Distances in Space

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Note: There’s a tl;dr at the bottom of the article.

Space is Big

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”

Douglas Adams,
The Hitchhiker’s Guide to the Galaxy

From A to B in Perspective

Be forewarned: This article will contain quite a few references to the space sim game Elite:Dangerous,  seeing as it’s aimed primarily at players of that particular game, but may hopefully also be of some interest to others.

When you go flying around in a star system, supercruising your way from planet to planet, the distances you travel may seem somewhat intangible. Usually you see them measured in light seconds (ls), but what is that, really? Well, on the scale of a star system, with its planets and their orbits, and even companion stars, our homely and familiar units of measurement such as metres and kilometres (or feet and miles) become so small, and the numbers of them which we’d need to convey even the smallest of interplanetary distances so staggeringly great so as to be almost meaningless, even to the wildest of human imaginations, so that we’ll need something else to use as our yard stick. This is where the light second comes into play.

For clarity, “light” in this context does not relate to weight or mass, as in “not heavy”, but rather “light” as in photons emitted from a star, or any other light source, which travels at a certain speed, which just so happens to be the speed of light.

Just like a light year is the distance a beam of light in the near vacuum of space will travel in one year, unless something gets in the way, a light second is the distance a beam of light travels in just one second.

1 ls = 299,792 km (186,438 miles)

But how far is that, really? In order to have a sense of scale, we need something familiar to compare with. The problem with that is, we don’t really have anything actually familiar that is also on that scale. The whole of the Earth is only 12,756 km across (7,933 miles), and 40,075 km around the middle (24,992 miles). That’s hardly enough when a beam of light, if persuaded to move in a circle, would go around the Earth’s equator a whopping seven and a half times in a single second, yet still take hours to reach some of the more distant planets (1 hour 19 minutes to Saturn, for example).

Your Friend The Astronomical Unit

1 AU = 149,598,023 km (93,034 miles)

Enter the Astronomical Unit (AU). The AU is the average distance between the Sun and the Earth. Measured in kilometres or miles, this is in itself a mind bender of a large number, but we’ll handle that in a bit. The main thing is that it relates to something which is at least vaguely familiar, and connects with something else with which we’re reasonably familiar, i.e. the Sun and the Earth.

Mind you, if you haven’t lived on Earth, you may not have a clue what I’m on about, but something along the same lines may also apply to you, your local star and your home world. Just humour me, nod along and pay attention.

Since an AU is close to 150 million km, and the speed of light is close to 300,000 km per second, this makes the time taken by a beam of light to travel from the Sun to us, close to 500 seconds (8 minutes and 20 seconds). This is a rather convenient number, reasonably round, and not too big. And in this case it measures the size of our nearest planetary neighbourhood.

So if you exit your hyperspace jump in, say, Gateway, and are headed for, say, the space station Dublin Citadel, which is just 299 ls from the star (some three fifths of an AU), or a little less than Venus is from Sol, then that’s practically a short stroll down the street and shouldn’t take too long. If, on the other hand, you’re going to New Chernobyl, which orbits one of the moons of the fourth and outermost planet in the Gateway system, then that’s a trip of some 6,536 ls, close to 13 AU, or about a third longer than Saturn’s distance from Sol, which will take a good bit longer, so you’d better use a spaceship.

So in other words, anything 500 ls or less is fairly close, whilst anything much bigger than that is comparatively far. And to figure out the number of AU from a number of light seconds, it’s just a matter of dividing by 500.

number of light seconds / 500 = number of astronomical units

Interplanetary Distances of Sol

For comparison with our own neighbourhood of planets, I’ve provided this simple table of distances from the Sun.

These are average orbital distances. Due to orbits being elliptical rather than circular, their apogee and perigee will be somewhat larger and smaller, in some cases very much so.

Ceres / Asteroid Belt2.81 39723’17”
Jupiter5.22 59743’17”
Saturn9.54 7821h19’42”
Uranus19.29 5902h39’50”
Neptune30.115 0254h10’25”
Pluto / Kuiper Belt39.019 7005h28’20”
Heliosphere Bow Shock230114 77031h52’50”
The Moon (from Earth)0.002561.3
Voyager 1 (as of 1-Jan-2019)14471,85619h57’36”

So when you’re on your way to earn yourself that free Anaconda, you may find it amusing to ponder that Hutton Orbital is 328 times as far from Alpha Centauri, as Pluto is from the Sun, and that New Horizons, which in 2006 was the fastest spaceship ever built by humans, took almost ten years to get there.

In Closing

Hopefully this will help bring some perspective to and appreciation for the otherwise quite unimaginable dimensions of the star systems that we traverse with such leisurely ease. Put on some of your grooviest music, and cruise on.

Thanks for reading, and fly safe.

Appendix: Some More Or Less Relevant Links


  1. Divide distance in ls by 500 to get number of AU.
  2. If you really need a tl;dr then you probably aren’t very interested in what this article has to say in the first place.