L is for Light-year

I know, I know, I’m way behind on my A to Z posts. Life hit me hard this month but I’m doing my best to start catching up. I plan to post for two letters a day until I’m back on track. Thank you for reading :) 

Everyone has heard of a light-year, most of us from a science fiction book or movie, but very few understand exactly what it is and how it is used in astronomy. The inclusion of the word year often confuses people into thinking it is a measure of time but a light-year is actually a measure of distance.

Think of it like when you are on your way to a friends house and you tell someone you are about “10 minutes away”, you substituted time for distance to give them a better idea of how far away you are.

In order to make sure everyone is measuring distances in space the same way the light-year has been defined by the International Astronomical Union (IAU) as the distance that a beam of light travels in vacuum in one Julian year. Astronomers use the light-year mainly for convenience. The Julian Calendar moves slightly slower than the Gregorian Calendar we use everyday, 365.25 days as opposed to the 365.2425-day.

We all know light can travel very, very fast, faster than anything we know, so light-years are often used to express great distances like those between stars or galaxies. It is especially used when writing or speaking in non-specialist or popular science circles. My guess is that it is very hard for us humans to imagine such large distances but when the distance is described in years it takes to get there at the fastest speed possible we get a better idea of the what is farther away than what.

1 light-year = 9460730472580800 metres (exactly)
9.461 petametres
5.878625 trillion miles
63241.077 astronomical units
0.306601 parsecs

The unit usually used in professional astrometry, the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies, is the parsec. A parsec is a measure of approximately 3.26 light-year.

To get an idea of how fast light travels let’s look at a few distances:

  • Traveling at the speed of light, you would encircle the globe of Earth almost eight times in one second.
  • Traveling at light speed it would take about a second-and-a-half to reach the moon.
  • From here to the sun is about 8 light-minutes. This is also known as the astronomical unit (au).
  • The nearest known star, Proxima Centauri, is a little over 4 light-years away.
  • Our own Milky Way galaxy is about 100,000 light-years across. The center of it is about 26,000 light-years away from us.
  • The Andromeda Galaxy is approximately 2.5 million light-years away.
  • From the Earth to the edge of the visible universe is about 45.7 gigalight-years in any direction.

Illustration, to scale, of Earth’s distance to the Sun. By LucasVB (Own work) [Public domain], via Wikimedia Commons

First used a few years after the successful measurement of the distance to a star by Friedrich Bessel in 1838. The star was 61 Cygni. He did recognize that readers would enjoy, and probably better understand, a unit of time rather than distance, but he refrained from using the light-year as a unit. It’s possible he thought it would degrade the accuracy of his measurements.

And anyway, the speed of light was not yet precisely known in 1838; its value changed in 1849 and again in 1862. It wasn’t even considered to be a fundamental constant of nature.

The light-year unit  did appear, however, in 1851 in a German popular astronomical article by Otto Ule. The paradox of a distance unit name ending on year was explained by Ule by comparing it to a hiking road hour (Wegstunde). A contemporary German popular astronomical book also noticed that light-year is an odd name. In 1868 an English journal labelled the light-year as a unit used by the Germans.

Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical.*

Whether or not it is a good name, it is here to stay. The human mind has a very hard time imagining the immense distances between celestial objects, but it can understand taking so many years to get from one place to another. It has become quite a popular term and anyone wanting to have a better understanding of astronomy has to understand it.


*Light-year history via Wikipedia

Image: In 1957, Laika, a stray dog from the streets of Moscow, became the first animal launched into orbit as an occupant of the Soviet spacecraft Sputnik 2, paving the way for human spaceflight. This photograph shows her in a flight harness. 

Laika died within hours from overheating, possibly caused by a failure of the central R-7 sustainer to separate from the payload. The true cause and time of her death were not made public until 2002; instead, it was widely reported that she died when her oxygen ran out on day six or, as the Soviet government initially claimed, she was euthanized prior to oxygen depletion.

On April 11, 2008, Russian officials unveiled a monument to Laika. A small monument in her honour was built near the military research facility in Moscow which prepared Laika’s flight to space. It features a dog standing on top of a rocket.


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Hello! My name is Lisa. I find the human condition fascinating and I often write stuff about that. I blog at zenandpi.com but you can also find me on Twitter, Tumblr, and Instagram, and if you like what I do, consider signing up for my newsletter. Thanks :)

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