To describe such a huge range, astronomers need a way to avoid confusing terms like "a billion trillion" and "a millionth". The distances and sizes of of the objects astronomers study vary from very small, including atoms and atomic nuclei, to very large including galaxies, clusters of galaxies and the size of the universe. 1 pc = 3.09 x 10 13 km = 3.26 lyįor even greater distances, astronomers use kiloparsecs and megaparsecs (abbreviated kpc and Mpc). A parsec is the distance at which 1 AU subtends an angle of 1 arcsec. This is because its definition is closely related to a method of measuring the distances between stars. Many astronomers prefer to use parsecs (abbreviated pc) to measure distance to stars. This means light from Proxima Centauri takes 4.2 years to travel to Earth. Proxima Centauri is the nearest star to Earth (other than the Sun) and is 4.2 light-years away. A light-year is the distance that light travels in a vacuum in one year: 1 ly = 9.5 x 10 12 km = 63,240 AU To measure the distances between stars, astronomers often use light-years (abbreviated ly). The distance from the Sun to the center of the Milky Way is approximately 1.7 x 10 9 AU. Jupiter is about 5.2 AU from the Sun and Pluto is about 39.5 AU from the Sun. An astronomical unit is the average distance between the Earth and the Sun: 1 AU = 1.496 x 10 8 km = 93 million miles However, the distances and sizes in the universe can be so big, that astronomers have invented more units to describe distance.ĭistances in the solar system are often measured in astronomical units (abbreviated AU). They often use meters for length, kilograms for mass, and seconds for time. To keep itself informed of and study critically the status of physics.Astronomers use many of the same units of measurement as other scientists.To assist industries, national and other agencies in their developmental tasks by precision measurements, calibration, development of devices, processes, and other allied problems related to physics.To identify and conduct after due consideration, research in areas of physics which are most appropriate to the needs of the nation and for the advancement of the field.To establish, maintain and improve continuously by research, for the benefit of the nation,.To strengthen and advance physics-based research and development for the overall development of science and technology in the country.The weights and balances used in local markets and other areas are expected to be certified by the Department of Weights and Measures of the local government.It is the responsibility of the NPL to calibrate the measurement standards in these laboratories at different levels.Responsibility of National Physical Laboratory (NPL): Physical quantities can only be added if they have the same Units Currently Accepted for Use with the SI System: NameĬalculation of number of kilometers in 20 miles:Ĭonversion factors a mile = 5280 ft, 1 ft = 12 in, 1 in = 2.54 cm, 1 m = 100 cm, 1 km = 1000 m.Ģ0 miles = 20 × 5280 ft = 20 × 5280 × 12 inches Units outside SI but Frequently Used in Physics: Name shake is used to measure very small time.Extremely small areas are measured in barn.Mass is measured in slug, metric ton, quintal.1 chandrashekhar unit = 1.4 times mass of the Sun. Chandra Shekhar unit is practical unit of measuring large masses.One a.m.u.: It is defined as 1/12 th of the mass of one atom. 1 a.m.u.A light-year (ly), parsec (pc) are used to measure the distance between stars and the astronomical unit is used to measure the distance between planets. Note that light-year (ly) and parsec (pc) are units of distances and not of time.
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