# What exactly is a Light Year?

At the recent Starscape Exhibitions a common question was “how far away are these deep-sky objects?”.  I gave a rough answer in terms of Light Years, and the questioner would walk away satisfied, but it was clear that they really didn’t have a clue about the immensity of distance implied by the term Light Year – so here’s a very short piece just to try and put things into context.

Before going into the numbers, I would like to remind you of Arthur C Clarke’s “First Law” as I will be touching on this in what follows. Clarke says,

“When a distinguished but elderly scientist says that something is possible, (s)he is almost certainly right.  When (s)he says it is impossible, (s)he is very probably wrong.”

And with that, onto the explanation…

### Distance Not Time

A Light Year is a measure of distance, not of time.  It is the distance travelled by light in one year as it barrels along at the not insignificant velocity of 186,000 miles (300,000 kilometers) per second.

Let’s start by looking at the numbers.

186,000 miles per second = 6.7 x 108 miles per hour = 5.87 x 1012 miles per year ((102 = 100, 103 = 1,000 and so on.))

It would take one second for a beam of light to travel seven and a half times around the Earth at the equator.

It takes light 1.3 seconds to travel from the moon to the Earth, 8.3 minutes to travel from the Sun to the Earth, and 5 hours and 20 minutes to travel from the Earth to Pluto.

A Light Year is equivalent to 5.87 x 1012 miles (or, if you like lots of zeros,  5,870,000,000,000 miles).

### The Nearest Star

After the Sun, the nearest star to us is Proxima Centauri (visible from the Southern Hemisphere) at a distance of 4.22 Light Years.  For arguments sake, suppose we can travel a million miles in a day (approximately 42,000 miles per hour).   At that rate it would take us 67,817 years to reach Proxima Centauri.

Let’s go faster, let’s travel at 100,000 miles per hour.  Travelling at 100,000 miles per hour it would only take us 28,253 years to reach Proxima Centauri.

The above velocities are so much slower than the velocity of light that we can’t rely on Einstein and Special Relativity making these journeys appear any shorter.  Even at these “low” velocities, tiny fragments of interstellar material would still cause considerable damage to our spacecraft.

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### Clarkes First Law

So, even with Clarke’s warning I have to say that although I am neither distinguished nor elderly, I think I am a reasonable approximation to a scientist.

I believe that if there is a God, then this Being has been amazingly clever in placing any other [non-terrestrial] civilizations at such vast distances that we will never be able to physically meet [yes – I am saying this is impossible].  This is to me actually very reassuring.

After all, history shows us that we haven’t fared too well in meeting other slightly more diverse cultures on the same planet, so what chance do we have of having a pleasant interaction with completely alien species?

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