A light-year is – as you probably know – a very very long way. About 9.5 trillion kilometres or 6 trillion miles. Light years are useful measures of distance because space is so very very big, so it’s easier to talk about the distance between stars as light years – which as well as being a distance also give us the time it takes for light from the star to get here. Measuring a distance that vast is of course impossible (you’d need a very long ruler!), so instead scientists measure the speed of light (in km per second and multiply that by the number of seconds in a ‘standard’ year). The story of how scientists have measured the speed of light is incredibly interesting and started with Galileo trying to time how long it took between him flashing a light on top of a hill and then seeing his assistant on top of another hill flashing his light in reply. Of course light travels too fast for this experiment so all Galileo was measuring was the human reaction times – but he had the right idea! The real hero in the story is Danish astronomer Roemer in the 17th century. There’s a really nice description of how Roemer measured the speed of light (and brilliantly got very close to the right answer) here: http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html
I think it is actually very complicated indeed. we can measure the distant to “nearby” things like the sun, the moon and the planets by using trigonometry and newtons laws.. For more distant objects we can use the same ideas. but it is a lot less accurate. Just looking at size and distribution of stars in milky way gives us a starting point.
We can improve these estimate by knowing how the distance affects starts brightness and some from their ‘redshift’. From all this we know that the milky way is 100,000 light years across and 1000 light years thick.. but those are such huge distances that they only make sense from very far away.
A light year is roughly 6 trillion miles! Since the circumference of the earth is about 25,000 miles, this means light could travel all the way around the earth 7 and a half times in one second. Phew!
Here is how you calculate it:
You start with the speed of light, which is about 186,000 miles per second. Then you multiply that by 3,600 seconds in an hour. Then you multiply that by 24 hours in a day. Then you multiply that by 365 days in a year. And you get:
Funnily enough in astrometry the parsec is used most commonly as a unit of measurement, because it can be more easily derived from, and compared with, observational data. A parsec is approximately 3.26 light-years.
Obiously a light year can’t be measured directly, so the distances have to be compared to a point of reference. For nearby stars, the star’s parallax is measured. The smaller the parallax, the farther away the star is. Parallax is the apparent change in position, of a star, compared to the far-away background, as Earth moves from one side of its orbit, to the other.