Question: how did scientists come up with what distance a light year is because a year is a measurement of time not space and why is it called a light year

Sheila Castilho answered on 4 Nov 2018:

A light-year is a unit of distance and it is used to express astronomical distances – like distances to stars and other distances on a galactic scale. Because it includes the word “year”, sometimes people confuse it with a unit of time.

But basically a light-year is the distance that light can travel in one year. Because in space the distances are so big (astronomical!) using kilometres or miles is not practical.

So to calculate that you have to take the velocity that the light moves (300,000 kilometres each second) and multiply that by the number of seconds in one year (31557600 seconds), so then you’ll have around 10 trillion kilometres that light can travel in a year – so a light-year!. So because this number is too big to use (10,000,000,000,000 kilometres), scientists came up with the light-year measure to make things easier to calculate.
For example, according to NASA, we can say that the Milky Way Galaxy is about 150,000 light-years across, instead of using all the 0s!

So think always that a light-year is how far the light travels in a year!