How phase difference can be measured using lissajous figures

Hello everybody, I am a student and this is my first post...

If you connect the signal leads to the two identical signals, having some phase difference then
- you'll get a lissajous figure and it keeps on rotating
-so my doubt is that how can you apply the relation

phase difference = sin inverse(h1/h2)
when you cannot measure h1 and h2(because the pattern is not stationary)

please help me even if my problem is trivial