# 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
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: Oct 4, 2004
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!

Thanks for the new online cosmic calculator. Other people here may want to check it out. The special feature is that it plots functions of redshift---like the distance now of an object we are now seeing with redshift z (the comov. dist. associated with z). So every time you use the calculator it gives you extra information--quickly plotting a bunch of interesting curves.

Thanks also for reminding me about HOGG'S DISTANCE SCALES tutorial!
http://nedwww.ipac.caltech.edu/level5/Hogg/Hogg_contents.html
This is about 10 years old and I have one or two minor reservations about how he words things but it is very useful---a good link to have handy. Many posters may want to check it out.

About angular size distance----more explicitly angular diameter distance. (Ned Wright, in his calculator, calls it angular size distance so I tend to stick with that terminology.)
One nice thing about angular size distance is that one can consider it as the distance then, when the object or material emitted the light that we are now seeing.

So both comov. distance and ang.size distance have nice interpretations as the real physical distance to the object or material resepectively now and then----now at the instant we receive the light and then when the light was emitted.