Light year

A light year (l.y.) is a unit of distance, measuring how far light will travel in one year. The speed of light is extremely fast (in fact it is the fastest speed permitted by the laws of physics), equal to 299,792,458 m/s. Therefore, if light travels 299,792,458 meters in one second, the distance light will travel in a year is equal to 9.460730472 x 1015 meters.[1]

This is an incredible distance! The following are some ways to help quantify the size of a light year and put it in perspective:

  • It is approximately 63000x the distance from the Earth to the Sun,
  • One light year is ~ 236 million times the circumference of the Earth.
  • Voyager 1, launched in 1977, is the object that humans have sent the farthest away, almost 18 billion kilometers![2] A light year is still 530x that distance!

To give some idea of astronomical sizes in terms of light years:

  • The nearest star to Earth, Proxima Centauri,[3] is about 4.22 l.y. away
  • The center of our galaxy, the Milky Way, is about 26000 l.y. away[4]
  • The nearest galaxy, Andromeda, is approximately 2.5 million l.y. away[5]
  • The diameter of the observable universe is about 92 billion l.y.[6]


For Further Reading


  1. [math]ly=299792458\frac{m}{s}\times \frac{3600s}{hr} \times \frac{24hr}{day} \times \frac{365.25days}{yr} \times 1yr = 9.460730472\times10^{15}m[/math]
  2. NASA's Voyager Probes to Leave Solar System by 2016 [online] accessed June 1st, 2016.
  3. NASA, Hubble's New Shot of Proxima Centauri, our Nearest Neighbour [Online], Available:
  4. NASA, The Milky Way [Online], Available:
  5. Space Facts, Andromeda Facts [Online], Available:
  6. Space, How Big is the Universe? [Online], Available: