What is the difference between a light-year and a parsec?

What is the difference between a light-year and a parsec?

• A. A light-year is distance light travels in one year; a parsec is about 3.26 light-years and relates to parallax measurement.
• B. A light-year is time; a parsec is distance traveled by light in a year.
• C. A light-year is the distance the Sun travels in a year.
• D. A parsec is the distance light travels in a century.

Answer: (A)

The main idea is that these two units measure distance, but they are defined differently. A light-year is simply the distance that light travels in one year, about 9.46 trillion kilometers. A parsec, on the other hand, is defined from parallax: the distance at which 1 astronomical unit subtends an angle of 1 arcsecond. That distance works out to about 3.26 light-years (roughly 3.086×10^13 kilometers). So a light-year uses a time-based idea (how far light goes in a year), while a parsec uses a geometric parallax measurement to define a distance. This is why a parsec is especially handy for expressing stellar distances, since it ties directly to how we measure them.

The main idea is that these two units measure distance, but they are defined differently. A light-year is simply the distance that light travels in one year, about 9.46 trillion kilometers. A parsec, on the other hand, is defined from parallax: the distance at which 1 astronomical unit subtends an angle of 1 arcsecond. That distance works out to about 3.26 light-years (roughly 3.086×10^13 kilometers). So a light-year uses a time-based idea (how far light goes in a year), while a parsec uses a geometric parallax measurement to define a distance. This is why a parsec is especially handy for expressing stellar distances, since it ties directly to how we measure them.