Light Year & Parsec

7.6 + 11.9 - Be able to use ... light year (l.y.), parsec (pc)

13.11 - Understand the term parsec (pc)

Light travels at approximately 300,000 km a second. The Moon is 380,000 km away so we see light from the Moon as it was just over a second ago. When you see videos of astronauts on the Moon you might notice at least a second delay in the communication between them and mission control.

In distance terms we might say that the Sun is 8 light minutes away and Neptune is 4 light hours away.

The length is time it takes light to travel in an Earth Year is called a light year. This distance is over 9 trillion kilometres.

When we view the Andromeda Galaxy we are seeing light that left there 2 million years ago. Writing the distance to an object like this in kilometres, or even astronomical units, would be impractical so we use light years instead.

A parsec is another measurement used in astronomy. It is used to determine the distances to stars. It is the distance that a star would have to move that would cover an arc second in the sky as viewed from Earth.

1 parsec is 3.26 light years.

See Also