Parallax and parsec

What is parsec and why do professional astronomers use it instead of the well-known and intuitive light-year for distances larger than the Solar System?

Parsec (pc) is the short form of "parallax of one second". It originates from the oldest method for estimating the distance of nearby stars - the parallax method.

In essence, (stellar) parallax in astronomy refers to the apparent shift in position of a nearby star (in the night sky) when the Earth is in opposite sides of its orbit. The shift is compared to the background stars which are much farther away and are assumed to be fixed. By simple trigonometry, one can easily calculate the distance to the star (d), assuming the average distance between the Earth and the Sun (1 au; 1 astronomical unit) and the parallax angle (θ) are known.

The parsec is defined as the distance to a star if the parallax angle is exactly 1 arcsecond, where 1 arcsecond (1") is 1/3600 of a degree. Since this angle is extremely small (equivalent to the size of a thumbnail held 2 km away), the figure above is not to scale. In SI units, 1 pc = 3.086e16 m. 1 pc is also approximately equal to 2.06e5 au, or 3.26 light-years.

Due to turbulence in the Earth's atmosphere, parallax angles of less than 0.01" are very difficult to measure on Earth. This limits Earth-based telescopes to measuring distances to stars about 100 pc away [1].

The parsec has become the standard unit of distance for professional astronomers. Apart from "pc", astronomers use "kpc" for measuring distances within a galaxy (kpc = kiloparsec) and "Mpc" for distances between galaxies (Mpc = megaparsec). Gpc (gigaparsec) is used for measuring the largest scales in cosmology.

If you are interested in reading articles on astronomy like this, follow my Instagram account @astro.snippets!

Reference:
[1] https://lco.global/spacebook/distance/parallax-and-distance-measurement/