WAIT, THE LIGHT-YEAR IS NO GOOD EITHER?

We've used lightyears for this article because they are (by the standards of cosmology) simple to understand. It’s how far light travels, in a vacuum, in a year, right? Sure, the word “year” is confusing because it normally measures time... but you can grok it. But as well as being confusing, a light-year is really hard to measure

accurately. So cosmologists came up with a better unit for interstellar distances. They call it... the parallaxsecond! ... Oh. You may remember Han Solo in once claimed the Millennium Falcon as “the ship that did the Kessel Run in less than twelve parsecs!” Parsec is the nickname of the parallax-second. Anyway, we still can't decide which is harder: Explaining what a parallax second is, or explaining why it’s better than a light-year. For science.

The parsec – about 3.26 light years, by the way - is certainly an “easier” and “more natural” unit. If you're only making observations from Earth that it - it’s all about gigantic skinny triangles in the sky, okay? But if you don’t immediately understand why it's important that the parsec is “the distance at which one astronomical unit subtends an angle of one arcsecond”, well that’s probably healthy, actually. The important part is, even though the parsec is better for observers on Earth, it's not necessarily superior to the light-year for space travellers. Why? Because both the light-year and the parsec rely on Earth’s peculiar and almost certainly unique orbit. The light-year is partly defined by how long it takes Earth to orbit the Sun (the year). And the parsec relies on the “astronomical unit”, which is the distance from the Earth to the Sun… or rather the average distance… or rather an internationally defined 149,597,870,700 metres.

While it’s hard to predict what an interstellar culture will use (since we’re so far from interstellar travel right now), it’s likely that a scientific unit - either the metre or its sucessor - will become the only measure of distance. As for the metre: in the case of the distance to Alpha Centauri, we’re talking 41.34 petametres. Eh, put it like that, it doesn’t sound so far…