Expressing Large Distances: Light-Years and Parsecs 

When expressing large distances in astronomy, the light-year is often used by the public, while the parsec is most often used by professionals.

What is a light-year?

1 light-year is the distance light travels in one year

• You may have heard that the speed of light (in a vacuum) is the ultimate speed limit in the universe.  The speed of light in a vacuum, denoted by c,  is 3 x 10⁵ km/sec.

• So how far does light travel in 1 year?  Consider how many seconds there are in 1 year:

Note that 31536000 seconds is 3.1536000 x 10⁷ seconds

• So in 1 year light travels 3 x 10⁵ km/sec x 3.1536000 x 10⁷ sec, or 9.4608 x 10¹² km

Do you see how to multiply in scientific notation?

What is a parsec?

• For now, simply consider 1 parsec to be 3.26 light-years!