# A critizism to Ned's calculator

1. Oct 4, 2004

### meteor

I've noticed what it seems to be an error in Ned Wright's calculator
http://www.astro.ucla.edu/~wright/CosmoCalc.html

First off, I want to present a beautiful theorem that relates 3 of the more often used distances in astronomy. Is called Etherington reciprocity theorem (Etherington,1933). It can be found here
http://arxiv.org/PS_cache/gr-qc/pdf/9910/9910014.pdf [Broken]
and here
http://nedwww.ipac.caltech.edu/level5/Hogg/Hogg7.html
the theorem is
DL=(1+z)*DS=(1+z)2*DD

DL is luminosity distance
DS is angular size distance
DD is angular diameter distance

If you go to the calculator, and introducing any value of redshift, you don't get the angular size distance, how is indicated, but the angular diameter distance! Just see the value of the luminosity distance and introduce it in Etherington theorem
I'm right in my claims?

Last edited by a moderator: May 1, 2017
2. Oct 4, 2004

### marcus

You (and NW both) are right.
There is some difference in usage among cosmologists.
"angular size distance" doesn't, unfortunately, always mean the same thing.

the Hogg tutorial is great. thanks for linking to it and reminding everybody.
I think he is admirably clear about the different kinds of distance,
have be reading that tutorial some this afternoon. more tomorrow

3. Sep 13, 2008

### tata

Well you are totally right about Angular Diameter Distance.
I found a new cosmology calculator which gives plots as well.
http://icosmos.co.uk

It is a nice toy i like it actually...

4. Sep 14, 2008

### marcus

Tata, I owe you for two things already, and it is just your first post!