Astronomical unit of distance
Astronomical unit is a unit of measurement equal to 149.6 million kilometers, the mean
distance from the center of the earth to the center of the sun.
A light-year (or light year, abbreviation: ly) is a unit of length used informally to express
astronomical distances. It is well over 9 trillion kilometres (or about 6 trillion miles). As defined by
the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in
one Julian year. Because it includes the word year, the term light-year is sometimes misinterpreted
as a unit of time.
A parsec (symbol: pc) is a unit of length used to measure large distances to objects outside the
Solar System. One parsec is the distance at which one astronomical unit subtends an angle of
one arcsecond. A parsec is equal to about 3.26 light-years (31 trillion kilometres or 19 trillion
miles) in length
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