Illustrated Guide to Astronomy


A light-year is an astronomical unit of distance equal to 9.5 trillion kilometers (5.9 trillion miles), defined as the distance light travels in an Earth year.



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The universe is big and the distances between the objects in it can be surprisingly large too. A light-year is defined as the the distance light can travel in one Earth year. It can be calculated by multiplying the speed of light (3 x 108 m/s or 300,000,000 m/s) by the length of a year (365.25 days or 3.15 x 107 seconds). Light years to meters: 9.45 x 1015 m, or 9.45 trillion kilometers!

An advantage to using the light-year as a unit of distance is that it makes us realise that light doesn’t move instantaneously. If a galaxy is a million light-years away, it means that the light has taken a million years to reach Earth. This means that when we observe a galaxy that is a million light years away, we are actually looking at the galaxy a million years ago.

For distances within a star system, the light-year is normally too big a distance to use. For example, the distance from the Sun to Earth is 0.00000156 light-years. The unit is mostly suited to intergalactic distances. Astronomers prefer to use the astronomical distance unit the parsec, but the light-year has survived as a popular unit. The term 'light-year' has been mentioned in many popular science fiction works, such as Star Trek.

This term is often confused as a unit of time because it includes the word 'year'. Sometimes people will write things like “technology is light-years ahead”; this can add to confusion about the term. A light-year is a unit of distance, like miles or kilometers—only on a much larger scale.

Distance to Astronomical Objects

  • Alpha Centauri: 4.4 light-years
  • Andromeda Galaxy: 2.5 million light-years
  • The Moon: 40.4 x 10-9 light-years
  • The Center of the Milky Way: 26 million light-years

Other Astronomical Units of Distance

  • astronomical unit (AU), the distance from sun to Earth - 150 million km
  • parsec, the parallax of one arcsecond - 31 trillion km
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