Distances in the Universe

1. Astronomical Unit

1.1. An astronomical unit (㍳) is a unit of length defined as the average Earth–Sun distance.

1.2. 1 Astronomical Unit = 149 597 871 kilometers

2. Parsec

2.1. The parsec (symbol: pc) is a unit of length used in astronomy, and it represents "an abbreviated form of 'a distance corresponding to a parallax of one arcsecond'." A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond (1⁄3,600 of a degree). In other words, imagine three straight lines forming a triangle between the Earth, the Sun and a distant object, as follows: line 1 connects the Earth and the Sun, line 2, perpendicular to the first line, connects the Sun and the object, and line 3 connects the object to the Earth. Now, if the angle at the object between lines 2 and 3 is exactly one arcsecond, then the object's distance from the Sun would be exactly one parsec.

2.2. 1 pc is to about 30.9 trillion kilometers or 3.26 light-years.

3. Light Year

3.1. A light-year, also light year or lightyear (symbol: ly), is an astronomical unit of length and defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.

3.2. 1ly = 10 trillion kilometres