I need to explain the concept of a light-year to a non technical audience. Actually a presentation about planets and galaxies. It is quite difficult (for them) to comprehend the speed of light itself ,leave alone the distance it travels in one year. Has anyone had to do this or can someone please give me a few ideas of how we can describe this ?

Or is there another way to describe sizes of galaxies (e.g. some galaxies are 15 lightyears in diameter) and the huge distances between them in some easy to understand way.

Thanks Kevin

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    $\begingroup$ "some galaxies are 15 lightyears in diameter" Er...15,000 light years is still quite a small galaxy, and 150,000 light years is only a bit larger than our Milky Way. google.com/search?q=size+of+the+milkyway $\endgroup$ – dmckee Jul 8 '13 at 13:37
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    $\begingroup$ apod.nasa.gov/apod/ap120312.html - this is an interactive show about the scale of things. For the time dimension, use this: chronozoomproject.org $\endgroup$ – Deer Hunter Jul 8 '13 at 13:37
  • $\begingroup$ You might find useful examples here: en.wikipedia.org/wiki/Orders_of_magnitude_%28length%29 also here: htwins.net/scale2 $\endgroup$ – Michael Brown Jul 8 '13 at 13:38
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    $\begingroup$ You could start with an everyday example: a commuter going to town by train consistently taking an hour to get there. Then the commuter could think of the town as one-train-hour away. $\endgroup$ – anna v Jul 8 '13 at 13:42
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    $\begingroup$ Do you have a priori evidence that your audience is challenged on this issue? Don't underestimate the average person's ability to grasp ideas of "big" and "fast." (Or rather, don't overestimate how much scientists intuit such things. As an astronomer, my understanding of "light year" is nothing more than "the distance light, which is really fast, goes in a year.") Throwing a dozen different analogies at an audience is guaranteed to lose them. $\endgroup$ – user10851 Jul 8 '13 at 23:35


Step 0: Preparation is key. Have your story well-prepared, the numbers ready, and the "smart questions" factor taken care of.

Step 1: Start by just writing down the number, nice, big and boring: $1\,\text{ly} = 9.4605284 \cdot 10^{15}$ meters. Explain what its definition is (the distance a ray of light in vacuum travels in a year), but don't spend more than a minute on this.

Step 2: This number, and its definition, will be completely meaningless to most people. Break this all down into things people have experience with. If you manage to come up with the most powerful way possible to get the number across, you'll likely spend well over 10 minutes on it.

It might be necessary to apply step 2 recursively, until only tangible numbers and definitions down at monkey-level are left.


Your will have to help your audience develop a "feel" for the concept, an intuition, something which is not easy to accomplish -- in all honesty, how long did it take you to develop an intuition and full comprehension of the term, beyond just a conceptual and numerical understanding? Things like numbers are very easy to write down, but numbers are an abstract concept. Abstract concepts are for the "left cerebral hemisphere"; they should be avoided if you want to appeal to the "right cerebral hemisphere" and breed an intuition for a concept.

All great lecturers I've ever had, in fact, all lecturers considered great I've ever seen, use the following rule of thumb: you cannot spend too much time on making people truly understand the basics.

To do this well, and with a big audience, you'll have to come up with a variety of ways, designed to keep everyone interested all the time, and explain the concept in a few different, independent ways using only terms and concepts people already have an intuition for.

With something like the lightyear, it's all too easy to slip into things like the distance between the Earth and the Sun, or the Earth and the Moon, the maximum distance you can travel around Earth's equator, the distance to Pluto, etc. (or god forbid, a log scale animation). To an audience of complete laymen, this will all make very little sense. Most likely you'll quickly lose their interest, unless you have pretty pictures showing...but that's not the kind of presentation I'd advise anyone to give.

It's usually best to use a few every-day examples, each worded in a few different ways, to cover everyone's experiences, and cover gaps from any previous examples that may have been left in people's minds.

An example.

The most "obvious" thing would seem to express the lightyear in terms of hours of travel by airplane, as that is a pretty common means of transportation, and usually covers the most distance in the least amount of time. However, while some people have a well-developed sense of distances traveled by aircraft (pilots, for example), most people actually do not know how far they've actually flown -- ask them to convert the distance traveled by an hour on an airplane to the number of hours walked on foot, using just their intuition and no numbers, and most people will fail horribly. These kinds of distances are already too big to truly wrap your head around.

So, you'll have to start even more down to Earth. Assuming you're from a country where cars are very common, most people in your audience will have driven in a car on the highway for an hour. Most people will at some point in their life, have also walked this distance, or rode it on a bicycle, or traversed it somehow with other, even more down-to-Earth means of transportation. Therefore, most people will have an intuition for this amount of distance.

These distances are unfortunately pretty tiny compared to the lightyear. At 130$\,$km/h, they would have to drive their cars for 8.3 million years. As you may have guessed, most people don't have a feeling for how long a million years is, so, you'll have to come up with a way to break that down to a small and understandable time span.

You can propose that their whole town/city, easily consisting of 1,000,000 people, each take a single car, and start driving. These are of course special cars; they do not stop, they do not need to refuel; they just go and drive on for 24 hours a day, 7 days a week. Now, the combined distance all 1,000,000 cars travel in 8.3 years, is one light year.

There is still room for improvement, as most people have never counted to 1,000,000 and therefore have no intuition for how much a million truly is. You can break that down again and come to your final story outline:

  • At the exact same instant in time, the entire population of 100 towns, each consisting of 100 people, steps into their cars (show a picture of 100 towns, with a zoom-on on a single town, showing 100 people).
  • They all own the same, highly specialized car; these cars never have to re-fuel, they can drive on auto-pilot which allows their drivers to sleep at night, and they have the capacity to store enough food and water for their occupant for the entire duration of the journey.
  • They all drive at 130 km/h, for 24 hours per day, never stopping for anything (show one of those huge traffic jams on a 2 x 8-lane highway).
  • They all continue to drive this way, for 8.5 years (time is always tough; you can make this more tangible by saying something like: "Take a moment and think about what you were doing in winter of 2004. Imagine from then till now, you were driving your car on the highway", etc.).
  • All the distances covered by all cars put back-to-back, is roughly equal to one percent of a lightyear.

As we are very graphically oriented creatures and images and pictures are usually the easiest things to digest, it's best to accompany this story with lots of pictures, drawings, scale models, etc., that all follow the same basic principles. Allow everyone plenty of time to digest each step of the story; this will allow them to truly appreciate the vastness of the lightyear (and, if you continue on to million-lightyear distances between galaxies, the size of the universe).

Use at least two different examples, that are as different as you can dream them up. You can take any every-day example (commercial airliners, time between thunder and lightning, distance you can see to the horizon when standing on the beach, etc.) and run the numbers. The key is to try and break down every quantity along the way to something everyone can grasp.


A good way to get some perspective about a lightyear is to scale everything down to everyday lengths. Start with the definition of a lightyear: it is the distance that light travels in 1 Julian year of 365.25 days = 31,557,600 seconds.

This picture shows the distance between the Earth and the Moon to scale. It would take light 1.29 seconds to cross this distance (384,000 km), which means that a lightyear is 24.6 million times the Earth-Moon distance.

That is still not very illuminating, so let's scale everything down to 1 ten billionth, and build a model of the solar system with everyday objects. You can ask a few volunteers in the audience to hold objects that represent the Sun, Earth, Moon, etc and stand at the appropriate distances.

  • The diameter of the Earth (12,000 km) shrinks to 1.2 mm, about the head of a pin. The Moon becomes tiny, only a quarter of the diameter of the Earth; you could use a small nail. The distance Earth-Moon is then only 38 mm.

  • On this scale, the Sun has a diameter of about 10 cm (the size of a grapefruit), and the distance between the Earth and the Sun is about 15 meters. It takes light 499 seconds to cross this distance.

  • Mars would be at a distance of 23 meters from the Sun. You probably won't have enough room to show Jupiter: it has the size of a big marble, located at a distance of 78 meters.

  • The average distance of Pluto is 587 meters. It takes light less than 20,000 seconds to travel this distance.

  • Finally, one lightyear corresponds with 946 km. And the nearest star, Proxima Centauri, is located at a distance of more than 4000 km.

I think this will give your audience an idea how large a lightyear is, and how empty space is.

  • $\begingroup$ While a good demonstration in itself, don't you think this teaches people more about the emptiness of space, rather than the vastness of a lightyear? A subtle, but IMHO important difference... $\endgroup$ – Rody Oldenhuis Jul 9 '13 at 5:43
  • $\begingroup$ @Rody No, I think people can grasp how far away 900 km is, and how vast it is compared to the meter-scale dimensions of the solar system and the millimeter-scale size of planets. The fact that it also demonstrates the emptiness of space is a bonus. I've used this analogy a few times at dinner parties, and people like it (or so they tell me). $\endgroup$ – Pulsar Jul 9 '13 at 6:01
  • $\begingroup$ Sure, the 950km is still OK (although I think it's already on the edge), but what I think is lacking here is people's true understanding of the size of the Earth. No-one is born with this kind of sense of scale, and very few will ever develop a sense for the size of the Earth. A powerful demonstration (+1), that just has one missing step. $\endgroup$ – Rody Oldenhuis Jul 9 '13 at 7:35
  • $\begingroup$ @RodyOldenhuis If you are giving a talk in town X you can use google maps to find a well known city about 950km from there. I've just done that for where I'm at and I think most people would be able to appreciate the distance. It's a good day's drive. $\endgroup$ – Michael Brown Jul 9 '13 at 8:13
  • $\begingroup$ @MichaelBrown: as I said, this is not what I'm disputing (although for many people, there already is an abstraction in place when talking even about 950 km. Go cycle that distance, and your perspective will change for sure). What I am disputing is that no-one will have an intuition for ~42 times that distance (around Earth's equator). When confronted with such numbers, there usually is some gasping and a sense of awe, but usually not a good intuitive sense of the actual "amount of ground" that lies on the equator. Numerical versus emotional understanding; not the same thing. $\endgroup$ – Rody Oldenhuis Jul 9 '13 at 8:54

I would start by explaining what a lightyear is in it's simplest terms, e.g. The distance light travels in 365 days. Then I would move on to the distance of a light second, light minute, hour, day etc.

anna v uses a great method of explaining the concept. Perhaps a combination of that and what I've already mentioned would work well.


As a layman myself, I think it would help to start by explaining that light is made up of photons. For many people light doesn't have any substance. You could build up from 186,000 miles per second to mph, x60 x60 = 669 million miles an hour. Light from the sun reaches us in 8 minutes, which could be a useful yardstick that gives some sort of perspective. Then suggest they imagine how far a photon can travel in a year. Perhaps you could explain that the nearest possibly inhabitable planet is 22 light years away, which doesn't seem too far. But with current spacecraft able to travel no faster than 17,500 miles an hour, that would take...a long time. Another way of looking at it might be to imagine how fast a car must be travelling to an ant. It's gone in a flash and yet we might feel we are "only" going at 10 miles an hour, i.e. it's all relative. We travel in a few minutes further than an ant can travel in its lifetime. If they're really non-technical I wouldn't use powers, just lots of zeros, unless you explain them first.


protected by Qmechanic Jul 8 '13 at 23:25

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