What is a Light-Year?

The distances to our nearest stars, galaxies and nebulae are huge - astronomical even!  If we used our normal units of measurement such as kilometres or miles, the number of digits would be huge too. So to make the numbers easier to handle, the concept of the light-year is used.

In 1838, the German astronomer Friedrich Wilhelm Bessel was the first to use the light-year as a unit of measurement in astronomy. He measured the distance to the binary star 61 Cygni as 10.3 light-years.

The light-year is not a unit of time although it sounds like it might be - it's a unit of length or distance, just like kilometres and miles. As defined by the International Astronomical Union, a light-year is:

'the distance that light travels in a vacuum in one year'

The light-year measures about 9.46 trillion kilometres or 5.88 trillion miles.  If these numbers were written using our normal units of length, that would be 946,000,000,000,000 kilometres or 588,000,000,000,000 miles.

I think you can see now why we use light-years rather than kilometres or miles! But even though the term is still very much in use, another larger unit the parsec (which is equal to 3.26 light-years) is actually much more popular in the scientific community.

Here are some approximate distances to some of the better-known night sky objects:

Deep Sky Objects:

• Andromeda Galaxy - 2.537 million light-years 
• Orion Nebula - 1,344 light-years 
• Betelgeuse - 642.5 light-years 
• Polaris - 433 light-years
• Capella - 42.92 light-years
• Arcturus - 36.66 light-years 
• Vega - 25.05 light-years 
• Sirius - 8.61 light-years 

Our Solar System:

• Sun - 150 million kilometres (93,000,000 miles) 
• Mercury - 77 million kilometres (48 million miles ) 
• Venus - 40 million kilometres (25 million miles) 
• Moon - 384,400 km (238,900 miles)
• Mars - 225 million kilometres (140 million miles)
• Jupiter -  778 million kilometres (484 million miles)
• Saturn - 1.44 billion km (894 million miles)
Uranus - 2.75 billion km (1.71 billion miles)
• Neptune - 4.5 billion kilometres (2.8 billion miles)

Note: all the distances to the planets in our solar system are approximate averages as they vary with orbits. The picture above does not show these distances to scale, although the relative planet sizes are correct.