Why is the ninth month called September?
interstellar distance measure, 1913, from first elements of parallax second. It is the distance at which an object has parallax (viewed from Earth) of one second of arc, or about 3.26 light-years.
A unit of astronomical length equal to 3.26 light-years. It is based on the distance from Earth at which a star would have a parallax of one second of arc. Its metric equivalent is about 30.8 trillion km (19.1 trillion mi). It is used in measuring distances in interstellar and intergalactic space. The closest star to Earth, Alpha Centauri, is about 1.3 parsecs away. Compare astronomical unit, light-year.