The parsec is defined as being equal to the length of the adjacent leg (opposite leg being 1 AU) of an extremely elongated imaginary right triangle in space. Please help improve it by merging similar text or removing repeated statements. This section may contain content that is repetitive or redundant of text elsewhere in the article. This corresponds to the small-angle definition of the parsec found in many astronomical references. In August 2015, the International Astronomical Union (IAU) passed Resolution B2 which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly 648 000 / π au, or approximately 30.856 775 814 913 673 ×10 15 metres (based on the IAU 2012 definition of the astronomical unit). Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs (kpc) for the more distant objects within and around the Milky Way, megaparsecs (Mpc) for mid-distance galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies. Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. The word parsec is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913 to make calculations of astronomical distances from only raw observational data easy for astronomers. Most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand. The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun. The parsec unit is obtained by the use of parallax and trigonometry, and is defined as the distance at which 1 au subtends an angle of one arcsecond ( 1 / 3600 of a degree). 30.9 trillion kilometres (19.2 trillion miles). The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (au), i.e. For example, if a star is 1,000 light years away from Earth, and astronomer sees the star's light in a telescope, he or she is seeing the star as it was 1,000 years ago because it takes 1,000 years for the star's light to reach the Earth.A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale) Understanding light years and parsecs can be tricky. It is equal to one billion parsecs! Rewind A parsec is 3.26 light years and takes the angle the object is to the Earth's position into account. Astronomers more often use a unit called a parsec to measure distances in space. Light years, however, are not the largest units to measure distance. The resulting number would be larger than most calculators could display. To find out the actual distance in miles between the Milky Way and the Andromeda Galaxy, you could multiply 5.88 trillion by 2.5 million. The nearest galaxy to our Milky Way, however, called the Andromeda Galaxy is 2.5 million light years away. It has been determined that Alpha Centauri is 4.22 light years away. For example the nearest star other than the sun to our solar system is called Alpha Centauri. When scientists use light years to describe the distances between objects in space, the actual numerical distances are so vast, they're inconceivable to the human mind. The sun, however, is only 0.00001581 light years from Earth! Why are Light Years Used? Hypothetically, if you were to drive non-stop at 60 miles-per-hour, it would take 177 years to make it to the sun. For example, the sun is over 93,000,000 miles from Earth. If you tried to use them to calculate smaller distances, you'd get very small decimals. With 8,760 hours in a year, a light year is equal to 5.88 trillion miles! Light years are normally only used to calculate huge distances. Since light travels at the speed of 186,282 miles per second, it travels over 670,000,000 miles per hour. It is defined as the distance that light travels in 365.25 days. A light year is a unit designed to measure distances between objects in outer space.
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