Question

What is a parsec and why is it necessary when a light year measures distance as well?

Answer 1

Parsec is the distance of circular arc of 1 AU that subtends 1 second at Sun's center. Precisely, parsec =206264.8 AU = 3.27925 light years. The name suggests the contexts for use.

Explanation:

Despite that parsec is not very large compared to LY, it is approximately 2.E+05 AU. Mega parsec-AU conversion is simple for a few significant digits approximations. Mega means million. Also, a circular arc distant i parsec from Sun, that subtends 1 deg at Sun, would measure 3600 AU. These is my best explanation. .