<p>With the Tully Fisher relation, the <a href="http://www2.astro.psu.edu/~caryl/a480/lecture8_10.pdf" rel="nofollow">inclination angle</a> is determined from the major-minor axis ratio where $sin(i) = \sqrt{1-q^2} $ and q is the inverse of your ratio. </p>

<p>You need to apply the sine of the angle to the rotational velocity because the measure is based on our line of sight observation which must be converted to the true 3D velocity.</p>

CC BY-SA 3.0
2014-09-24T23:37:55.117Z
Thank you Aaron! This was indeed helpful especially the detailed attached notes! T-F is looking less mysterious slowly but surely :)
Hari Sheldon
CC BY-SA 3.0
2014-09-24T06:44:20.377Z
2014-09-24T06:44:20.377Z
Aaron
<p>Can anyone explain by looking at the solution in the pic, how did the cos (i) came about?
I guess they are getting this from the major-minor axis info in the question, but I am not sure about the derivation.</p>

<p>Also v=300/sin(i), what is this formula?</p>

<p>Many thanks :)</p>

<p><img src="https://i.sstatic.net/ABPqs.jpg" alt="enter image description here"></p>

CC BY-SA 3.0
2014-09-24T05:28:04.857Z
I think it is just an assumption on the inclination of the plane of the galaxy with respect to the line of sight of the observer.
Py-ser
CC BY-SA 3.0
2014-09-24T23:39:13.400Z
you were correct! its about inclination. The answer by Aaron has attached notes (if you are curious) :)
Hari Sheldon
CC BY-SA 3.0
2014-09-25T01:50:12.880Z
Yeah, but as @Aaron showed, it was not just "an assumption", you can derive it from the major-minor axis ratio. It was weird, indeed, that that parameter was not used in the solution...
Py-ser
<galaxy><astrophysics>
Using Tully Fisher to measure Distance Problem
Hari Sheldon
# Using Tully Fisher to measure Distance Problem

(Asked by: username_0 on 2014-09-24)

Can anyone explain by looking at the solution in the pic, how did the cos (i) came about?
I guess they are getting this from the major-minor axis info in the question, but I am not sure about the derivation.

Also v=300/sin(i), what is this formula?

Many thanks :)

![enter image description here](https://i.sstatic.net/ABPqs.jpg)

**Tags:** galaxy, astrophysics

> **Comments:**
> **username_1**: I think it is just an assumption on the inclination of the plane of the galaxy with respect to the line of sight of the observer.  
> **username_0**: you were correct! its about inclination. The answer by username_2 has attached notes (if you are curious) :)  
> **username_1**: Yeah, but as @username_2 showed, it was not just "an assumption", you can derive it from the major-minor axis ratio. It was weird, indeed, that that parameter was not used in the solution...  

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## 1 answers

### Answer by username_2 on 2014-09-24. Score: 2 | Accepted answer

With the Tully Fisher relation, the [inclination angle](http://www2.astro.psu.edu/~caryl/a480/lecture8_10.pdf) is determined from the major-minor axis ratio where $sin(i) = \sqrt{1-q^2} $ and q is the inverse of your ratio.

You need to apply the sine of the angle to the rotational velocity because the measure is based on our line of sight observation which must be converted to the true 3D velocity.

> **Comments:**
> **username_0**: Thank you username_2! This was indeed helpful especially the detailed attached notes! T-F is looking less mysterious slowly but surely :)  

---

