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What are (i) light year (ii) astronomica...

What are (i) light year (ii) astronomical unit and (iii) parsec?

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Each of them is a unit of length.
(i) 1 light year (1y) is the distance travelled by light in vacuum in 1 year.
`1 Iy = (365.25xx24xx60xx60s)xx(3xx10^(8) m//s)`
`~~ 9.47 xx10^(15)` m
It is often rounded off as `1 Iy ~ 10^(16)`m.
(ii) 1 astronomical unit (AU) is the mean distance between the sun and the earth. 1 AU `~~ 1.5 xx10^(11)`m.
1 parsec (pc) is the distance at which 1 AU subtends an angle of 1 arc-second (1..). 1 pc `~~ 3.1 xx10^(16)` m.
Light year and parsec are used as very large units of length. Astronomical distances are usually expressed in 1y or pc.
`lpc ~~ 3.26 `ly
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