agricultural and industrial output by designated quotas for a limited period of usually five years. Nations other than the former USSR and the Soviet bloc members, especially developing countries, have adopted such plans for four, five, or more years. Joseph Stalin, in 1928, launched the first Five-Year Plan; it was designed to industrialize the USSR in the shortest possible time and, in the process, to expedite the collectivization of farms. The plan, put into action ruthlessly, aimed at making the USSR self-sufficient and emphasized heavy industry at the expense of consumer goods. It covered the period from 1928 to 1933, but was officially considered completed in 1932. The second Five-Year Plan (1933-37) continued and expanded the first. The third plan (1938-42) was interrupted by World War II. The fourth covered the years 1946-50, the fifth 1951-55. The sixth plan (1956-60) was discarded in 1957, primarily because it overcommitted available resources and could not be fulfilled. It was replaced by a Seven-Year Plan (1959-65), which fell far short of estimated increases in agricultural (especially wheat) production. The Seven-Year Plan was considered the start of a longer period (20 years) devoted to the establishment of the material and technical basis of a Communist society. The late 1960s and early 1970s saw increased emphasis placed on consumer goods, and the 9th Five-Year Plan (1971-75) for the first time gave priority to light industry rather than heavy industry. The agricultural sector still grew far less than projected in the 10th (1976-80) and 11th (1981-85) Five-Year Plans, and overall economic performance was poor. The 12th and final Five-Year Plan (1986-90) projected increases in consumer goods and energy savings, but the economy began to slide, shrinking by 4% in 1990. The dissolution of the Soviet Union made the formation of a 13th Five-Year Plan a moot point.

Time required for the Earth to travel once around the Sun, slightly less than 365 14 days. This fractional number makes necessary the periodic adjustment of days in any calendar that is to be kept in step with the seasons. In the Gregorian calendar, a common year contains 365 days, and every fourth year is a leap year of 366 days except for any year that is divisible by 100 but not divisible by 400 (e.g., 1900 was not a leap year).

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Distance traveled by light moving in a vacuum in one year, at its accepted speed of 186,282 mi/second (299,792 km/second). It equals about 5.9 trillion mi (9.5 trillion km), 63,240 astronomical units, or 0.307 parsec.

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