# Physics KS3/KS4: Professor Brian Cox - Why do astronomers use 'light years' to measure distance in space?

**This video discusses measurements in astronomy and explains why astronomers do not use standard forms like metres or kilometres to measure distance, but instead refer to light years.**

The definition is given as the distance light travels in one year, and the scale of magnitude is highlighted by converting a light year into kilometres to demonstrate that light years simply make the numbers more manageable.

### Teacher notes

** Points for discussion **

*A common challenge with space physics is the proportions involved. Very large (and very small) entities can be challenging for students to grasp as they are so hard to visualise. When taking about the distances from the Earth to the sun, or distances to other stars, the measurements are vast. It is really useful for students to appreciate that light years are used as a unit of measurement simply to make the numbers more manageable.*

** Suggested activities **

*For KS3, after watching the film, allow students to practise writing out the distance of a light year in kilometres so that they can experience the potential difficulty in dealing with such large numbers.**Students could also practise calculating other astronomical proportions in kilometres to further emphasise the point.**The film could also provide an opportunity to practise using standard form, if at an appropriate stage of the curriculum. Calculating using light years instead of standard form could then be done to demonstrate how using light years simplifies the process.**Any models or visualisations that can be used to demonstrate the relative distances may be useful to support an awareness of order of magnitude and scale.*

** Curriculum information **

*Suitable for KS3 England, Wales and NI and CfE Scotland, S1-3.*