# Using Tully Fisher to measure Distance Problem

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 :)

• 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 Sep 24 '14 at 5:28
• you were correct! its about inclination. The answer by Aaron has attached notes (if you are curious) :) – Hari Sheldon Sep 24 '14 at 23:39
• 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 Sep 25 '14 at 1:50

With the Tully Fisher relation, the inclination angle is determined from the major-minor axis ratio where $sin(i) = \sqrt{1-q^2}$ and q is the inverse of your ratio.