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

Very long astronomical distances are approximated using the “Cosmic distance ladder.”

It’s called a “ladder” because we slowly built it up rung by rung. For objects that are close enough that the movement of the earth itself can cause a perceptible change in their relative location, we can use trigonometry.

We can then learn a lot about those nearby objects (such as how bright they are), and when we see similar objects farther away, we can calculate their distance using observations of how they have relatively dimmed. This works particularly well for certain kinds of supernovae, which are known to have a particular peak brightness.

This lets us measure the distance of many more exotic things that are even farther away, which scientists have put on the distance ladder using a variety of physical properties. These include gravitational waves, light spectra, and even theories about the typical width of a galaxy.

None of these methods are airtight, and you could probably find serious scientists who disagree with a given distance measurement, especially for things outside our own galaxy.