How do they calculate light-years?

Question

For example, how do they know that the Eistein Ring is “12 billion light-years” away? Why not 10 or 20? How do they arrive at that number?

In: 2

Answer 1

The Earth goes around the Sun, so, at different times of year, the Earth is in a different place.

An astronomer can measure the position of an object in the sky and record the angle. If the astronomer does this when the Earth is at two different positions, the astronomer gets two slightly different angles.

You can then draw a triangle, using the imaginary line between the Earth’s positions on the two days measurements were taken as the bottom, and angles measured on the two dates to be the angles that the sides meet the bottom. Where the sides of the triangle meet is where the thing you are measuring is.

Now you have a triangle where you know the length of 1 side and all the angles inside, you can use geometry to figure out the length of the other sides of the triangle (distance to the thing from Earth).

A light year is just a distance. It’s about 5.9 trillion miles.