## How do you find the distance to the galaxies having equal diameter?

I am reading Richard Feynman's book Vol I. There he tells us how people usually used to find out the distance to some distant objects. I understand triangulation for objects that are near to the earth or for stars that are near to our solar system. But how did they use the same principle to find out the distance to those galaxies(which have the same diameter of our galaxy) that are far away from the earth ?

Here's the excerpt-

"Knowing the size of our own galaxy, we have a key to the measurement of still larger distances- the distances to others galaxies. Figure 5-7 is a photograph of a galaxy, which has much the same shape as our own. Probably it is the same size, too. (Other evidence supports the idea that galaxies are all about the same size.) If it is the same size as ours, we can tell its distance. We measure the angle it subtends in the sky; we know its diameter and we compute its distance -triangulation again"

Can someone help me?

 Mentor Blog Entries: 9

## How do you find the distance to the galaxies having equal diameter?

 Quote by iVenky But how did they use the same principle to find out the distance to those galaxies(which have the same diameter of our galaxy) that are far away from the earth ?
Take a pen and a piece of paper. Draw an elongated equilateral triangle with vertex A being the observer, and vertices B and C marking the edges of the observed galaxy.
Simple measurements allow us to name the angle at vertex A - it is the angular size of an object on the celestial sphere. For example, for the Andromeda galaxy it's over 3 degrees.
By assuming that the observed galaxy is of the same size, we are given the length of the distance BC - the same as the size of Milky Way, so ~100 000 ly.

This gives us enough information to calculate the distance using simple trigonometric relations. In the case of our Andromeda example, this nets a result of ~2 million ly, which is not a bad approximation for the ~2,5 million ly of actual distance.