Astronomers now prefer to quote distances between stars in Parsecs, rather than the older Light Years. This is because it makes some calculations much simpler.
The navigation instruments on Jane’s eight-footer are all calibrated in Light Years. So why did Space Fleet decide to go back to the older unit? The answer is very simple.
Look out of the window at some object in the distance, such as a tree on the horizon. Now move your head from side to side. The tree will appear to move around in the window. If you have something like a sextant which lets you measure angles very accurately, and you know how far you are moving your head, you can work out how far it is from you to the window.
Parsecs work like that. As the Earth goes around the sun nearer stars seem to wobble when compared against very distant ones. If you know the size of Earth’s orbit, and use a telescope to measure the wobble you can work out how far away the stars are.
One problem. One of the constants involved in the definition of the parsec is the size of Earth’s orbit around the sun. That’s all right, unless you are on another planet circling another star. Jane’s home planet, Mercia, orbits a type F star over two hundred light years away in the general direction of Alpha Cygni.
On the other hand the Light Year is the distance light travels in 365.25 days, exactly. A day is defined as 24 x 60 x 60 seconds, and a second is defined in terms of the vibrations of the caesium atom.
The point is that, if you know the definition, you can go anywhere in the universe and do an experiment to measure a light year and get the same result.
And that’s why Space Fleet work in light years.